Measurement of Static AM-PM Conversion in Frequency Multipliers

The generation of very narrow spectral lines in the far-infrared by frequency synthesis from VHF precision sources requires very stringent specifications on the spectral purity of the source and on the phase noise introduced by the synthesizer. The dc measurements of the AM-PM conversion in different multiplier stages are presented in this paper: stages employing transistors, varactors and step-recovery diodes are examined. The results show that a few degrees per dB of input level variation are typical for the AM-PM conversion reported to the input in a simple, carefully built and well tuned multiplier stage employing any of the mentioned solid state devices. This value is shown to be unlikely to degrade more than the expected n2 factor the spectral purity of a signal with AM noise as low with respect to PM noise as it is in the output of a good quartz crystal controlled oscillator; however, such a conversion could become a source of phase noise, with degradation of the spectral purity, for a signal with a slightly worse AM noise.