When people consider upgrading from the RTL-SDR, there are three mid priced software defined radios that come to most peoples minds: The Airspy, the SDRplay RSP and the HackRF. These three are all in the price range of $150 to $300 USD. In this post we will review the three units and compare them against each other on various tests.
Note that this is a very long review. If you don’t want to read all of this very long post then just scroll down to the conclusions at the end.
What makes a good SDR?
In this review we will only consider RX performance. So first we will define some terminology, features and specifications that are required for a good RX SDR.
SNR – When receiving a signal the main metric we want to measure is the “Signal to Noise” (SNR) ratio. This is the peak signal strength minus the noise floor strength.
Bandwidth – A larger bandwidth means more signals on the screen at once, and more software decimation (better SNR). The downside is that greater CPU power is needed for higher bandwidths.
Alias Free Bandwidth – The bandwidth on SDR displays tends to roll off at the edges, and also display aliased or images of other signals. The alias free bandwidth is the actual usable bandwidth and is usually smaller than the advertised bandwidth.
Sensitivity – More sensitive radios will be able to hear weaker stations easier, and produce high SNR values.
ADC – Analogue to digital converter. The main component in an SDR. It samples an analogue signal and turns it into digital bits. The higher the bit size of the ADC the more accurate it can be when sampling.
Dynamic Range – This is directly related to ADC bit size, but is also affected by DSP software processing. Dynamic range is the ability of an SDR to receive weak signals when strong signals are nearby. The need for high dynamic range can be alleviated by using RF filtering. Overloading occurs when a strong signal starts to saturate the ADC because the dynamic range was not high enough.
Images/Aliasing – Bad SDRs are more likely to overload and show images of strong signals at frequencies that they should not be at. This can be fixed with filtering or by using a higher dynamic range/higher bit receiver.
Noise/Interference – Good SDRs should not receive anything without an antenna attached. If they receive signals without an antenna, then interfering signals may be entering directly through the circuit board, making it impossible to filter them out. Good SDRs will also cope well with things like USB interference.
RF Filtering/Preselection – A high performance SDR will have multiple preselector filters that switch in depending on the frequency you are listening to.
Center DC Spike – A good SDR should have the I/Q parts balanced so that there is no DC spike in the center.
Phase Noise – Phase noise performance is determined by the quality of the crystal oscillators used. Lower phase noise oscillators means better SNR for narrowband signals and less reciprocal mixing. Reciprocal mixing is when high phase noise causes a weak signal to be lost in the phase noise of a nearby strong signal.
Frequency Stability – We should expect the receiver to stay on frequency and not drift when the temperature changes. To achieve this a TCXO or similar stable oscillator should be used.
RF Design – The overall design of the system. For example, how many lossy components such as switches are used in the RF path. As the design complexity increases usually more components are added to the RF path which can reduce RX performance.
Software – The hardware is only half of an SDR. The software the unit is compatible with can make or break an SDRs usefulness.
Next we will introduce each device and its advertised specifications and features:
Device Introduction and Advertised Specifications & Features
| Airspy | SDR Play RSP | HackRF | |
| Price (USD) | $199 / $ 249 USD (with Spyverter) + shipping ($5-$20). |
$149 USD + shipping ($20-$30 world, free shipping in the USA) £99 + VAT + ~£10 shipping for EU. |
$299 USD + shipping |
| Freq. Range (MHz) | 24 – 1800 0 – 1800 (with Spyverter addon) |
0.1 – 2000 | 0.1 – 6000 |
| ADC Bits | 12 (10.4 ENOB) | 12 (10.4 ENOB) | 8 |
| Bandwidth (MHz) | 10 (9 MHz usable) | 8 (7 MHz usable) | 20 |
| TX | No | No | Yes (half duplex) |
| Dynamic Range (Claimed)(dB) | 80 | 67 | ~48 |
| Clock Precision (PPM) | 0.5 PPM low phase noise TCXO | 10 PPM XO | 30 PPM XO |
| Frontend Filters | Front end tracking IF filter on the R820T2 chip. | 8 switched preselection filters + switchable IF filter on MSI001 chip | Two very wide preselection filters – 2.3 GHz LPF, 2.7 GHz HPF |
| ADC, Frontend Chips | LPC4370 ARM, R820T2 | MSi2500, MSi001 | MAX5864, RFFC5071 |
| Additional Features | 4.5v bias tee, external clock input, expansion headers. | LNA on the front end | 5v bias tee, LNA on front end, external clock input, expansion headers. |
| Notes |
The Airspy is designed by Benjamin Vernoux & Youssef Touil who is also the author of the popular SDR# software. Of note is that there has been a misconception going around that the Airspy is an RTL-SDR/RTL2832U device. This is not true; there are no RTL2832U chips in the Airspy. The confusion may come from the fact that they both use the R820T2 tuner. The RTL2832U chip is the main bottleneck in RTL-SDR devices, not the R820T2. When coupled with a better ADC, the R820T2 works well and can be used to its full potential. The Airspy team write that they sell units mostly to universities, governments and professional RF users. However, they also have a sizable number of amateur users. |
The SDR Play Radio Spectrum Processor (RSP) is designed by UK based engineers who appear to be affiliated with Mirics, a UK based producer of SDR RF microchips. The chips used in the SDRplay RSP are dedicated SDR chips which were designed for a wide variety of applications such as DVB-T tuners. The RSP uses these chips and improves on their front end capabilities by adding an LNA and filters in order to create a device capable of general SDR use. Initially when writing this review we had deep problems with the imaging of strong signals on the RSP. However, a recent Dec 22 update to the drivers has fixed this imaging problem tremendously. The SDRplay is currently selling about 1000 units a month according to electronicsweekly.com. |
The HackRF is designed by Micheal Ossmann a computer security researcher who was given a development grant from DARPA. His company is called “Great Scott Gadgets”. The HackRF’s most unique feature when compared to the other two SDR’s is that it is capable of both receiving and transmitting. There is also a clone called the HackRF Blue out on the market which is about $100 cheaper, but they don’t seem to have stock or be producing these any more. |
From the specs it is clear from the ADC sizes that both the Airspy and SDRplay RSP are in a different class of RX performance when compared to the HackRF. However, people always compare the Airspy and SDRplay with the HackRF due to their similar price range, so we will continue to compare the three here in our review, but with more of a focus on comparing the Airspy and SDRplay RSP.
In order to use the Airspy on HF (0 – 30 MHz) frequencies a $50 add on called the Spyverter is required. This is an upconverter that is designed for use with the Airspy’s high dynamic range and bias tee power port. However, one hassle is that the Spyverter must be connected/disconnected each time you want to switch between HF and VHF/UHF reception as it does not have VHF/UHF passthrough. The RSP and HackRF on the other hand can receive HF to UHF without the need of an upconverter or the need to change ports. A single port for HF to UHF can be very useful if you have a remote antenna switcher.
Post continues. Note that this is a long post with many images.
System Hardware Requirements
| Airspy | SDRplay RSP | HackRF |
|
Requires a fast modern PC. The Airspy website suggests at least a 3rd gen Intel i3 2.4 GHz processor. The creators have also suggested elsewhere to look up your CPU score on PassMark and ensure that your CPU score is above 3500. You will also need to ensure that you have a high quality high speed USB 2.0 port and controller. Some controllers are known to be buggy and are unable to provide the full required bandwidth. Update your USB drivers if you have issues. Requirements can be reduced slightly by using the “bit packing” feature of the Airspy. Overall CPU requirements are much higher than the other two options because the Airspy only supports a 10 MSPS (10MHz) sample rate. There is a 2.5 MHz sample rate available, but they write that it may not operate that well at the moment. The Airspy also uses a different USB mode of operation compared to the RSP, which requires greater CPU power. |
No requirements are given, but we estimate that specs significantly lower than what is needed by the Airspy. The RSP also supports various smaller bandwidths which can reduce CPU requirements. From online reports it seems to run fine on older PC’s like Core2Duos, though the maximum sample rate may be restricted. |
No specific requirements given, other than the need for a good Hi-Speed USB Port for running the HackRF at higher sample rates. At the highest sample rate of 20 MSPS we estimate that CPU requirements similar to the Airspy are needed. |
Initially we had trouble running the Airspy on our main PC at its largest bandwidth of 10 MHz. There was significant crackling and jitter on the spectrum due to lost packets. Either our older Intel i5-750 2.67 GHz CPU (passmark 3732 but overclocked to 3.33 GHz) is not fast enough for this high amount of data, or our USB chipset is not good. After updating the USB drivers to the latest version the problem improved, but stuttering was still present. Our problems were solved when we discovered the bit packing feature. With this feature enabled the Airspy worked fine at 10 MHz. This feature reduces the amount of data needed to be transferred by packing the bits resulting in less data transfer for the same results. We also tested the Airspy on a more modern Intel i5-3470 @ 3.2 GHz with passmark score of 6568, and it ran perfectly at 10 MSPS without the need for bit packing, but strangely this CPU saw dropped packets when bit packing was turned on. An Intel i7 laptop on the other hand had no problem running the Airspy with bit packing off. On an i5-4200U @ 1.60 GHz CPU (passmark 3280) laptop the Airspy stuttered even with bit packing turned on.
The RSP worked fine on all the PCs that we tried on (all i5 and i7’s mentioned above) and it ran well at the highest rate of 8 MHz.
On our i5-750 and i5-4200U CPUs we were unable to run the HackRF at 20 MHz without significant crackling and jitter on the spectrum due to lost packets. At the next lowest setting of 16 MHz it ran fine. On the i5-3470 the HackRF ran fine at 20 MHz.
We monitored the USB usage using Windows performance monitor and discovered that the Airspy (with bit packing at 10 MHz) used 30,000,000 bytes per second, the RSP (at 8 MHz) 25,000,000 bytes per second and the HackRF (at 20 MHz) 40,000,000 bytes per second.
It was mentioned to us by the SDRplay team that the main reason that the RSP works well on older hardware is its use of an “isochronous mode” USB driver which is in contrast to the Airspy’s “bulk mode” driver. An isochronous driver reserves the necessary USB bandwidth, whereas a bulk mode driver does not. Thus there is a greater risk of packet loss with a bulk mode driver. However, the disadvantage of an isochronous driver is that there is no means to know how many samples were lost if the system has a transfer error. This means that with an isochronous driver it is impossible to implement coherent receivers, which is one of the Airspy’s use cases with it’s external clock input.
We also note that the Airspy sends raw ADC samples to the PC and then must do the IQ conversion on the PC. In low IF mode the RSP does the same, but in Zero IF mode it sends IQ data.
Physical Appearance and External Design
| Airspy | SDRplay RSP | HackRF | |
| Dimensions | 5.3 x 2.5 x 3.9 cm | 9.7 x 8.0 x 3 cm | 12.2 x 7.6 x 1.7 cm |
| Weight | 65 g | 110 g | 100 g |
| Ant. Connector | SMA | SMA (F-Type on older units) | SMA |
| USB Connector | Micro USB | USB B | Micro USB |
| Case/Shell | Aluminium | Plastic | Plastic |
| Additional Connectors | MCX CLKIN connector | None | SMA F for CLKIN and CLKOUT |
| Buttons | None | None | RESET and DFU (Firmware flash) buttons. |
| Notes |
– |
Previous versions of the SDR Play (like our purchased unit) used an F-Type antenna connector. |
– |
The Airspy is the smallest device with the SDRplay and HackRF being close to the same size.
With RF devices a good enclosure is recommended to help keep any strong RF interference out of the signal path. A conductive metal enclosure is best so that a faraday cage is created. Out of the three units, only the Airspy comes with an aluminium enclosure, and a good electrical connection is made to the enclosure via a nut on the SMA connector and through edge tracks on the PCB. The HackRF and RSP both come in plastic cases and so have no shielding.
All three SDR’s use a standard SMA connector. Previous versions of the RSP, like the unit we used in this review came with F-Type connectors. We don’t like the F-type connector because it is less commonly used in the radio field and it has poorer RF insertion loss properties, so it is good to see that they changed the connector in the newer versions. There were no other changes to the RSP in the upgrade apart from the case and connector, so we believe this change should not significantly affect the review.
We’re not fans of the micro USB ports used on the Airspy and HackRF as they tend to easily cause loose connections with a bit of movement, but due to size constraints we understand why they were used. The HackRF does indeed sometimes disconnect when we move the cable or device around without care, but we have not had this trouble with the Airspy. Micro USB connectors are also easily broken off PCBs with cable strain, however the Airspy and HackRF casings appear to provide adequate strain relief. We also note that Airspy R2 also has a vastly improved micro USB connector that is through hole mounted onto the PCB and looks almost impossible to break. Regardless, as with any connector we would advise against applying too much strain.
The USB B connector on the RSP is sturdy and does not easily come loose. It is also easier to find high quality shielded USB cables with USB B connectors.
The Airspy and HackRF both have external clock inputs. This allows these devices to be used with more accurate clocks, such as GPS synchronised ones. It also allows them to be used as coherent receivers (many receivers using the same clock source) for various applications such as doppler direction finding and passive radar setups.
Installation and ease of use for general frequency browsing on Windows
| Airspy | SDRplay RSP | HackRF |
|
Installation involves simply plugging the Airspy into the USB port of the PC and letting Windows plug and play automatically install the drivers. The user can then open up SDR#, select Airspy from the menu and push start. |
Setting up the RSP first requires installation of it’s drivers from the SDRplay website. As the RSP has no official software, the second step requires the installation of a plugin for SDR#, or other supported software such as HDSDR. The entire set up procedure is simple, but it is not plug and play. |
Installation on Windows is similar to installing an RTL-SDR. Just run zadig and install the WinUSB driver for the HackRF. Then it can run on Windows with SDR#. |
All the SDR’s were easy to install on Windows, but the Airspy was the easiest with its plug and play operation.
Available Software
| Airspy | SDRplay RSP | HackRF |
|
Native compatibility with SDR# on Windows. Also compatible with HDSDR, SDR-Console and GQRX on Linux. Good support for some other software that is most often used with the RTL-SDR:
Also has unofficially developed ExtIO interface allowing it to work with any app supporting ExtIO. For example:
Good support for the Raspberry Pi 2, especially for ADS-B where they have developed an official 20MSPS ADS-B decoder which claims performance as good as or better than a dedicated Beast ADS-B receiver. Linux and Mac open source drivers available, but no SDR# support. |
Compatible with SDR#, HDSDR and SDR-Console through plugins. Though compatibility with SDR# is restricted as third party plugins cannot be used. The RSP is also compatible with any app supporting ExtIO. For example:
Linux drivers available and plugins for SoapySDR, CubicSDR, Pothos and GNU Radio are available. CubicSDR also runs with the RSP on the Mac. Eventual improved support planned for Raspberry Pi 2 and Android. |
Compatible with SDR#, HDSDR and SDR-Console on Windows, GQRX on Linux and RF Analyzer on Android. Several programs in code form on GitHub, but not many “plug and play” apps. Designed to be used more with software like GNU Radio. |
One advantage to using the Airspy with its SDR# software is the “decimation” feature. With wideband SDR’s it can be difficult to spot or tune into narrowband signals. You can use the zoom feature, but when zooming you lose resolution. The decimation feature reduces the visible bandwidth, but keeps the resolution high, allowing weak signals to be easily discerned from the noise. An added bonus is that the effective number of ADC bits is increased with decimation, meaning that signals can have higher visual SNR (full audio decimation is performed automatically). This makes the Airspy very good at browsing and fine tuning on small narrowband signals.
The SDRplay RSP can achieve a similar effect by reducing the bandwidth displayed, however this means that a lower sample rate is used, and thus less decimation occurs.
The SDRplay and HackRF do not have the decimation software feature yet, though the SDRplay team write that they have the decimation feature scheduled for an upcoming API update. The RTL-SDR has this feature through a third party plugin written by Vasilli so it seems feasible that this feature can be easily implemented.
Estimated Bill of Materials Cost
These are just very rough guesses and they could be wildly inaccurate. Note that these costs are for parts only, and they do not take into account manufacturing costs, engineering time costs and support staff overhead costs etc.
| Airspy | SDR Play RSP | HackRF |
|
LPC4370 ~ $8 SI5351C ~ $3 R820T2 ~ $1? Other components, connectors, passives, PCB, case, cable etc ~$25 Total Cost ~$37? |
Mirics FlexTV dongle (dongle uses the same MSi001 and MSi2500 chipsets as the RSP) claimed $5 BOM. Filters + Switches ~$10 Connectors, Passives, PCB, case ~$10 LNA ~ $1 Total Cost ~$26? |
MAX2837 ~$10 MAX5864 ADC~ $6 RFFC5072 Mixer ~$13 LPC4320FBD144 Processor ~ $5 XC2C64A-7VQG100C FPGA ~ $3 Switches ~ $10 SI5351C ~ $3 Other components, connectors, passives, PCB etc ~$20 Total Cost ~ $70? |
RF Design
| Airspy | SDR Play RSP | HackRF |
|
The RF chain in the Airspy goes Input -> R820T2 -> LPC4370. The LPC4370 has a 12 bit ADC. The website claims a NF of 3.5 dB and an IIP3 of 35 dBm. However this noise figure is probably taken at maximum gain, and the IIP3 taken at zero gain. The Airspy does not have preselectors, apart from an internal IF filter in the R820T2 chip. It seems that the design approach of the Airspy is to optimize for signal linearity and to avoid overloading through natural high dynamic range. |
The RF chain in the RSP goes Input -> Switch-> MGA-68563 LNA-> Switch -> Filter -> MSi001 -> MSi2500. The MSi2500 has a 12 bit ADC The RSP is designed with switching RF filters and an MGA-68563 LNA right at the front end which is only active for signals that are above 60 MHz. The MSi001 tuner chip has a second LNA inside it. The MGA-68563 is used as a preamp to overcome the losses in the filters and presumably to lower the noise figure of the internal MSi001 tuner LNA’s. The MGA-68563 has a 1 dB noise figure, 19.7 dB gain and a 20 dBm OIP3. According to the SDRplay team the MSI001 has a NF of around 4.5 dB in VHF and UHF mode. The RSP has 8 switched front front end filters that are automatically selected, as well as an adjustable IF filter inside the MSi001 chip which can help to overcome interference from strong in band signals. To use this internal filter the IF bandwidth must be reduced in the RSP configuration screen. This means that when reducing the IF filter size you will see less that 8 MHz. In conclusion it seems that the SDRplay tries to optimise itself for sensitivity by using a front end LNA, and focuses more on overcoming the effects of overloading via switched filter banks. |
The RF chain in the HackRF is much longer longer and more complex. The front end goes Input -> Switch -> MGA81563 Amp (optional) -> Switch -> Switch -> Switch -> LPF/HPF -> Switch -> Switch -> RFFC5072 Mixer -> Switch -> Switch -> MAX2837 -> MAX5864 ADC -> LPC43XX processor. The MAX5864 has an 8 bit ADC. In order to get the HackRF to perform over such a wide range and to RX and TX, many diode switches are placed into the RF chain. Each of these switches causes a 0.35 – 0.5 dB signal loss which can explain why the HackRF has fairly poor sensitivity. The HackRF also has no real filtering, but technically there is a 2.3 GHz LPF and a 2.7 GHz HPF. In conclusion the HackRF has rather poor RX specs, with only an 8 bit ADC and several lossy switches. |
We are of the opinion that adding an extra LNA right at the front end of a receiver (like what is done on the RSP and HackRF) is generally a bad idea. This is because an LNA at the front end will not reduce the noise figure as much as an external LNA placed near the antenna would. Additionally, if there is a built in LNA placed near the front end, then this ruins the system for optimal performance if an external LNA were to be used. If we were to place a second LNA near the antenna to overcome coax losses, then the linearity (IP3) of the system would be further degraded due to the additional internal LNA, possibly resulting in more overloading and intermodulation. The RSP designers decided to add an extra LNA at the front end to overcome the preselector filter insertion loss, but we think perhaps adding a bias tee and supplying an external LNA would have been better for performance, although more cumbersome. The option to bypass the front end LNA during operation would also be beneficial.
See this previous post for more information on proper LNA placement.
SDR Controls
Each SDR has its gain and sample rate controlled through an on screen interface.
Airspy
The Airspy can run in sensitivity, linearity or free mode. The sensitivity and linearity modes simply choose an optimal set of values for the IF/Mixer/LNA gains which can be controlled manually in free mode. The sensitivity mode uses more gain on the LNA which can come at the expense of reduced linearity and thus more intermodulation. The Linearity mode uses less LNA gain, and more IF/Mixer gain which can reduce intermodulation, but at the expense of a few dB’s of SNR. The Airspy quickstart guide suggests the following procedure for setting the gain:
- Start with the minimum gain
- Increase the gain until the noise floor rises by about 5dB
- Fine tune to maximize the SNR (the bar graph on the right)
The sample rate drop box allows you to choose between the two sample rates available for the Airspy which are 10 MSPS and 2.5 MSPS. The 2.5 MSPS option is still experimental and is known to have USB noise problems on some PCs.
The decimation feature allows you to reduce the visible bandwidth while at the same time increasing the visible SNR. This allows you to easily spot and tune into weaker signals.
The Bias-Tee check box allows you to turn on the 4.5V bias tee. The SpyVerter option automatically sets the frequency offset to -120 MHz for easy operation with the SpyVerter upconverter.
There is no option to set the PPM offset (the PPM box is for adjusting the Spyverter upconverter only) as the 0.5 PPM TCXO used on the Airspy should not need any offset adjustment.
Airspy Controls
SDRplay RSP
For the SDRplay we used a beta version of their SDR# 1400+ plugin for their latest API which fixed most of the RSP’s imaging problems. It was a beta released just to us to do this review sooner so it was a little buggy in terms of crashes. SDRplay recommend using their ExtIO plugin instead, but to get fair comparison screenshots we wanted to use the same SDR# version on both Airspy and RSP tests. To be sure the plugin wasn’t affecting RX results we compared it to their officially released ExtIO plugin for older versions of SDR# and HDSDR and we saw no changes in terms of signal performance.
The gain tuning method used by the SDRplay is a little different compared to most SDRs like the Airspy and the HackRF. Here instead of tuning by adding gain, they use a gain reduction (GR) method which reduces the gain by some amount from the total available gain. As you adjust the gain slider amplification in the mixer and IF stages change automatically.
There is also the option to toggle on or off the internal Mirics chip LNA. This is the LNA in the tuner chip, and not the front end LNA which is always on for VHF+ frequencies. Turning this LNA on can help to reduce the noise floor, but also may cause additional imaging problems.
If you prefer to use the automatic gain control (AGC) then you can control the “setpoint” value which will try and keep the noise floor at the specified value.
In the IF Amplifier section you can also choose between Zero IF and Low IF modes. The Low IF mode appears to produce less unwanted images, however if the Low IF mode is selected then the largest bandwidth available is 1.536 MHz. The IF bandwidth setting lets you specify the size of the IF filter used in the MSI001 chip. Setting a lower value will reduce the amount of visible bandwidth, but can help to block out in band interferers.
The ADC sample rate can also be adjusted independently of the IF bandwidth, but must be equal to or larger than the IF bandwidth.
SDRplay Controls
HackRF
The HackRF has only two gain slider control options, LNA and VGA, but there is also an option to enable the MGA-81563 front end amplifier with the Amp check box.
The sample rates available are 8, 10, 12.5, 16 and 20 MSPS.
HackRF controls
Real World Signal Reception Tests (VHF/UHF)
The methodology of these tests was to tune to a known signal and adjust the gain settings until the best SNR was obtained on that signal. We then compared the SNR achievable by each SDR (sensitivity) and took note of any undesired effects such as interference and intermodulation effects (dynamic range).
For a better screenshot comparison we also reduced the visible bandwidth of each SDR down to 6.4 MHz. This is simply a crop and does not change the FFT resolution. The Airspy, RSP and HackRF were running at 10 MHz, 8 MHz and 8 MHz respectively, but the spectrum were zoomed in to 6.4 MHz. FFT resolution for each test was set at 32k.
One important thing to note is that we cannot easily compare SNR values if the bandwidth of each device is different. Reducing the visible bandwidth increases the FFT density which causes the visible SNR to rise. Doubling the bandwidth causes a 3dB drop in visible bandwidth. So with the same signal viewed at 10 MHz and 5 MHz, the SDR with the 5 MHz would show an SNR level 3dB higher than on the 10 MHz SDR. Therefore the difference in SNR in a stable signal when shown at 10 and 8 MHz bandwidths is 3 * (10 – 8) /5 = 1.2 dB. In all these tests that we run at 10 MHz and 8MHz bandwidths, we must reward the Airspy SNR readings by +1.2 dB to get a more accurate comparison. This is a small change and probably within our measurement margin of error, but it still should be noted.
We tested each SDR within 5 minutes of each other to ensure the signal conditions did not change too dramatically, but just in case we repeated the test several times to ensure that the relative SNR’s where stable across the test time period. We could not test the units at the same time when connected to an antenna splitter because 1) Each of the SDR’s output their own noise which interfered with the other units and 2) our PC was not powerful enough to run all three at once.
In all tests we used a roof mounted Diamond D130 discone antenna. It was placed in a suburban area with good reception from several radio towers that were about 15-30 kms away. Some of the tests we performed include:
- No overload test. We optimised the gain settings to provide maximum SNR of a desired signal whilst not increasing too far as to cause images from overloading.
- Overloading allowed test. In these tests we optimized the desired signal gain for max SNR on our desired signal, and we did not care if overloaded signals showed up as long as they did not affect the desired signal.
- Test with BCFM filter. Since we had strong broadcsat FM in our area we repeated some of the overloading allowed tests with a BCFM trap in place.
- Test with LNA. Here we tested with an external LNA. The LNA we used was the LNA4ALL which has an approximate 18-23dB gain at most frequencies.
- Test with LNA and BCFM filter. Here we tested with an external LNA and a BCFM filter in front of it.
- Test with LNA and high loss coax. Here we used more lossy coax so that the gain from the LNA was not so harsh.
75 MHz Police
Here we tried to listen to a police frequency at 75 MHz. There was strong broadcast FM interference at 88 – 108 MHz that would cause problems if we increased the gains too much.
Max SNR without Overloading Test
In this test we adjusted the gains to get the highest SNR without having interference that would wipe out the band.
| Airspy | RSP | HackRF |
|
Here the Airspy was able to obtain the best results, receiving with the highest SNR and with a fairly clean spectrum. Here Linearity mode worked the best, with the Sensitivity mode working poorly due to the broadcast FM interference. |
The RSP was set the GR 62dB with the internal LNA turned off. It was able to receive well too, but with an SNR about 10dB lower than the Airspy and with more interference shown and some mild imaging across the center. Increasing the gain further caused the RSP to overload and display broadcast FM interference all over the band. The reason the RSP performed more poorly in this test is possibly because this 75 MHz frequency is covered in the same 60 – 120 MHz bandpass filter that covers the broadcast FM band. This means that the broadcast FM band is not blocked out at all while tuned to 75 MHz. |
The HackRF performed the worst, overloading the easiest and having the lowest SNR, but the signal was still very easily copyable. Surprisingly even with interfering stations nearby turning on the front end amplifier improved the signal significantly without causing too much interference. |
Below we show the type of interference that started to show up when the gains were increased too far. All three units showed similar interference.
Broadcast band interference showing on the RSP when the gain was increased too far.
Discussion
The results showed that the Airspy was able to easily receive the police signal the best, with the RSP coming in second. Increasing the gain too far caused overloading issues and the noise floor to rise. We think that the RSP could not perform as well as the Airspy since it relies more on adequate filtering to improve dynamic range than the Airspy does. Since the RSP filter active at 75 MHz is a 60 – 120 MHz bandpass filter, overloading effects from broadcast FM (BCFM) can easily be seen as it is not filtered out.
RSP Bandpass Filter for 60 – 120 MHz
Broadcast Band FM (BCFM) Radio
Here we tested each radio on it’s ability to receive broadcast band FM. Note that we offset the IF tuned area slightly in order to capture the peak and the noise floor bottom more accurately in SDR#’s SNR measurement tool.
Max SNR Test
| Airspy | RSP | HackRF |
|
The Airspy was able to receive best under Sensitivity mode with little to no interference or imaging, but it was consistently about 3-6 dB less sensitive that the RSP. |
The RSP consistently received the best in this test, usually getting SNR values about 3-6 DB’s higher than the Airspy. The best reception occurred at a GR 55 with the Mirics LNA turned off. |
The HackRF received the poorest, consistently being having about 10-15 dB’s or SNR less. With the front end amplifier turned on the band became overloaded. |
LNA Test
We also tested BCFM reception with an external LNA connected. It was placed before an extra 3M length of RG174 coax cable.
| Airspy | RSP | HackRF |
| The Airspy performed similarly to when used without an LNA. | Here the RSP showed problems with overloading even when the GR was reduced to the very lowest level and the Mirics LNA was turned off. |
The HackRF performed similarly to when used without an LNA. |
LNA with higher loss cable test
We also did another test for the Airspy and RSP with the LNA connected to some much higher loss cable. To get higher loss we added in a 12dB attenuator as well as 10m of RG174 which has a loss of about 3dB at 100 MHz. The total loss is about 15dB and the gain from the LNA4ALL is about 22dB, so the total left over gain is 7dB.
| Airspy | RSP |
| The Airspy performed well with the LNA and lossy cable as in the last test. Mild interference was seen between strong stations. | With less overall gain in front of it the RSP performed much better as was almost identical to the Airspy. There was higher noise levels from overloaded signals between the strong stations though. |
Discussion
It was interesting to see in this test that the Airspy could not reach a maximum SNR as high as the RSP could. However, as predicted, the overall performance of the RSP was reduced too much when an external LNA was added, making it unable to receive properly even with the RSP gain set at its minimum value. Reducing the external gain through more loss helped stabilise the RSP results. It is likely that the LNA4ALL itself was overloaded by the BCFM band in these tests.
It was mentioned by Yousseff that a better metric for BCFM comparison is the MPX spectrum of the FM signal, so below we do some MPX comparisons.
BCFM MPX Comparison
Before we published this article we gave the draft to the creators of the Airspy and RSP for review. Yousseff, co-creator of the Airspy suggested that we look at the MPX spectrum of the BCFM signal as his concern was that the stronger SNR shown by the RSP was not accurate as its SNR may be higher due to non-linearities mixing into the actual signal.
To test the MPX spectrum we used HDSDR with the unofficial Airspy ExtIO plugin and the RSP with it’s official ExtIO plugin. We piped the audio out through stereo mix and looked at it in SDR#. This allowed us to view the broadcast FM MPX spectrum, which allowed us to see things like the stereo pilot tone and RDS. We then measured the SNR of the stereo pilot tone.
92.6 MHz Strong Station
This was a strong classical music station.
| Airspy | RSP |
| The Airspy had an SNR of 64.8+1.2=66dB in the frequency spectrum and an MPX SNR of 64.8dB. | The RSP had an SNR of 71.5dB in the frequency spectrum and an MPX SNR of 65.7dB. |
Overall the MPX SNR was nearly identical between the two SDR’s. We also checked other similarly strong stations and all gave similar results, with the RSP showing higher SNR on the frequency spectrum, but both showing near identical MPX SNR’s.
87.6 MHz Weak Station
At 87.6 MHz there was a weak station and we repeated the MPX test. Here we note that when we pushed the spectrum to the side the Airspy performed better and was able to reach a frequency spectrum SNR nearby identical to the RSP.
| Airspy | RSP |
| Here the Airspy had an SNR of 29.4+1.2=30.6 dB in the frequency spectrum and an MPX SNR of 45 dB | The RSP had an SNR of 30 dB in the frequency spectrum and an MPX SNR of 44.4 dB. |
104.2 MHz Weak Station
At 104.2 MHz there was another low power FM station.
| Airspy | RSP |
| Here the Airspy has an SNR of 31.2+1.2 = 32.4dB in the frequency spectrum and an MPX SNR of 43.5 dB. | The RSP had an SNR of 41.6 dB in the frequency spectrum and an MPX SNR of 44.5 dB. |
Discussion
When comparing the MPX spectrum on both strong and weak stations we found that although the RSP showed a higher SNR in the frequency spectrum, the SNR in the MPX spectrum appears to be nearly identical to the Airspy’s.
150 MHz
Here at 150 MHz there are some taxi radio signals and also some trunking signals. Since this frequency is very close to a powerful pager at 157 MHz, and also still close to the strong broadcast FM band, it is easy to see effects from overloading.
Max SNR with no Overloading Allowed
In this first test we attempted to maximise the SNR on all three radios, whilst ensuring that the spectrum was kept clean of any overloading effects.
| Airspy | RSP | HackRF |
|
Here the Airspy was able to receive about 10 dB better than the RSP. We note that the Sensitivity and Linearity tuning modes did not work well at all. Here we need to manually tune each gain setting to get the best results. |
With the RSP we could not increase the gain further than [GR60 Mirics LNA off] before overloading effects were seen. |
The HackRF performed the worst. It was able to receive best with the front end amplifier turned on. |
Max SNR with Overloading Allowed
In the next test we tried to maximise SNR by adjusting the gains without regard to the overloading effects – as long as these effects did not completely wipe out the signal.
| Airspy | RSP | HackRF |
|
In the previous no interference test the Airspy was already near the limit of maximum SNR. Increasing the gain further started to cause the noise floor to rise and cause the SNR to drop. We were only able to increase the gain a little further. By increasing the gain a little more we started to see mild broadcast FM band interference that would only show up when the pager transmitted, as well as some other imaging effects. |
The RSP was able to reach a SNR almost as good as the Airspy’s after the gain was increased to [GR36 Mirics LNA off], but it came at the expense of seeing some bad broadcast FM band interference show up whenever the pager transmitted. |
We were also able to increase the HackRF’s SNR by increasing the LNA gain further, but at the expense of some pretty heavy interference of the same type as seen on the RSP. |
Max SNR with BCFM Filter
In this next test we placed a broadcast FM block filter in front of each SDR and observed the effect. We attempted to tune for max SNR regardless of any overloading effects, as long as they did not affect the signal of interest.
| Airspy | RSP | HackRF |
|
With the Airspy and broadcast FM block filter we were able to pump up the gains a little further and increase the SNR by about 1 dB over the last test. The mild broadcast band interference that appeared when the pager transmitted no longer showed up and we saw no other effects from the nearby pager. |
With the BCFM blocking filter in place the RSP was able to achieve a 3dB+ increase in SNR over the previous test, and without any BCFM overloading effects shown. However, one important problem was that when two pager frequencies transmitted at the same time, then there was very bad interference that affected our signals of interest (see the bottom of the image). We were unable to get rid of this interference for all gain settings, so we just had to ignore it and tune for best SNR. There was also images of the nearby pager that showed at 150, 151, 151.5, 152.5 and 154 MHz. |
Oddly, the HackRF SNR levels performed worse with the BCFM filter in place, but the BCFM interference was nullified. We are unsure of the reason why. |
LNA Test
Here we connected an LNA in front of 6M of RG174 cable and observed the results.
| Airspy | RSP | HackRF |
| With an LNA in front broadcast FM interference was increased a lot. The max SNR was now only 59.7+1.2=60.9dB. | Here the RSP had similar interference problems to the Airspy and when it was run without the BCFM filter. It reached a max SNR of around 56.4dB. |
The HackRF’s performance did not change too much with the LNA. There was still lots of interference. |
LNA with BCFM Filter
This time we connected a BCFM filter in front of the LNA and retested.
| Airspy | RSP | HackRF |
| With the BCFM interference removed the Airspy performed well once again, with only mild pager interference showing up. | The RSP still had the same problem with the pager interference mixing into the desired signals. | The HackRF performed better with the BCFM filter in place, but there was still significant interference whenever the pager transmitted. |
LNA with high loss cable
The RSP team suggested that an LNA should only be used to overcome the cable loss. So here we added a 12dB attenuator and 10M of RG174 cabling which gives a 3.5dB loss to get a total loss of about 15.5 dB at 150 MHz. The LNA4ALL has about 23.5dB gain at 150 MHz, giving a total extra gain of 8 dB.
| Airspy | RSP |
| Like in the other tests the Airspy saw significant FM interference, though it was less than with higher external gain used. | Similarly the RSP still had high FM interference present, much higher than the Airspy. At this frequency cable losses are low and the total extra gain of 8dB was probably still too high for the RSP to work well. |
LNA with high loss cable and BCFM Filter
In this test we added a BCFM filter in front of the LNA.
| Airspy | RSP |
| With the BCFM filter in place the Airspy performed very well. | With the BCFM filter and lower external gain the RSP no longer seemed to show the problem with the pager signal mixing into the desired signal. However, there was still pager images visible in the spectrum. |
Discussion
Here at 150 MHz there is strong BCFM signals at 88-108 MHz and a strong pager at 156 MHz. In this environment the Airspy won out as the best receiver. The Airspy was able to get the best SNR for our target signal without any significant interference from overloading. The RSP struggled with overloading interference from BCFM, and even when a BCFM block filter was used it had problems with pager interference destroying our signal of interest. By looking at the bandpass filter used on the RSP when tuned to 150 MHz, we can see that the BCFM band attenuation may not be strong enough to help. Also it does not block out the 156 MHz pager.
With an LNA used all receivers had issues with BCFM interference, and it is likely that the LNA4ALL was overloaded. Adding a BCFM block filter improved things for the Airspy, but although BCFM interference for the RSP and HackRF was reduced, pager interference which was not blocked out now strongly affected the RSP and HackRF.
The 120 – 250 MHz RSP filter active when tuned to 150 MHz.
Marine And Pager Signals
In our next test we moved up to 161 MHz and tried to receive a fairly weak marine weather station (NOAA weather equivalent). This station is very close to the 157 MHz pager, which can easily wipe out the marine signal if the RF gain is set too high.
Max SNR with no Overloading
In this first test we tried to maximise the SNR of the marine signal whilst not allowing overloading effects to occur across the spectrum.
| Airspy | RSP | HackRF |
|
Here the Airspy had the best SNR of around 17.4+1.2 = 18.6 dB, however there was some mild overloading effects present just next to the pager signal which would not go away, as well as a very mild image near 162 MHz. |
The RSP was able to reach an SNR of around 15.4 dB without much interference showing up, although the spectrum was not as flat as the Airspy’s. |
The HackRF had the lowest SNR of 12 dB, and had some bad overloading effects show up near the pager signal. Some FM interference also showed but we did not reduce the gain further as it was only mild. |
Max SNR with overloading
In the next test we tried to maximise the SNR without regard to the effects of overloading – as long as it did not wipe out the marine weather signal.
| Airspy | RSP | HackRF |
|
Here the Airspy was able to reach a maximum gain of 28.5+1.2=29.7 dB’s and only pager intereference was seen, none from the BCFM band. |
The RSP showed some pretty heavy BCFM and pager interference but luckily the marine signal was not too affected by this as it was not underneath any interference. It was able to reach a maximum SNR of around 32.1 db, and was consistently about 2 dB higher than the Airspy. |
The HackRF could only get a max SNR of around 22.7 dB. |
Max SNR with overloading and BCFM filter
In this next test we added a broadcast FM band filter in front of the LNA and tuned for best SNR regardless of the overloading effects shown.
| Airspy | RSP | HackRF |
|
Here the Airspy produced results very similar to without the BCFM filter. It appears that the Airspy was not affected by BCFM interference in this test, only by the strong pager. |
The SDRplay on the other hand showed a good improvement in terms of a reduction in BCFM interference from overloading. With the BCFM filter in place only pager interference shows. |
Similarly the HackRF showed a good improvement with no more BCFM interference shown. |
Max SNR with LNA
Here we added a 6M length of RG174 and placed an LNA4ALL at its beginning to try and overcome the loss.
| Airspy | RSP | HackRF |
|
Here the Airspy performed similarly to when used without a LNA. |
The RSP used a gain setting of [GR50 Mirics LNA off] and couldn’t achieve and SNR as good as without the LNA. Increasing the gain further caused very bad overloading. Overall performance was similar to the Airspy. |
The HackRF also showed interference very similar to without the LNA. |
Max SNR with LNA and BCFM Filter
Here we placed a BCFM filter in front of the LNA.
| Airspy | RSP | HackRF |
|
The Airspy performed very similarly in terms of SNR as when used without the BCFM filter, but the nature of the pager interference changed. It became more clumped up nearer to the actual pager frequency. |
The RSP seemed to work much worse with the BCFM filter in place. The SNR couldn’t reach as high as when used without it. The gain used was [GR62 Mirics LNA off] and it was optimized for the best SNR. The BCFM filter also caused an odd rise in the noise floor around the pager. |
With the HackRF the BCFM interference was removed, but heavy pager interference remained. We weren’t able to pump up the gain as much as when used without the BCFM filter, but we could achieve a higher SNR with the filter in place. |
Max SNR with LNA high loss cable and BCFM Filter
| Airspy | RSP |
| Here the Airspy reached a maximum SNR of around 23.8+1.2=25 dB. | The RSP reached a slightly higher maximum SNR of 27.5 dB, but had a bit more pager interference. The lower external gain seemed to reduce the problems seen in the last LNA test. |
Discussion
Again in this test the Airspy showed that it was able to receive well without any effects of overloading showing up. The Airspy was pretty immune to the effects of BCFM overloading at this frequency, but still had trouble with the pager overloading. The RSP had issues with BCFM and Pager overloading.
Adding a BCFM block filter had no affect on the Airspy which was already unaffected by BCFM interference. On the RSP it removed the BCFM interference but caused the pager interference to have more effect.
Adding an LNA did not help much with such a strong pager nearby causing overloading to occur at much lower gain levels on all three SDRs. Placing a BCFM filter in front of the LNA had almost no affect on the Airspy once again. Interestingly the BCFM + LNA combo with the RSP caused a big increase in overloading effects, the reason why we are unsure. When less external gain was used due to the high loss cable the RSP’s performance improved.
At 162 MHz, as with the last test at 150 MHz, the RSP used its 120-250 MHz bandpass filter which may not have been sufficient to fully block out BCFM and of course it could not block out the pager.
463 MHz
At 463 MHz we have a business band full of various types of trunked radios as well as some telemetry signals.
Max SNR with no Overloading
Here we tried to maximise the SNR of a target signal without any overloading showing up.
| Airspy | RSP | HackRF |
|
Here the Airspy performed the best, reaching an SNR value of 46.3+1.2 = 47.5 dB. To get good results we had to use the Free mode and carefully tune the gains manually. The sensitivity and linearity modes could not reach a SNR value anywhere near as good at 46 dB. |
The RSP was able to reach a SNR of 38.5 dB. Increasing the gain further caused BCFM interference to show. The gain that gave the best SNR without overloading was GR66 with the Mirics LNA turned on. Mild BCFM interference was showing at this gain level. |
The HackRF could only reach a max SNR of 34.7 dB. |
In all three tests increasing the gain further than those used caused a decrease in SNR from overloading, so the max SNR with overloading allowed test was skipped.
Max SNR with BCFM filter
Here we tried to maximise SNR of the target signal with a BCFM block filter in place.
| Airspy | RSP | HackRF |
|
With the Airspy using the BCFM block filter we were now able to get an SNR up 49.4+1.2 = 50.6 dB. We were also able to use the sensitivity gain setting which produced the best SNR result. There was mild interference from the strong pager shown at around 460.5 MHz. |
The RSP was now able to reach an SNR similar to that of the Airspy, at 49.7 dB’s. The max SNR was obtained at GR51 with the Mirics LNA turned on. However, there appears to be some halo effect on the signals which may be phase noise or non-linear mixing. |
The HackRF was now able to reach an SNR of 44 dB’s. Some pager interference was seen below 462 MHz whenever it transmitted. |
Max SNR with LNA
In this next test we placed an extra 6M of RG174 coax cabling, and an LNA4ALL before that to try and mitigate the losses.
| Airspy | RSP | HackRF |
|
Here the max SNR that could be obtained with the Airspy was only 32.6+1.2 = 33.8 dB, and there was some pretty decent BCFM interference that showed up at all gain levels. |
The RSP reached a similar gain of 31.1 dB and also had BCFM interference showing up. Its gain was set to [GR62, Mirics LNA off]. |
Surprisingly the HackRF worked the best here, reaching a gain of 34.2 dB’s with very little BCFM interference shown. |
Max SNR with LNA and BCFM Filter
In the next test we added a broadcast FM filter in front of the LNA4ALL.
| Airspy | RSP | HackRF |
|
The Airspy was now able to reach a SNR of 52.4 dB, with no BCFM interference shown. Only mild pager interference was seen at 460.5 MHz. |
The RSP had some trouble receiving with the BCFM filter in place. Even at the lowest gain setting severe pager intermodulation occurred whenever the pager transmitted. The gain in the image was set to the lowest possible, [GR78, Mirics LNA off]. |
The HackRF also could not reach a higher SNR with the BCFM filter in place. Increasing the gain any further caused full band overloading. |
Max SNR with LNA and BCFM Filter and High Loss Coax
Here we used longer coax cable and a 12 dB attenuator to simulate a long cable run. At 463 MHz the total loss in this test is about 12 dB from the attenuator plus 6.828 dB loss from 10M of RG174, giving a total loss of 18.828 dBs. At 463 MHz the LNA4all gives about 23.5 dB of gain, so the total external gain is 23.5-18.828 = 4.672 dB’s of gain remaining.
| Airspy | RSP |
| With the lower external gain the Airspy performed much better, with almost no external interference visible. | The RSP still had the same problems with BCFM interference as with the lower loss cable test. |
Max SNR with LNA and BCFM Filter and High Loss Coax and BCFM Filter
| Airspy | RSP |
| In this test the Airspy had an SNR of about 51.2 + 1.2 = 52.4 dB. No interference or overloading was noticed. | The RSP reached an SNR of 50.6 dB. Unlike the test with low loss coax, no interference or overloading was noticed. |
Discussion
Once again the Airspy was the best receiver as it was able to reach higher SNR values without overloading effects showing up. When we added a BCFM block filter in front of the receivers the max reachable SNR levels became nearly identical, though the RSP showed a problem that looked something like phase noise to us. The fact that the BCFM block filter improved results on the RSP seems to indicate that its internal 420 – 1000 MHz bandpass filter did not completely block out the interfering BCFM signals.
Interestingly adding an LNA into the system caused large amount of BCFM interference to show up on both the Airspy and RSP causing issues by making the max SNR obtainable much lower. Adding a BCFM filter in front of the LNA solved this overloading problem completely on the Airspy. On the RSP the addition of the BCFM filter solved the BCFM overloading problem, but instead we got stronger pager interference which was not seen on the Airspy. The interference was seen even with the RSP set to the lowest gain levels.
When using higher loss coax after the LNA the Airspy performed well with just the LNA, but the RSP still had BCFM issues. After adding the BCFM filter in before the LNA both the Airspy and RSP performed well.
420 to 1000 MHz Bandpass filter on the RSP.
860 MHz
This band again is a business band which several TETRA wide signals around.
Max SNR without Overloading
Here we tried to receive TETRA signals at 860 MHz. We adjusted the gains until we obtained maximum SNR on a target signal whilst stopping before overloading occurred.
| Airspy | RSP | HackRF |
| Here the Airspy reached an SNR of about 31.5+1.2 =32.7 dB’s and was about 6 dB less sensitive compared to the RSP. No matter what gain settings we applied we could not get it to match the SNR of the RSP. |
The RSP had the highest SNR and reached a max SNR of 38.8 dB. The gain used for the max SNR was [GR 52 Mirics LNA on]. |
The HackRF was able to reach a SNR of 30.3 dB’s. |
Increasing the gains further simply raised the noise floor and lowered SNR, so we did not perform the max SNR with overloading allowed tests. Although the RSP was getting higher SNR results we are unsure if this is accurate as there is some odd rise in the noise floor on the signals. It looks like phase noise or perhaps non-linear mixing might be pushing the measured SNR up, but we could not confirm as we could not find a TETRA decoder that would show the bit error rates.
Max SNR with BCFM Filter
Next we tried with the BCFM filter in place.
| Airspy | RSP | HackRF |
|
With the BCFM filter in place we were able to push the gain a little higher before full band overloading occurred, and get a higher SNR on our target signal of 35.1+1.2 = 36.3 dB. Increasing the gain further caused pager interference to show up, and mild pager interference can be seen on this Airspy screenshot already (mild rise on noise floor when the pager transmitted). |
The RSP remained at a similar SNR level of 39.6 dB, about 1 dB higher than without the filter but within variance tolerances. However, with the filter in place effects that look like phase noise or non-linear mixing started to show up on most signals. |
The HackRF saw poorer performance with the BCFM filter in place, with the target signal max SNR being reduced to around 26.1 dB. |
Since the Airspy’s SNR improved with this filter we can conclude that the Airspy was being desensitized slightly by BCFM interference.
Max SNR with LNA
Here we added an LNA and 5M of RG174 and recorded the results.
| Airspy | RSP | HackRF |
|
With the LNA in place the Airspy SNR was boosted up to 36.3+1.2 = 37.5 dB, just about matching the SNR of the RSP without the LNA. |
The RSP’s SNR was reduced to 37.8 dB with a gain setting of GR46 and the internal Mirics LNA turned off. |
Interestingly the HackRF was able to reach the highest SNR now with an SNR of 38.7 dB. |
Max SNR with LNA and BCFM Filter Before LNA
Here we placed a BCFM filter before the LNA.
| Airspy | RSP | HackRF |
|
For the Airspy placing the BCFM filter before the LNA significantly worsened reception. The max SNR was now only 30.5+1.2 = 31.7 dB and pager interference started showing up in the form of noise floor rises whenever it transmitted. |
Similarly, with the RSP reception was worsened with the BCFM filter in place. Now pager interference showed up strongly whenever it transmitted and the max SNR was significantly reduced to 18.7 dB. Gain settings where GR65 internal Mirics LNA off. |
The HackRF saw similar problems with the max SNR reducing to 21.5 dB and massive noise floor rises whenever the pager transmitted. |
We’re not sure why placing the filter before the LNA caused trouble in this test.
Max SNR with LNA and BCFM Filter After LNA
Here we decided to see how reception was affected if we placed the BCFM filter after the external LNA.
| Airspy | RSP | HackRF |
|
The Airspy worked much better with the BCFM filter placed after the LNA. Now the SNR was increased to around 34.6 + 1.2 = 35.8 dB. |
Similarly the RSP saw an improvement and it’s SNR was increased to 38.9 dB. The gains used were GR36 internal Mirics LNA off. |
The HackRF was also improved getting a max SNR of 29.6 dB. |
Max SNR with LNA and high loss cable
Here we used longer coax cable and a 12 dB attenuator to simulate a long cable run. At 858 MHz with a 12 dB attenuator and 10 dB of loss from 10m of RG174 cable we have a total loss of 22 dB. The LNA4ALL has a gain of about 20 dB at 858 MHz. Thus there is a total of 22 – 20 = 2dB of gain left over.
| Airspy | RSP |
| The Airspy reached an SNR of 23.2+1.2=24.4dB and showed no signs of any sort of interference. | The RSP only reached a maximum SNR of 17.8 dB. Unlike in the other tests the RSP started showing signs of BCFM interference with this longer cable for some reason. |
Max SNR with LNA and high cable and BCFM filter
In this test we added the BCFM filter before the LNA4ALL.
| Airspy | RSP |
| The Airspy now reached a maximum SNR of 28.7+1.2 = 29.9 dB. The spectrum was clean. | The RSP now reached a maximum SNR of 33.8 dB and the spectrum was also clean. |
Discussion
At these frequencies the RSP seemed to be more sensitive and appeared to be much less affected by BCFM and pager interference, and so this time the RSP was able to reach higher SNR values. By adding a BCFM filter in front of the receivers the Airspy’s max reachable SNR became closer to that of the RSP’s, but there was still about a 3dB difference. It seems that at this frequency the Airspy was more affected by the BCFM band than the RSP was.
With an LNA added the maximum SNRs reachable by the Airspy and RSP were both reduced significantly as overloading occurred much easier, but both SDRs were almost identical in performance. When we added a BCFM filter in front of the LNA, reception on both receivers worsened a lot, especially on the RSP. When we placed the BCFM filter after the LNA reception improved, and the RSP was able to reach signal levels about 3 dB higher than the Airspy once again. We’re unsure of why this would happen.
With higher loss cable used after the LNA the RSP suffered some BCFM interference and again we are unsure why. The Airspy performed normally, but both suffered heavy loss from the cable. We think that the estimated cable loss was perhaps about 10dB higher than estimated at this frequency. With the BCFM filter placed before the LNA4ALL both the Airspy and RSP performed similarly.
The same built in bandpass filter used in the RSP as with the 460 MHz test is used in this 860 MHz test so it is interesting to see that this frequency is much less affected by BCFM overloading than the 460 MHz band is.
L-Band Test
Here we tested the three SDR’s on their ability to receive L-band satellites such as Inmarsat with an L-band patch antenna.
Max SNR Test
| Airspy | RSP | HackRF |
|
The Airspy was able to reach a max SNR of about 7.4+1.2 = 8.6 dB. |
The RSP was able to reach a max SNR of about 8.1 dB. It’s looks a bit lower due to the signal jumping up and down when taking the screenshot, but overall the RSP seemed to be about ~1 dB more sensitive than the Airspy at L-band. |
The HackRF also received decently with an SNR at 8 dB, but some interference was seen. |
Max SNR with LNA
With an LNA attached just before the receiver, without any extra run of coax cable.
| Airspy | RSP | HackRF |
|
The Airspy was able to reach a higher SNR of 13.9+1.2 = 15.1 dB. |
Strangely, with an LNA connected the RSP was not able to receive L-band signals at all. This could be due to possible overload from BCFM or DVB-T signals. |
The HackRF had its SNR boosted to 9.7 dB, but there was interference seen from what looks like other L-band signals, a DVB-T signal, and perhaps various trunking signals from 460 MHz. |
Discussion
The RSP again showed that it has good sensitivity that is on par or slightly better than the Airspy at this frequency. However, when an LNA was added the RSP could no longer receive L-band signals at all! We guess that the RSP may have been overloaded by strong DVB-T signals at 500 MHz. We guess this because it looks like we can see the edge of a DVB-T signal ghosting in the waterfall image.
L-Band Test 2
Because of the poor results the RSP had with the LNA we decided to recheck results at another location with very weak BCFM and other terrestrial signals like DVB-T so that the RSP would not overload.
Max SNR Test
| Airspy | RSP | HackRF |
|
Without the LNA the Airspy has a SNR of about 3.5+1.2 = 4.7 dB. |
The RSP had an SNR of 4.6 dB. |
The HackRF had an SNR of 2.8 dB. |
Max SNR with LNA
| Airspy | RSP | HackRF |
|
With the LNA connected the Airspy once again showed a significantly increased SNR, now showing 7.1+1.2 = 8.3 dB. |
This time with very little interference present the RSP worked fine with the LNA and got 6.3 dB. |
The HackRFwas able to reach a higher SNR of 6.4 dB. |
Discussion
In this location without strong earth based signals the RSP worked fine with the LNA in place and received the expected boost in SNR. From these results it appears that without an external LNA the RSP works a little better, but with an external LNA signal on both SDRs is improved, but better on the Airspy.
Real World HF Tests
In these tests we tested the Airspy and RSP on the HF (0 – 30 MHz) band. We mostly ignored the HackRF for these tests as its performance on HF was quite bad and not really worth our time to compare. The HackRF may perform better with the use of an upconverter, like the SpyVerter but we did not test this. The HackRF is also very difficult to use at HF frequencies due to its lack of decimation or bandwidth options lower than 8 MHz. This means that the visual FFT resolution is very bad, making it difficult to visually identify signals.
The Airspy can only receive HF with the help of an upconverter. We used the recommend partner upconverter, the Spyverter and powered it via the Airspy’s bias tee. Using an upconverter introduces some losses, probably around 6dB, so our expectation was that the RSP would have better SNR at HF.
We also noticed that the RSP had significant imaging problems when the Zero IF (ZIF) mode was used. However, when the Low IF (LIF) mode was used there were no images. In the LIF mode the maximum bandwidth is restricted to 1.536 MHz, but for HF this is okay, since we need to use lower bandwidth’s to be able to accurately view the very narrow band HF signals. Thus all HF tests performed on the RSP will mainly use the LIF mode to get the best results, but we did use ZIF mode in some tests where no images were seen.
AM Band
Here we tested the maximum SNR obtainable on the AM band with a long wire antenna. We ran the RSP at 1.536 MHz (actual bandwidth was actually around 1.646 MHz) and the Airspy at 10 MHz with decimation 4 or 2.5 MHz, cropped down to 1.6 MHz. The FFT interpolation addition for the Airspy was then 3 * (2.5 – 1.6) / 1.25 = +2.16 dB.
Even with the interpolation adjustment for the Airspy, the results showed that the RSP and Airspy had very similar results as their spectrum’s were almost identical.
One major issue with the Airspy that we had to address was what seemed to be USB or general PC noise. Originally, we had been running both the RSP and Airspy through a 10 meter long active USB extension cable. However, on the Airspy the use of this active extension cable caused some interference issues. The RSP also experienced a little interference from the active cable, but it was almost negligible. This interference only appeared on some bands, like the AM band and did not occur for VHF and above frequencies.
When the active USB cable was not used, and the device was plugged directly into the PC the noise disappeared. However the Airspy was also affected by which USB port was used. Some USB ports were more noisy than others, but this did not cause as much noise as with the active USB cable.
We also tested both SDR’s with a galvanic isolator (the one from Nobu). This isolates the antenna from the device and PC, but at the same time introduces a little loss. Reception in the AM band was significantly improved with the galvanic isolator, but we didn’t notice much difference when it was used on the other bands.
We demonstrate the PC interference issue in the images below.
Discussion
The USB noise issue originally made us think that the Airspy/SV was poorer than the RSP at BCAM reception. However, once we stopped using the active USB extension cable the RSP and Airspy were pretty much identical, with the Airspy/SV maybe being a little more sensitive. Adding in a galvanic isolator helped solve even more noise problems, but even with the isolator both the Airspy and RSP had similar performance.
6 MHz
For this test we used a Magnetic Loop antenna.
Here the Airspy/SV and RSP seemed to perform very similarly and we could not really determine any difference between the two in terms of sensitivity or overloading. We also tested the HackRF here but it was always about 10 dB poorer than the Airspy and RSP.
We also redid this test with a long wire antenna on the Airspy and RSP. Again the results where almost identical.
8 MHz
He we tested reception on a STANAG signal. Again in this test we could not see any real difference between the Airspy/SV and RSP. The SNR’s were nearly identical on average (we must take an error of +-6dB in these images as the HF signals in this band were rising and falling in strength fairly fast over a period of seconds).
9 MHz
In this test we used a long wire antenna and again results were pretty much identical. We note that 9 MHz was the only band other than BCAM that was significantly affected on the Airspy by the active USB cable issue.
11 MHz
This test was performed with a long wire antenna and once again no real difference between the Airspy and RSP could be detected.
15 MHz
At 15 MHz there was again no real discernible difference between the Airspy and RSP.
18 MHz
Once again, no real discernible difference could be seen.
Discussion
It seems that both receivers were pretty much identical on these HF tests. The Airspy however had some noise issues when running it with an active USB cable and was sensitive to which USB port was used.
The RSP should be used in LIF mode to get the best results as the ZIF modes produced significant imaging, however this is okay as there is unlikely to be a situation where you need 8 MHz of bandwidth for HF.
Strong Signal Test: Airspy v.s. SDRplay RSP
Leif has already done some good tests regarding the dynamic range on the Airspy and the SDRplay RSP which we show again below. The test he did was to inject a strong interfering signal into the signal path and measure its effects on a nearby broadcast FM station. As the interfering signal increases in strength we can expect the FM signal to be degraded and fall into the noise floor. SDR’s with better dynamic range specs will only degrade at stronger levels of interference.
Leif’s results showed that the Airspy generally wins in terms of dynamic range. However, the SDRplay team has critiqued this test as Leif did not use the official drivers released by the SDRplay team which they write should give better performance. He also did not use the IF filter bandwidth reduction which might improve the RSP’s max dynamic range by about 5 dB, at the expense of reducing the bandwidth.
Dynamic range tests by Leif. 500 kHz and 1 MHz offsets. 
Dynamic range tests by Leif. 2 MHz and 5 MHz offsets.
Below are the results for the two tone tests done by Leif. Leif also made some two tone tests where the two tones were arranged for the 3rd order intermodulation product to appear in the passband of the desired signal. Here the Airspy was still better in most tests, but the SDRplay was better in some too.
Two tone comparison 
Two tone test
Our own loss of reception (Dynamic Range) test
In this test we wanted to compare the dynamic range of the Airspy and RSP ourselves. To do this we devised a simple test involving a DMR trunking channel and an interferer generated by the HackRF. The trunking channel was received by an antenna whilst the HackRF was used to inject a simulated GSM interferer at various offsets from the DMR signal. We used the DMR decoder program to monitor the DMR signal, and steadily increased the interferer gain on the HackRF. When a loss of synchronisation occurred on the DMR signal we recorded the HackRF gain value at which this happened. During the test we were free to adjust the Airspy and RSP gain settings and center frequencies (to move intermodulation products out of the way) to try and obtain the best reception possible.
Please remember when testing like this only the relative measurements are meaningful, the absolute numbers mean nothing. The test is simply to show which receiver works better in the presence of strong signals, not to show any quantitative difference.
| Offset (MHz) | Airspy (Higher is better) | RSP (Higher is better) |
| -400 | MAX+ | MAX+ |
| -300 | MAX+ | 35 |
| -200 | MAX+ | 21 |
| -100 | 36 | 20 |
| -50 | 23 (Interefer Harmonic directly on top of signal) | 14 |
| -10 | 35 | 22 |
| -9 | 33 | 22 |
| -8 | 29 | 20 |
| -7 | 29 | 20 |
| -6 | 26 | 20 |
| -5 | 26 | 20 |
| -4 | 27 | 20 |
| -3 | 26 | 20 |
| -2 | 27 | 20 |
| -1 | 23 | 18 |
| -0.5 | 17 | 15 |
| 0.5 | 17 | 15 |
| 1 | 23 | 18 |
| 2 | 25 | 20 |
| 3 | 26 | 20 |
| 4 | 25 | 20 |
| 5 | 24 | 20 |
| 6 | 26 | 20 |
| 7 | 30 | 20 |
| 8 | 30 | 20 |
| 9 | 32 | 20 |
| 10 | 29 | 20 |
| 50 | 22 (Interefer Harmonic directly on top of signal) | 16 |
| 100 | 30 | 17 |
| 200 | 35 | 14 |
| 300 | MAX+ | 16 |
| 400 | 46 | 18 |
| 500 | MAX+ | 22 |
| 600 | 43 | 30 |
| 700 | MAX+ | 44 |
| 800 | MAX+ | 44 |
| 900 | MAX+ | 44 |
| 1000 | MAX+ | 44 |
In these tests the Airspy performed significantly better. It was able to tolerate much stronger interference at all offsets.
The RSP has the option to reduce its IF filter down to 200 kHz in order to improve dynamic range by better blocking in band interferers. However, we didn’t really see much improvement when reducing the bandwidth like this. There was maybe at most a 1-2 dB improvement.
The RSP had issues with the strong interferer, even at offsets far from the centre. We believe that this problem is caused by reciprocal mixing, which is when the phase noise of the local oscillator mixes with a strong signal (the interferer) and causes interference to other weak signals. The Airspy with its low phase noise clock exhibited this problem significantly less.
HackRF TX gain settings.
Other Tests
Power Usage
The current consumed by the SDR is important if you wish to use it on a battery powered device. From the results we see that the RSP is the most power efficient device, with the Airspy and HackRF requiring about double the current.
| Airspy | RSP | HackRF |
|
0.34A 0.4A (with SpyVerter) |
0.16A |
0.4A 0.44A (LNA On) |
PPM Drift and Offset Test
To test the oscillator drift we tuned to a 1.5 GHz L-Band signal and watched the drift for about 40 minutes using spectrum lab. A stable signal is important for decoding signals as many digital decoders cannot handle signals that drift too fast.
The Airspy with its temperature compensated oscillator (TCXO) had an initial 0 PPM offset and drifted the least as is expected with the TCXO. It drifted about 500 Hz over 40 minutes from a cold start giving a PPM drift of about 0.3 PPM, which is within the 0.5 PPM TCXO spec. After 5 minutes of warm up the drift was only about 150 Hz, which is about 0.1 PPM.
The RSP has a crystal oscillator rated at 10 PPM. Our unit had an initial offset of -5 PPM and drifted about 1.6 kHz after 40 minutes from a cold start giving a PPM drift of about 1 PPM. After about 10 minutes the RSP drift stabilised down to about 0.1 PPM. Even though it does not use a TCXO the RSP drift is quite low, probably because it is power efficient and does not generate much internal heat, as well as having a much large PCB to dissipate the heat into. However, since no TCXO is used external temperature changes from night to day for example could affect drift.
The HackRF has a crystal oscillator rated at 30PPM. Our unit had an initial offset of -18 PPM and a larger drift of about 4 kHz after 40 minutes from a cold start which is about 3 PPM. Like the RSP it also does not use a TCXO. But compared to the RSP its current usage is much higher, possibly creating more heat which makes the oscillator drift much more.
Screenshots of the HackRF at 20 MSPS
As a bonus to highlight a good feature of the HackRF we show some screenshots showing wideband reception of some signals with the HackRF running at 20 MSPS.
HackRF Receiving Broadcast FM 
HackRF Receiving the GSM Band
Conclusion
It is clear that there is no overall “winner”, each SDR has their own strengths and weaknesses and what you choose will depend on your needs and budget. The Airspy clearly works significantly better in tough RF environments than the RSP. However the RSP comes in at over $50 to $100 less and does not require an add on upconverter to listen to HF. The HackRF has poor RX performance, but has the widest bandwidth, tunable range and can transmit.
If we were to choose a unit we would say between the Airspy/RSP and HackRF, pick the Airspy/RSP if you are interested in scanning, DXing or just browsing the radio spectrum. Pick the HackRF if you are more interested in experimenting with locally generated radio signals/devices (such as for reverse engineering wireless devices). Between the Airspy and RSP, pick the Airspy if you live in suburban/city areas and want a the best reception, or pick the RSP if you live rural or are more concerned about budget.
In table form we make the following recommendations:
| Airspy | SDRplay RSP | HackRF |
|
Advantages: The Airspy is the clear winner in terms of overall RX performance. Its natural high dynamic range allows for excellent SNR and reception of weak signals when in the presence of nearby strong signals. There are very few to no images caused by strong signals in the Airspy so the spectrum is very clean. It also works well with external filters and LNA’s and has good official software support for Windows and the Raspberry Pi. Disadvantages: Costs $50 more than the RSP. And another $50 if you want HF capability. Plus shipping costs. Slightly less upper frequency range than the RSP. Needs a fast modern PC to run. Should you buy it? The Airspy costs $199 USD, or $249 USD if you buy the SpyVerter for HF, plus $5-$20 shipping depending where in the world you are. The Airspy is the best for users in need of the best RX performance. This is the best unit if you live in a tough RF environment like in a city or suburbia or intend to use an external LNA. Note that you will need a fast PC to run the Airspy. Marketed more towards professional RF users, but also has a strong amateur/scanner user community. |
Advantages: The RSP is the winner in terms being the cheapest all-in-one RX unit that is much better than an RTL-SDR. The RSP can tune to HF frequencies out of the box without an add on, has a higher top frequency of 2 GHz, doesn’t need a high end PC and is $50 USD cheaper with free shipping in the USA (or $100+ cheaper if you only consider the Airspy & Spyverter combo for HF). Disadvantages: Its capabilities in the presence of very strong signals are not as good as the Airspy so overloading in suburban/city settings is a problem. Also, there is no official software so you are tied to third party options. Should you buy it? The RSP costs $149 USD with free shipping in the USA, or £99 + VAT + ~£10 shipping in the EU. The RSP is the best for users who want a low cost all in one RX device with decent, but not great RX performance. The RSP does VLF to UHF, and can work with slower PCs. But don’t buy this unit if you have problems with strong signals in your area or if you want to use an external LNA. Mainly marketed towards amateur/scanner users. |
Advantages: The HackRF is the winner in terms of being an all rounder. It can TX, it has the widest bandwidth and frequency range. Disadvantages: Its RX performance is poor compared to the Airspy or RSP. Needs a modern PC for higher bandwidths. General RX software support isn’t great. Should you buy it? The HackRF costs $299 USD + shipping costs. The HackRF is the best for RF experimenters/people who want an all in one RX/TX device and don’t need great RX performance for DXing. It is great for reverse engineering wireless devices. We think it is more designed to be used with custom software written in GNU Radio or Python. It’s main selling point is is wide frequency range, wide bandwdith and TX capability. Don’t buy the HackRF if you are looking for RX performance better than an RTL-SDR or want a DX radio. Marketed more towards the hacker/security/electronics or reverse engineering crowd. |
Replies from the Manufacturers of the Airspy and SDR Play
Before we posted this review we sent a copy to the manufacturers of the Airspy and the SDRplay RSP so that they could fact check our review for mistakes or bad testing methods. Here are their responses:
Initially we were confused about what sort of data came out of the RSP. The SDRplay team wrote:
The RSP does deliver raw samples from the ADC, but as the MSi001 is capable of delivering analog I/Q signals, you need dual ADCs to sample the output from the tuner. The max sample rate for the Airspy single channel ADC is 20 MS/s, which is necessary to deliver 10 MHz of bandwidth without major aliasing problems. The Dual ADCs on the RSP can each sample in excess of 10 MS/s which together means that the USB throughput needs to be 2 x 10 MS/s x 12 bits, which is the same as the Airspy. The Airspy then needs to de-rotate the sampled IF to digital I/Q, whereas the RSP does not need to do this as the information is in I/Q form. If the RSP is used in low IF mode, then only a single ADC is used and the USB throughput is halved, but as correctly observed, the IF bandwidth is limited to 1.536 MHz.
Regarding our LNA tests the SDRplay team wrote the following which prompted us to do LNA testing with higher loss coax cable:
Regarding the external LNA [LNA4ALL], we appreciate that people use off-the shelf LNAs, but a 20 dB LNA of gain is in excess of what is really necessary to overcome the loss of the coax cable. Our point is that when using an external LNA, the system still needs to be ‘designed’ or people might be tempted to cascade these LNAs mistakenly thinking that 40 dB of gain will improve performance. If the cable loss was in fact 14 dB (maybe 10-15m of cable), the performance of the RSP would actually be better than if it is only 4-5 dB.
Regarding the LNA tests the Airspy team write:
[In the LNA tests] the LNA [LNA4ALL] might not have enough dynamic range in a high performance setup since it overloads before the Airspy.
The Airspy team suggested that the LNA should improve SNR on the Airspy, but that the LNA we used was not suitable for our environment due to it overloading on BCFM signals. They suggested that we should have used a LNA with a much higher dynamic range such as the PGA103+.
When asked about the PC requirements of the SDR the SDRplay team wrote:
The benefit of the isochronous mode driver is that it reserves the necessary USB bandwidth, something that does not happen with a bulk mode driver. As a consequence, depending what else the PC is actually doing, there is a greater risk of buffer overflows and packet losses with a bulk mode driver than with an isochronous mode driver. We do occasionally get complaints from developers that we chose not to use a WinUSB (bulk mode) driver, but the reason for doing this was to open up the range of platforms capable of using the device’s full capability.
When we asked the Airspy team why they did not use an isochronous driver they replied:
[With an Isochronous driver] you have no means to know how many samples you lost if your system had a transfer error – which means you can’t implement coherent receivers with it even if the sampling is synchronized
About Leif’s tests the SDRplay team wrote:
Regarding the tests performed by Leif Asbrink, our principle concern was that to interface the RSP to Linrad, he had used a driver which was not developed by either Mirics or SDRplay and was known to contain bugs which prevented proper control of the RSP and had a sub-optimal gain map. We felt that the use of this driver was likely to compromise the results he was able to achieve.
If you have any experiences or comments about one or more of these SDR’s, or if you find any mistakes we have made then please post about them in the comments section.
Disclaimer:
Although every care was taken to be accurate in this review, please note that we are not a professional RF testing agency. We bought the Airspy R1, SDRplay RSP and HackRF SDR’s with our own funds, however we received a complimentary Airspy R2 and Spyverter from the creators to use in our review.
La suite à lire sur le site de : RTL SDR Review: Airspy vs. SDRplay RSP vs. HackRF