[Elecraft] Clean CW signal

[Geoffrey Mackenzie-Kennedy]

Dave Walker wrote:


>I notice when I listen to my CW on a separate reciever that there appears 
>to be a slight husky modulation to the carrier.  It's not located near a 
>power supply - so I've eliminated magnetic field interference.  I do have a 
>manual MFJ tuner located 3 ft away. My antenna is a G5RV that runs across 
>the roof of the house.  I have a 6:1 balun at the 300/50ohm junction.
>
> I was wondering if this slight raspy noise on the carrier is normal or if 
> I have some issues with my setup.

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Does the modulation of the carrier change in any way if you change the 
sidetone frequency significantly and do you hear the same husky modulation 
on all bands? Is the modulation still there when transmitting into a dummy 
load and not your antenna?

My K2/100 is #3255, same vintage as yours but unmodified except for some 
receiver birdie killers, and on CW the sidetone generator frequency / phase 
modulates the carrier slightly. With the sidetone frequency that I usually 
use, approx 400 Hz, the first set of sidebands is approximately 90db below 
carrier which should not be an issue running barefoot, but could be with a 
linear. After building my K2 the problem had been worse, caused by U10 on 
the Control Board not meeting its crosstalk spec. - an unusual case I 
believe.

Although this might not be the cause of the modulation that you hear, the 
sidetone generator, the BFO varicap tuning and receiver IF filter varicaps 
circuits all share the Quad DAC chip U8 on the Control Board. Ideally each 
DAC should be completely isolated from the others, but in practice this is 
not the case which results in some sidetone signal hopping across to the BFO 
varicap circuit U10D during transmit, modulating the BFO. The same crosstalk 
problem can exist within the Op-amp U10 (Control Board), the cause of my 
problem.

If your other receiver can tune to the BFO frequency, it would be worthwhile 
to listen to the BFO signal during CW transmit into your antenna and into a 
dummy load.

73,
Geoff
GM4ESD