There might need to be some capacitive balance besides the resistive
balance. Might be that the carrier isn't as far down the side of the
filter on one sideband as the other and there's some leakage coupling
past the filter from the carrier oscillator by stray coupling or
inadequate power supply bypassing. With the different distance down the
slope of the filter, the phase delay would be different from one
sideband to the other and adjusting for a null on one wouldn't necessary
match on the other. When you set a null on one sideband you are not only
nulling the direct coupling but also the stray.
73, Jerry, K0CQ
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