Aigneryu
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Hi,
When your transmitter is on, the output of PA will be a carrier-modulated signal, which will be different from your VCO. So if you have some feedback path, the PA will still pull your VCO. So that is why the VCO frequency should avoid to be the same as your LO in a "integrated" "direct-conversion" (or low-IF). If you lock your VCO at 2x of the required LO, then you can prevent the pulling.
Sincerely,
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