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Re: [linrad] Quietening computer noise

Many thanks for the info.
The system is just a crude lashup at the moment as a starting point. The front end board is just sitting on the bench, with separate power supplies for that and the the LO which is a previously built sig gen. The PC is a cheap second hand box for the experiment (AUS$70 only) The antenna is just a long wire at present. That may be the best place to start. I will incorporate the front end board and the LO into a shielded case remote from the PC and filter the audio (Perhaps balance?) between them, and put up a decent antenna with a separate ground.


wb9uwa@xxxxxxxxxxx wrote:

Hi Richard,

I haven't seen it directly mentioned here yet, so it might be worth mentioning.
You did not say what you are using for an antenna.
Be very careful that the PC does not become part of that antenna.
If you just use a simple wire connected to the RX, your PC is sure to
be part of your ground plane and will generate noise. I am not so sure
that double sheilded coax is needed at HF, but it is important that
the antenna be remotely located and have its own local independant
ground, depending on antenna type. None of my outdoor HF antennas
pick up RFI from the PC.

73, Jim Shaffer, WB9UWA.

On 2 May 2004 at 20:58, 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

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