[R-390] Computer RFI and R390s

Mon, 11 Nov 2002 10:54:17 -0800

The best implementation is to use balanced shielded pair between the 390 and
the PC line input, with the ground shield connected at only one piece of
equipment to avoid ground loops. As a kinda useful connection, one may use
CAT5 cable, using a true color coded pair and then all others as a ground or
shield. Real shielded twisted pair is hard to find at the Shack. The modem
transformers are a good idea; they are extremely well made as Drew pointed
out. I used to design them and Ethernet transformers for a living. The 100B
Ethernet transformers on network adaptor cards are typically flat from
100kHz to 100MHz and very good impedance match to 100 Ohms. Quite good for
390 service. Not to mention, the modem and Ethernet transformers are
Hi-potted to better than 1500VAC Primary to secondary.

Bob

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Drew Papanek
Sent: Monday, November 11, 2002 10:00 AM
To: [email protected]
Subject: [R-390] Computer RFI and R390s

Richard,

Very good point regarding use of audio transformer for isolation.  Telephone
line coupling transformers salvaged from computer modems work well for this
and audio work.  I recently tested one flat within 3 dB over  30Hz - 40KHz
with 600 ohm source and load (has 1:1 turns ratio and electrostatic shield
between primary and secondary).

You can accomplish the same result by using the Line Output of the R-390x
(being balanced it is already isolated).

Drew

>From: "Richard Biddle" <[email protected]>
>Subject: RE:  [R-390] Computer RFI and R390s
>
>If you are connecting the R-390 to the computer audio card, you may also
>want to try a 600ohm/600ohm isolation transformer in the audio line.  I
>had a ground loop problem that cleared right up when I did this.  I also
>put a Topaz line isolation transformer on the primary of the radio
>before the variac.
>
> >How do you guys keep out computer hash from your listening/receiving?
> >Turning the computer off isn't viable, since one of the programs I use
>is a
> >RTTY program.
>
>73 de Richard W5SRB

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