Simple question (I hope!) I've been looking at AM radio alignment techniques recently and IF alignment in particular. The "generic" approach in a valve(tube) radio is to feed the IF frequency into the control grid of the mixer/frequency changer valve and monitor for peak output at the speaker whilst tuning the IF transformers. I would imagine the same technique is generally applicable in transistor radios also.
The control grid of the frequency changer is the RF input with the LO signal arriving on a separate grid. What I don't understand is how the LO signal doesn't interfere with the generated test IF signal fed into the control grid?
The general consensus says that you can perform an IF alignment leaving the LO signal in-place and changing the LO shouldn't affect the IFT output. But doesn't the mixer valve hetrodyne the two signals to produce the IF products (subsequently tuned by the IFTs). So, if you leave the LO signal as an input won't you get variable products and won't it affect the output?