A frequency conversion system includes a mixer, which is coupled to mix an input signal with a Local Oscillator (LO) signal, so as to produce an output signal. Control circuitry is configured to adjust an actual level of the LO signal provided to the mixer, so as to maintain the actual level substantially constant. A nulling signal generator is coupled to inject a nulling signal into the input signal prior to mixing with the LO signal adjusted by the control circuitry.