Distortion effects in switching-diode modulators arising from local-oscillator interference

The paper considers the situation of a switching-diode modulator driven by a local-oscillator signal derived from some source, typically a frequency synthetiser whose output contains both a significant level of random noise and also harmonically unrelated spurious tones at low level. The interaction of input signals to the modulator with the local-oscillator interference is considered, and expressions are derived for the output-signal levels arising from this interaction in terms of the local-oscillator signal/noise ratio. The performance is shown to be independent of local-oscillator source resistance and diode `offset¿ voltage. The performances of experimental Shottky-barrier modulators agree well with predictions.