[Elecraft] K3 in NAQP SSB

[Joe Subich, W4TV]

Tom writes: 

> Be especially aware that rigs with external 12V supplies can 
> develop what sounds exactly like RFI from "pin 1" problems.
> 
> This is because the power supply leads of the SSB radio are 
> current modulated at an audio rate. If the 12V supply has a 
> ground connection common to the negative lead post on the 
> supply (or through other gear) the V- lead voltage drop can 
> cause a "pin 1" issue from dc potential that varies at an 
> audio rate.

This is a particular problem with transceivers that have a 
single V- lead and only one "pin" for the V- connection (like 
the K3).  If the negative lead connection is not absolutely 
perfect other "ground" paths (e.g. the shield on a microphone 
cable) through accessory equipment will carry a portion of the 
return current.  

If the accessory device has an active output, a change of 
only a few millivolts in ground potential sue to the ground 
currents will directly modulate the transceiver's mic input.  
With typical microphone levels of less than 10 millivolts, 
even one millivolt of "current modulation" can appear to 
as a significant "RF feedback." 

The same issue can appear with CAT ports. Where the common 
(signal ground) of the CAT port includes excess impedance, 
the signal WILL seek a lower impedance return.  With RS-232, 
the driving signal is 20 to 30 V P-P (+/-10 to 15 V) ... 
when Pin 5 of a RS-232 port (or signal ground of a TTL 
input) includes several ohms of DC resistance, there is a 
significant chance that the data signal will couple into 
sensitive microphone circuits.  

73, 

   ... Joe, W4TV