[Elecraft] AGC behaviour

[Jim Brown]

On Wed, 24 Sep 2008 07:59:36 -0400, Monty Shultes wrote:

>I have very high white noise in my K3 on 75 meters when connected to the 
>serial port computer interface. Removing this connection greatly reduces the 
>noise.  If you use HRD, LP-BRIDGE, or any control software, you can also 
>hear a pulsing noise related to polling the K3 over the serial port.  I 
>believe this can be lessened, but have not tried as yet.  Here in Florida, 
>USA, the level of thunder storms creates a naturally high noise level in the 
>summer, but the effects described above are clearly noticeable.  I use the 
>Elecraft KUSB as serial interface.

I suspect that either the computer or the K3 are dumping RF trash onto your 
serial cable, which, like any antenna, is radiating it, and your 75M antenna is 
picking it up. I suggest that you wind the serial cable around a toroid to form 
a common mode choke tuned to 75M. Fair-Rite #31 is the best material to use, and 
a 2.4-inch o.d., 1.4-inch i.d. toroid is a convenient size. Fair-Rite #43 will 
work, but you'll need more turns. This size is commonly called FT-240 in the ham 
world. 

See http://audiosystemsgroup.com/RFI-Ham.pdf for tutorial information about how 
and why this works, as well as measured data for chokes with 1 - 14 turns around 
#31 and #43 toroids. Ferrite beads clamped onto the cable will NOT fix this 
problem on the HF bands. They WILL help on 2M. :) 

73,

Jim Brown K9YC