Major interference from computer

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rebelradio
Posts: 5
Joined: Thu Jan 14, 2021 8:15 pm

Major interference from computer

Post by rebelradio » Fri Jan 29, 2021 10:24 pm

I was playing with my RTL-SDR stick using my desktop computer. On many frequency ranges, the spectrum was "wavy", and there was strong audible interference, like this:
SDR_interference_original.png
Interference on desktop computer
SDR_interference_original.png (1.19 MiB) Viewed 1409 times
I was wondering what was causing this, could it come from my desktop computer or some other equipment. I tried changing the USB port but the interference didn't go away. Turning off the display didn't help either. But I noticed that it changed a bit when I did something on the computer, such as opening the browser or starting a Youtube video.

So the next easiest option was trying my laptop instead, and yes, no interference this time:
SDR_no_interference.png
No interference on laptop computer
SDR_no_interference.png (354.23 KiB) Viewed 1409 times
As soon as I booted my desktop computer, the interference returned as you can see:
SDR_interference1.png
Desktop computer was booted and interference started
SDR_interference1.png (445.26 KiB) Viewed 1409 times

The next interesting question is if my computer is faulty, or if this could be mitigated with proper setup.

The distance between the interference peaks is about 70 kHz which could match the operating frequency of the switch mode power supply. Or some on-board power converter on the motherboard or the GPU.

The case is made of metal but it's pretty thoroughly painted so the side panels and other parts may not form a good electrical connection to the main chassis (not sure about this, should check). And there are ventilation grills on multiple sides. However, I think these shouldn't have a big effect at these frequencies.

The IO shield is also missing, but more importantly, the front side is not metal at all - there's a big front door (plastic), another hatch (plastic), air filter and fan rack (plastic) with two fans (plastic and a small amount of other materials in the motors). Then comes the hard drive rack which is metal but it doesn't cover the pathway completely.

The power cord doesn't have any ferrite suppressors, perhaps I could try those. And there are display, mouse, keyboard and audio cables as well. Any of these could, in theory, radiate interference.


Moderators: Feel free to move this to a more appropriate subform, such as Signal identification, if you think that it would be beneficial.

ckoval7
Posts: 93
Joined: Fri Sep 27, 2019 8:54 pm

Re: Major interference from computer

Post by ckoval7 » Sat Jan 30, 2021 2:09 am

Is your desktop connected to the internet via ethernet? If not, you can just stop reading this post, otherwise, read on.

Is that the only device in that part of your home connected via ethernet? What happens if you unplug the ethernet cable? What happens if you plug it into your laptop, assuming it actually has the port for it.

alanzfq
Posts: 201
Joined: Thu Oct 04, 2018 11:18 am

Re: Major interference from computer

Post by alanzfq » Sat Jan 30, 2021 7:18 am

Which way is it getting in? Via USB?
Via the antenna? Where is it?
Alan

hotpaw2
Posts: 177
Joined: Sat Jan 14, 2017 11:07 pm
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Re: Major interference from computer

Post by hotpaw2 » Sun Jan 31, 2021 4:44 pm

You could try looping various cords and cables connected to your PC through a ferrite toroid, one at a time, and see which, if any, connector is leaking the RFI.

rebelradio
Posts: 5
Joined: Thu Jan 14, 2021 8:15 pm

Re: Major interference from computer

Post by rebelradio » Fri Feb 12, 2021 11:07 pm

I unplugged all cables except power and display cable from my desktop computer while using the RTL-SDR on the laptop, but the interference did not go away.

I also tried running Furmark and Linpack stress tests on the desktop computer to see if the interference changed when the power supply load increased, but the changes were not very significant (at least around 118 MHz).

I'll try to get some ferrite rings next to see if I can dampen the interference.

Is there software that can scan over a frequency range like a spectrum analyzer or measuring receiver does? It would be nice to see how the interference is distributed over frequencies.

alanzfq
Posts: 201
Joined: Thu Oct 04, 2018 11:18 am

Re: Major interference from computer

Post by alanzfq » Sun Feb 14, 2021 8:32 am

You did not answer the important questions.
Does it stop if the antenna is disconnected?
If so what is the antenna, where is it?
Alan

rebelradio
Posts: 5
Joined: Thu Jan 14, 2021 8:15 pm

Re: Major interference from computer

Post by rebelradio » Sun Feb 14, 2021 3:38 pm

alanzfq wrote:
Sun Feb 14, 2021 8:32 am
You did not answer the important questions.
Does it stop if the antenna is disconnected?
If so what is the antenna, where is it?
Alan
I disconnected the antenna, and the interference disappeared. Booting the desktop computer did not have any effect near 118 MHz when the antenna was disconnected.

I'm using the RTL-SDR VHF dipole antenna kit at the moment. It's quite close to the the desktop computer.

I also tried connecting the RTL-SDR to the TV antenna socket of my apartment. The interference was weaker this time, but it could still be seen.

alanzfq
Posts: 201
Joined: Thu Oct 04, 2018 11:18 am

Re: Major interference from computer

Post by alanzfq » Mon Feb 15, 2021 8:22 am

The first thing must be to get an extension cable and find a place for the antenna where it picks up least interference.
Then try the ideas posted earlier.
Try turning off the monitor, often the worst offender if the antenna is close.
Alan

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