EDIT/ADD: So far as a conclusion…Well, there are distinct spikes at 480/960Mhz on both ports (Same PC and ports are in close proximity mind you) and that USB 3.0 is just loud. That said, it appears that using a quality powered hub does in fact lower the noise so far as plugging the SDR directly into the PC goes. But using it also moved the radio a few feet away from the rest of the cables going to the motherboard (HDMI, eSata, Cat6, etc.). So far as the rest, can draw your own conclusions.
My approach was to connect the AirSpy mini to the RaspPi4 via a 6” usb cable that I MODIFIED by cutting into the cable, severing the red wire and powering the AirSpy with a separate 5.2 volt dc power supply.
Testing by measuring 30 days of data (back in pre-CoVid19 days) the total aircraft count was increased slightly (1-2% as I recall) compared to 30 days powering the AirSpy from the Pi4.
Using a USB hub may provide a similar benefit by powering the SDR from a separate source.
I do. Any particular test you’d like to see? The extension I used already had one built on, but I do have more that could be snapped on.
ADD:
I started messing around a bit more with lower gain since I wasn’t content with the results above as I’m of the opinion some of the distortion for lack of better terms may have been caused by excessive gain (look at that hair on the direct connect USB 3.0 above), so I decided to notch gain up slowly until the noise floor rose about 2db and here is that result:
960Mhz is the loudest from this USB 2.0 port by a longshot. 480Mhz barely sticks out, but 1440Mhz has a ring as well (another multiple of 480Mhz). Not much difference between plugging the radio into the USB 3.0 port this time except an added ring at 1200Mhz (which is a divisor of 2400Mhz), not sure if this that be from the USB 3.0 chipset itself? Plugging the radio in using the shielded extension cable with extra ferrites clamped onto it made zero difference in this round of tests.
Either way, more information to add to the pile for what it’s worth. We should probably get back on the subject of filters as EMI could be it’s own thread for sure.
Attenuation of a good cavity filter is around .8db (at center) and I think the FA filters come in about 1.5db or thereabouts from my tests. I’m not quite sure where the 5-10db figure is coming from. I haven’t seen that much even downstream of a filter and splitter in my own tests at least. NF is usually product of amplification, not bandpass filtering. Perhaps I’ve learned it all wrong?
The premise of putting a filter prior to LNA is so the LNA doesn’t saturate depending on the op3 of the amp and site interference. For some, it can be advantageous to do so, for others the insertion loss/attenuation prior to amplification outweighs the benefits. It all depends on site, so what may work for me, may not for the next person.
Negative. NF (Noise figure) is LNA/tuner related (relationship between input SNR and output SNR), attenuation and insertion loss are more related to filtering, fittings, and cable chains.
Ah - ok, question answered - thanks.
When I tried ferrites (power cable, USB and antenna lead), either they made no difference or it was detrimental (not at all what I expected). This suggests your USB cable is a lot better than mine.
I was running it this morning again, this time for my new outdoor setup
Installed is a Pi 3B with blue ProPlus Stick (connected directly) and a Jetvision Antenna connected via low-loss cable
Any suggestion if a filter would improve something?
SMA (female to male) 1090MHz Mode S filter. Designed to reduce interference and significantly improve valid mesage rate of ADS-B/Mode S receivers. This is the new 2019 version which features a narrower pass band frequency range of 1080MHz…
First update after the filter is active since two hours.
Gain setting of 40.2 untouched:
no significant change in results
Gain set to 49.6 (last step before -10)
messages > -3dbFS (the important trigger) remain below 5%
Range graph has been slightly increased, is more stable now
Positions/sec increased compared to the # of aircraft seen. This was the second indicator i was looking at while adjusting gain.
Overall i would say it’s an improvement. With gain 49.6 i normally have ratio of 10% for the messages > -3dbFS
Continue monitoring
Next update:
The last two hours with gain 49.6 the lines for messages received and positions/second are almost on the same level.
I haven’t seen this before. In the past the line for pos/s was a lot lower in comparison.
Living in Germany and having a shop in germany where it was “in stock” made the trick. I ordered early enough to have it picked up by DHL in the shop at the same day.
But seriously the device was working also without it. So do i “need” the filter? No. Do i “want” the filter? yes
I had one of these last year and it did give some improvement, particularly on the Flightaware orange dongle and the cheap eBay ones but sadly it has now failed and is effectively a 100dB attenuator!
Do you happen to have another Raspberry and antenna laying around? If so, (and you’re bored), maybe set the other system up on a temporary basis enough to gather data - run both side by side for 24 hours, then swap the new filter back into the mix and run them for another 24 hours and calculate the percentage difference between the two during those times. Only then will you be able to tell if there is a true gain or not with or without the filter. The other setup doesn’t need to be great, just enough to calculate percentages between the two… Otherwise it’s impossible to compare and analyze results properly. Comparing the two though Virtual Radar would be the easiest - this would enable you to log positions/packets/plane count, range etc. more accurately on both sides.
If can get the test antenna within the same general area, even better since results would be more accurate than if one was indoors and the other on the roof for instance.
Yes, i have another system. But with a different antenna. heading to the blind spot where my roof covers the reception.
Fact is that setting the gain to 49.6 was simply overloading the signal before using the filter.
I am not that critical on that. I have a good feeling and it was worth the 20 Euro.
Beside that the GSM signals went down clearly.
I ran the test as mentioned in the first post again, this time with filter.
Results compared. At least the filter does what it should do
