[Elecraft] RS232 Noise

[Jim Brown]

On Wed, 22 Apr 2009 19:11:51 -0400, Mike Harris wrote:

>This extra noise vanished when I removed the "RS232" cable from the 
K3. 
>The cable is actually the one I made up for the K2.

There are several problems in the setup. First, the K3 connector shells 
are not properly bonded to the chassis, so the cable shield is useless. 
Second, the cable Elecraft sold with the K2-series products used 
parallel conductors, not twisted pairs. That's a recipe for noise and 
RFI. 

I live in a fairly quiet area in the Santa Cruz Mountains, and lots of 
small electronics produce noise that I can hear, both HF and VHF. Much 
of that noise is radiated as a common mode signal on the various cables 
connected to the noise source. I have several suggestions for you. 

1) Build the serial cable shown in my RFI application note. 
http://audiosystemsgroup.com/RFI-Ham.pdf  It uses CAT5 cable. When VHF 
noise is a concern, shielded CAT5 may offer further improvements. 

2) Use ferrite chokes on every cable to kill common mode current. A 
single turn through a #43 core peaks the choking action around 150 MHz. 
Two-three turns moves the peak down to 6M and provides about three 
times the choking impedance. If you're concerned about both bands, use 
multiple cores in series. The equipment at both ends of any cable can 
be the noise source, so if the cable is longer than about 1/4 wave, use 
chokes at both ends. I did all of this in my station (and around my 
XYL's computers) and killed a lot of noise. 

3) Fix the pin 1 problem in the K3 by bonding the DB-connectors to the 
chassis. Pin 1 problems couple RFI in both directions -- that is, they 
let RF into equipment and they put RF onto cables that radiate it.  

73,

Jim Brown K9YC