One of the things I didn't like in the 20m proto BITX I'd built
was the presence of audio getting into the IF amp on transmit and IF
noise in to the audio on recieve. I'd gone the route of adding an
47uh RF choke from the intersection of the balance pot wiper to ground
which helped greatly. However, it always bothered me.
So I opened it up and tried a design change that I find pleasing and
also elimiates a number of components. As an engineer I'm the sort
that looks at a design and says "what if..?". The solution was
staring me in the face and also eliminates the PI attenuator as well.
The change is to turn the balanced modulator around and feed the IF
in on the transformer and RF (bfo) in at the balance potentiometer
via the .1uf capacitor. This eliminated the need for the 3 resistors
and the rf choke. I noted cleaner modulation, less RX noise and more
gain available in both RX and TX.
Might as well add this mod to the collection. I plan to use this
mod in the 6M version.
Allison
KB1GMX
