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General Category => General Radio Discussion => Topic started by: Antennae on January 31, 2015, 1857 UTC

Title: Need help with noise from computer monitor.
Post by: Antennae on January 31, 2015, 1857 UTC

I got a new SDR and my LCD computer monitor really creates some interference. When I turn off the monitor, the interference goes away. Monitor is connected via serial type cable.

My SDR has one cable, its a USB that goes to the computer.  I obtained 4 toroid ferrites, stacked them, and wrapped the USB cable about 8 times around them. This made the noise go down a lot.  And it refined the noise.

The noise appears in 60hZ increments.

I use an 8-plug surge protector on my computer.  If I plug the monitor and computer directly into the wall socket the noise is still there.

Is this a ground loop?  How can I alleviate this?  Thanks.








 

Title: Re: Need help with noise from computer monitor.
Post by: ChrisSmolinski on January 31, 2015, 1914 UTC

Do you mean 60 kHz, not 60 Hz, intervals for the noise?

You may want to try some ferrites on the video cable, I assume it is a 15 pin VGA cable?

How are you feeding the antenna connection to the SDR?

Title: Re: Need help with noise from computer monitor.
Post by: Antennae on January 31, 2015, 2116 UTC

Oop, yes, 60 kHz intervals for the noise. 

I tried ferrites with my 15pin VGA cable.  Two ferrites with the cable coiled twice.  Nothing noticeable changed. 

Both of my antennas are fed with 75ohm coax left over from old cable installation. Less than 10 yrs old cable.  They come into the room's sliding glass door, on the floor.  They run under the table with the monitor to my SDR which is on the floor.

Title: Re: Need help with noise from computer monitor.
Post by: ChrisSmolinski on January 31, 2015, 2248 UTC

Have you tried ferrites/chokes on the coax? I've also experimented with grounding the coax inside the shack, sometimes it helps (and sometimes it does not).

Title: Re: Need help with noise from computer monitor.
Post by: Antennae on February 01, 2015, 0210 UTC

I wrapped my antenna coax twice around 2 ferrites and some overall noise was attenuated but not the 60kHz stuff. 

I'll have to get back to you on the shack grounding.

Title: Re: Need help with noise from computer monitor.
Post by: VK3BVW on February 01, 2015, 0352 UTC

RFI is one of those devilish problems that Hams and SWLs contend with on a very regular basis. The causes of RFI can be many and varied - even out of the one offending piece of electronic equipment. Trial and error experimentation is the order of the day.

One of the best free documents on the web relating to solving any type of RFI problem is Jim Brown's (K9YC) detailed 66 page dissertation entitled A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing. Well worth having a look at this:

http://audiosystemsgroup.com/RFI-Ham.pdf (http://audiosystemsgroup.com/RFI-Ham.pdf)

The other website that may be helpful (and less technical) is called RFI TIPS AND TRICKS!  which was compiled by M.D.Lowell, N1LO:

http://www.hamuniverse.com/rfi.html (http://www.hamuniverse.com/rfi.html)

Hope this helps.

Rob VK3BVW

Title: Re: Need help with noise from computer monitor.
Post by: Antennae on February 01, 2015, 1632 UTC

Thanks much for the links! I'll check those out.

This morning I hooked it up to another computer, my laptop.  And the other computer's monitor still gave interference, but less.  It went away when I turned it off.  So this makes me think the monitor's bad vibes are getting into my coax which would be a common mode problem. 

I have a dream of the coax into a common mode choke, then into the SDR which is in a faraday cage, and then out of the SDR with the USB cord with ferrites that leads to the computer.   :)