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Re: [linrad] Quietening computer noise

Hello Richard,

Best to start with a computer certified to be FCC B rated for home use. 
I have had reasonable luck with
 HP computers and I like the Intel D865PERLL motherboard I am now using.

Considerations for making a PC RFI/EMI quiet:

1. Make sure the case is all metal and covers have a good metal to metal 
bond, use the cover screws.
2. Remove all unused ISA/PCI cards, replace the panel covers for unused 
expansion slots.
    if you removed one of the CDROM drives or other front panel 
accessible drive, plug up the hole with
    a shielding metal foil and tape.
3. Sniff out with a RF probe to the receiver input , any offending noise 
from locations in the shack.
    Anything powered up can emit noise, potentially. Locate all 
offending noise sources.
    Turn off all AC power to the facility and power the test receiver 
with a battery to sniff out offending noises.
    This is your baseline.
4. If noise is found on the power cable, choke it with a ferrite choke 
like a clamshell type.
5. Ditto for the USB-RS232-parallel cables-keyboard-mouse. Wireless mice 
may have a 432 MHz RF datalink.
6. Ditto for the video monitor cables, I find LCD monitors quieter then 
a CRT .
7. Put antennas high on a tower to physically separate the offending 
noise emitter (computer)
8. Use only a double shielded coaxial cable or a solid copper shielded 
cable like Heliax
    ( on TX RF will leak out of a braided shield cable and on RX, noise 
leaks in.)
9. If the ethernet cable is noisy, disconnect, be sure to sniff out the 
data com devices like routers and hubs etc.
    non-metallic fiber optic cables are very quiet and RFI proof, use 
fiber in your data network.
10 For multiple DC power supplies, bond all negative terminals to 
ground, station ground, ESG.
    ESG is electrical safety ground, the green wire.
11. Use differential balanced inputs to the audio cards, avoid mic (high 
gain )inputs to the sound card
12. electrically bond all chassis in the shack together to ground, ESG
13. Measure the contribution to ground current for all active devices 
bonded to ground.
    any contributor greater than a few 10s of milliamperes needs to 
removed or corrected.
14. Physically separate sensitive receivers from noise sources.
15. Route cables to prevent noisy cables being routed with clean RX cables.
     route coaxial cables separate from other cables, route audio/IF 
cables away from power and ETHERNET cables.
16. Improvements can be made to a threshold point, where noises outside 
of you control will dominate.
    You may have electrical power line noises, neighbor video game 
noises, etc. that you may not be able to control.
17 Implement  Leif's suggestions on quieting the Delta-44 soundcard.
18 Good Luck.

Stan, WA1ECF  system level EMC (electromagnetic compatibility), our 

Richard Hosking wrote:

> Dear all
> I have managed to recieve signals on HF using two SBL-1 mixers driven 
> by a quadrature LO (AD9854) and a couple of low noise opamps (NE5534). 
> Linrad runs in direct conversion I/Q mode. The main limiting factor at 
> present is noise from both computer and monitor.
> The sound card is bog standard and the machine is a PII 400 MHz 
> running RH9, but with only 64MB of RAM
> Questions
> The max filter bandwidth appears to be about 2 KHz on SSB, and 
> narrower on AM  - if I try to make it wider nothing happens.
> Is this due to RAM and processor limitations?
> How to get rid of that computer noise? ( I have made no attempt 
> whatever so far)
> Thanks in advance
> Richard