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Author Topic: Computer noise in receiver  (Read 5489 times)

Posts: 1

« on: October 07, 2011, 12:53:03 PM »

After grounding everything I am still getting computer and LCD monitor noise in my receiver.
It is also getting into all my audio cables.
Any ideas.

Posts: 376

« Reply #1 on: October 08, 2011, 02:34:17 AM »

Does the noise level change if you move your antenna further away?
How did you ground everything? The monitor and computer are already connected with wide braid in the monitor cable, but you also need to use wide braid between the radio and computer - preferably wide braid going to a single ground point. The ground wire in the three-prong power plug may not be sufficient for RF grounding.
If the noise is being radiated from the switch-mode power supplies, you might need an AC in-line filter, and perhaps ferrites.

Do you use shielded audio- and data cables between the radio and computer? Do you have galvanic separation of all lines with audio- and signal transformers?

I suggest browsing the RFI/EMI sub-forum here on eHam too.
« Last Edit: October 08, 2011, 02:41:12 AM by LA9XSA » Logged

Posts: 157

« Reply #2 on: October 19, 2011, 12:28:10 PM »

I'll go ahead and throw my $.02 in here.  Back before I got my General license I had my computer hooked up to my SW receiver with a Y cable that had a pair of RCA males on one end and a 3.5mm mini on the other, and it was unshielded.  Having just got my Tech. license and not knowing much about RFI, it took me a while to trace it to that Y cable. 

In my case, replacing the Y cable with a shielded one made no difference at all.  I was about to order some snap-on ferrite beads when I got my General and replaced the SW receiver with a transceiver.

Anyhow, I'm certainly no authority on RFI, but ferrite beads are relatively cheap and easy to try, assuming your noise source is your audio cable.  The first step of course is to figure out where it's coming from, and until you do that you're just guessing.

Posts: 6252

« Reply #3 on: October 19, 2011, 10:22:38 PM »

After grounding everything I am still getting computer and LCD monitor noise in my receiver.
It is also getting into all my audio cables.
Any ideas.

Some computers are made with basic parts that have quality issues.  For example, some computer power supplies are perfectly adaquate to run a computer, but put a radio receiver anywhere near them, and you'll hear all kinds of noise.  Replacing the power supply in the computer with a high quality name brand supply may solve the problem, but then again, it may not.  Sometimes its the cables themselves that are the problem.  I was bugged by a noisy audio system in my home and found that the simple fix of making sure the RCA plug ears made good contact with the RCA jack shell solved a lot of the noise issues.

Too much of the time its a question of trial and error.  Sometimes you've just got to bite the bullet and spend money for good cables and parts to solve the problem.

Posts: 2483

« Reply #4 on: October 21, 2011, 05:25:58 AM »

I tried to help a ham who had similar problems.  I took my old HP 141T spectrum analyzer to his house and connected it to his antenna.  When we looked at the radio spectrum on the CRT, it was obvious that the computer, monitor, plasma TV (or some combination thereof) were spewing RF all up and down the HF spectrum.  

We unplugged the different devices in turn to see which was the greatest offender.  The computer and the TV were the sources of the interferrence.  The monitor was "clean".  The only solution (in this case) was to turn the computer "off" and UNPLUG the TV when the ham wanted to operate.  Merely turning the TV off did not stop all of the interferrence.  I guess this was because some of the TV circuits are energized even when the TV is off.  On the other hand something inside the TV may have been arcing.  I did not try to find out.

The computer and TV were all in plastic cases.  Plastic is transparent to RF generated inside the units.  No amount of grounding, shielding, or bonding helped.

Dick  AD4U

Posts: 12629

« Reply #5 on: October 21, 2011, 06:39:45 AM »

 Merely turning the TV off did not stop all of the interferrence.  I guess this was because some of the TV circuits are energized even when the TV is off.

Some  modern HD TV's has a option in menus to "quick boot" or not. When in the quick boot mode, they keep some circuits energized but display off.

Ham since 1969....  Old School 20wpm REAL Extra Class..

Posts: 477

« Reply #6 on: October 27, 2011, 02:00:35 AM »

RFI can be from conduction or radiation.  The best solution is isolation and distance.  'Best' doesn't necessarily mean practical though.  The hard part is figuring the "how/what/when" of it.  Once you figure that out, the 'fixing' of it isn't as difficult. 
The 'conduction' part usually means shielding and isolation by means of transformers and/or capacitors and/or ferrite.  The 'distance' part (mainly for radiated noise) doesn't necessarily mean huge distances, a few feet can 'cure' it sometimes.
Which would 'cure' your problem?  I have no idea. 
Good luck.
 - 'Doc

Posts: 17484

« Reply #7 on: October 28, 2011, 02:55:55 PM »

What are you using for an antenna?  How far is it from the shack?

If you have common mode currents on the coax while transmitting, the coax shield is also
acting like an antenna on receive.  And that antenna runs much closer to the sources of
interference inside the house.  Do you still have the same level of interference when
you disconnect the antenna cable?  If so, try adding an effective balun / common mode choke
at the antenna.

OCFD / "Windom" antennas are one of the worst offenders for this.  We have one at the
county Emergency Operations Center, and had to add a choke near the feedpoint to reduce
the received noise from the computer network system.
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