Ceiling Fan AGC?

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noj
Posts: 4
Joined: Wed Sep 12, 2018 10:46 pm

Ceiling Fan AGC?

Post by noj » Wed Sep 12, 2018 11:03 pm

Hi,
I have an RTL-SDR v3 and kept having an issue with it suddenly dropping all RF gain. So basically you would be receiving signals visible on the waterfall in SDR# or HDSDR and suddenly you would lose all the signals and though the waterfall would continue to move it would be empty. You could see some traces, but it was if you turned the RF gain all the way down.

The only way I found to recover from this was to reboot the PC the dongle was connected to. This happened with three different computers, using Windows 10 and Ubuntu 18.04. Stopping/starting the program, removing/reinserting the dongle, etc would not help, you have to actually reboot.

Accidentally I happened to have the waterfall visible when I turned off a ceiling fan and observed the waterfall go quiet. I was able to repeat this, with both the ceiling in the same room as the computer/dongle and the fan in the adjoining room. I suspect other large electrical loads may be involved (water pump, AC)as well as it happens at times I am not turning anything off/on.

The computers are powered via a UPS and it doesn't trip or do anything odd, and I have no problem with the computers themselves powering off, freezing, etc. This makes me think maybe it is an RF spike rather than an AC spike. The dongle is hooked via coax to an outdoor bazooka wire antenna. Other than this issue everything works fine and I can listen for hours so it doesn't appear heat or use related.

I am wondering if anyone else has seen this and has any ideas on how to avoid it.

Thanks,

Jon

rtlsdrblog
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Joined: Mon Nov 19, 2012 11:54 pm

Re: Ceiling Fan AGC?

Post by rtlsdrblog » Fri Sep 14, 2018 1:39 am

That is certainly a very strange problem. I haven't heard of anyone experiencing anything similar before.

Do you happen to have some screenshots or ideally a video of the problem occurring? The fact that replugging the dongle doesn't help, and that it happens on multiple computers, makes me think that it's not a dongle problem or a software problem, but possibly is a problem with power supply cleanliness.

Possibly there is some interference occurring within your power supply. High frequency interference getting into the 5V line can cause issues with USB dongles. What if you try without the UPS?

noj
Posts: 4
Joined: Wed Sep 12, 2018 10:46 pm

Re: Ceiling Fan AGC?

Post by noj » Fri Sep 14, 2018 5:49 am

Thanks for the reply, I do have a screenshot and when I have some more time I can try some additional testing.

If you look at the screenshot you can see where I was receiving various signals and noise, then like I described it is like the RF gain instantly drops way down. The waterfall keeps moving and you can see some slight signs of signals, audio is just static.

You will also see a bunch of details I realize I left off like that I am using rtl_tcp, but I believe I can cause the same thing with SDR# and running spyserver.

Jon
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rtlsdrblog
Site Admin
Posts: 1790
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Re: Ceiling Fan AGC?

Post by rtlsdrblog » Mon Sep 17, 2018 5:31 am

Hmm it might just be an rtl_tcp issue then. I've seen rtl_tcp be quite buggy sometimes. But yeah see if it happens to SpyServer too, and ideally with a direct connection.

noj
Posts: 4
Joined: Wed Sep 12, 2018 10:46 pm

Re: Ceiling Fan AGC?

Post by noj » Tue Sep 18, 2018 1:44 pm

I managed to get some more testing done yesterday.

Tried CubicSDR on the client with SoapySDR on the server and had the same issue.

Removed the machine with the dongle from the UPS and plugged it directly into the wall, had the same issue.

Tried running CubicSDR on the same machine with the dongle and had the same issue.

Later today I am going to try my one remaining Windows 10 laptop again, the performance was never good there, but I believe it had the same issue. This will let me run on batteries and take the AC supply completely out of the picture.

The only other things I can think of is to eliminate a short USB extension cable, will do that when I move to the laptop. I am also going to try and cause it without an antenna attached, or with my VHF antenna.

noj
Posts: 4
Joined: Wed Sep 12, 2018 10:46 pm

Re: Ceiling Fan AGC?

Post by noj » Wed Sep 19, 2018 1:56 pm

I did some more testing yesterday with my Windows 10 laptop, after upgrading the Zadig drivers, and SDR# I was able to duplicate the issue both on and off AC power. I took the opportunity while on Windows to upgrade SDR-Console to the latest version and also saw the problem with that.

The last thing I tried was disconnecting the antenna and power cycling the fan. I was not able to duplicate the issue with the antenna disconnected. Since I have to reconnect the antenna to see if the gain is reduced kind of after the fact all I can say is it didn't seem to happen.

I feel with it happening on the laptop on battery power I have eliminated a power spike or line noise from being the issue. I am going to test more today with other antennas.

Also when testing with the laptop I found that removing and re-inserting the dongle is "resetting" it now. I don't think that was the case when this all started.

If this was an RF spike is there any sort of protective circuit in the dongle that would reduce the gain to prevent damage?

rtlsdrblog
Site Admin
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Re: Ceiling Fan AGC?

Post by rtlsdrblog » Thu Sep 20, 2018 11:37 am

If it's due to a very large RF spike you can add an attenuator, or if you can find out what frequency the spike is use a filter. Another option is a power limiter. e.g. https://www.seeedstudio.com/Power-Limiter-p-1444.html.

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