FlightAware Discussions

Ran into a bad RTL-SDR filtered preamp

I’ve been struggling for some time now to get my second (test) feeder up and running.
This feeder is a mirror image of my usual feeder with a Jetvision A3 antenna, Pi 4, Airspy mini, RTL-SDR filtered preamp and Sysmocom cavity filter.
Everything works but I’m seeing less traffic as usual even if I use the same antenna as my main feeder. I’ve been swapping several items around in my quest to find the culprit and it turned out to be the RTL-SDR filtered LNA, an item I just received 2 weeks ago.
I wanted to see the actual output signal so I hooked it up to my spectrum analyser and injected a 1090Mhz carrier through a 30dB attenuator.
At first I had my analyser set at a 50Mhz span and everything looked quite normal with a nice 28dB amplified 1090Mhz signal. I noticed some jitter at the left-hand side of the screen and expanded the span to 100Mhz and there it was, a huge parasitic oscillation @1058MHz that was always present, even without an input signal.
I tried to see if lowering the power supply voltage made a difference but the oscillation is even present at 1.5V
I guess there must be a feedback loop through the 1090MHz saw filters but the board is just to small to fiddle with smd coils and capacitors, any suggestions or do I just order a new one?

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I’d say that’s most likely a defective chip … other stuff seems less likely.

Just message them that the unit is bad you should get a replacement.

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I see that you are feeding the unit with an external PS.
Do you inject DC power into the spectrum analyzer probe? Does it have DC blocking?

Well, I unsoldered the resistor linking the DC power supply coming from the output connector (bias-T) to the LNA chip and soldered a wire directly to the LC low pass filter on the power line. This wire is going to a standard 5V USB-power supply (or an adjustable lab power supply for the test).
This way the Airspy doesn’t need to supply the >200mA needed to power the LNA and it’s much easier to test the output with a spectrum analyser without having to worry about powering the LNA and avoiding DC on the probe.

This is what I did to feed trough the coax. Not pretty, but it works. Components marked for clarity.
I guess you disconnected the R (0 ohm) that I rotated 90 degrees… Did you check your DC power for the oscillations?

As you can see, there’s already a small soldering hole present on the DC power line so that’s the ideal place to solder a jumper wire. I still wonder why there’s a green power led present as you can only see it with the case removed!
The parasitic oscillation has nothing to do with the power supply, I checked that with a 47µF tantalum capacitor. I did noticed that spraying some cooling spray on the saw filters will slighty change the frequency but that’s not unusual as it forms an oscillator with the amp around the bandpass frequency.

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The only external way to parasitic couple would be on the connection between the two stages.

Is this looking like what you have there?

I was wrong about removing the 0 Ohm resistor, in fact it was L2 on the diagram above that I removed.
I have an identical LNA for my main feeder with both the saw filters removed and bypassed since I already have a much better 4 pole cavity filter between my antenna and the LNA.

When bypassing, IMO you need to use the RC series group, like in their datasheet.

Good news, RTL-SDR will send me a new filtered LNA to replace the bad one :+1: