TenTec
[Top] [All Lists]

[TenTec] Computer Interface Question

To: <tentec@contesting.com>
Subject: [TenTec] Computer Interface Question
From: mark@microenh.com (Mark Erbaugh)
Date: Mon, 2 Apr 2001 23:14:27 -0400
I built what I think is a completely isolated interface between my computer
and my Pegasus accy connection.  I use 1:1 isolation transformers on both
audio out and audio in, I use a pot to drop the audio in to mic levels, and
I use an opto-isolator to interface the serial port RTS line to the PTT
circuitry.

At first It thought things were working well. When monitoring signals using
DigiPan, I saw no evidence of 60 Hz hum. One quick on-air test and I got a
'good looking signal' report. Well, tonight I was doing some fine tuning and
listening to my Pegasus on another receiver. I discovered some very audible
60 Hz hum on my tx signal when the input signal is very low. I suspect that
it was there during my brief on-air test, but masked by the PSK tones.
Further investigation reveals that the hum all but goes away if I turn off
my computer monitor.  I was able to greatly reduce the hum with the monitor
on by moving the interface box around.

The interface is built in a plastic box. The computer case is grounded to
the shack ground as is the Pegasus, but the interface box (plastic) is not.
I tried un-grounding the computer case, but that had no effect. I also
placed torriods on the monitor video cable and the cables to/from the
interface box with no effect.

It appears that somehow my monitor (a cheap Pixie 17" color montor) is
coupling with my interface. Would a metal box provide better shielding?
Other suggestions?

73,
Mark, N8ME


--
FAQ on WWW:               http://www.contesting.com/FAQ/tentec
Submissions:              tentec@contesting.com
Administrative requests:  tentec-REQUEST@contesting.com
Problems:                 owner-tentec@contesting.com


<Prev in Thread] Current Thread [Next in Thread>