Real World Signal Reception Tests (VHF/UHF)
The methodology of the tests in this review 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). All tests were performed in SDR#. We understand that SDR# is considered as better for the Airspy, but when comparing results with the RSP on HDSDR and SDR# we saw no changes in signal performance, so decided to use SDR# for side by side comparisons. The only disadvantage we can see is that third party plugins are not available for the RSP.
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 during this review 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.
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.
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.
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
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.
We also tested BCFM reception with an external LNA connected. It was placed before an extra 3M length of RG174 coax cable.
|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.
|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.|
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.
|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.
|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.
|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.|
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.
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.
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.
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.
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.
Here we connected an LNA in front of 6M of RG174 cable and observed the results.
|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.
|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.
|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.
|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.|
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.
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.
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.
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.
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.
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.
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
|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.|
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.
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.
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.
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.
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.
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.
|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
|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.|
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.
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.
|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.
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.
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.
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.
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.
|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.
|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.|
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.
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
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.
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.
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
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
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.
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.
HF Tests and conclusions continued on the next page.