Spurious phase modulation by crystal filters

A spurious phase modulation (PM) problem discovered during a satellite thermal vacuum test was traced to a supposedly passive component: a crystal filter. While a literature search showed that this phenomenon has been observed in the past, the effect does not seem to be widely known and is a possible problem in some current designs. The PM was noted in a passive four-pole lattice-type crystal bandpass filter at high drive levels over a narrow temperature range. The modulation sidebands were within the 3-dB bandwidth of the filter and 20 dB below the carrier.<
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