Help me test RTL-SDR

[Mario]

Hi

I have a 3m dish that I use to listen to hydrogen line. Recently I experienced a problem with it. The received signal strength jumps in intensity. Sometimes it's -80 dBm, sometimes -50dBm (-50 is around what it should be around). This happened suddenly, and if my memory works, it happened after one rainstorm (possibly with lightning as well). I didn't have lighning arrestor so maybe SDR got a bit of a overvoltage on the input.

I tried locating the problem. I made new coax cables and put new connectors and this seemed to work. But not for long. The problem appeared again. I think maybe it has something to do with RTL-SDR AGC instead of cables or connectors. Maybe both.

Last weekend I removed SDR from the dish and tested it in the house. AGC behaves quite strange:

I open GQRX and tune to 1420MHz. This is without LNA, just SDR receiver plugged into USB, and no antenna. I used manual gain settings. There was no difference in noise floor between using lowest gain setting and using highest gain setting. Maybe something gets amplified, I'm not sure, but it's not that much.

Checking "hardware AGC" does make a difference in noise floor and it goes up by a good amount.

I did the same thing on FM radio band and I can see the difference between lowest and highest gain setting and I can receive and hear stations.

Now, what I want to ask is, if any of you have enough time, can you please do the same experiment. RTL-SDR with no LNA and no antenna, tune to 1420MHz and change gain from lowest to highest and tell me if the noise floor goes up and down.

In the meantime, I've ordered new RTL-SDR, just in case.

[Alex P]

Hello Mario,

The most simple troubleshooting method is to have a spare LNA and SDR available.

 Tests  Input Open to 50 Ohm load should shift the noise floor Up several dB

( max gain on SDR )

Open to 50 Ohm  SAWBird LNA Input  via SDR    + 6 dB

Open to 50 Ohm   SDR Input  +1.2 dB

Regards,

Alex

[Mario]

Hi Alex, thanks for responding. I'll try that.