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Verification of Spectral Purity in the HIFI Local Oscillator

Many high efficiency varactor diode multipliers rectify a small amount of RF input power and act as a charge pump into the bias network. If the phase and the frequency of the pumped charge resonates with the bias circuit it will result in a forced parametric oscillation, which is as spectrally pure as the fundamental input signal. In HIFI these oscillations were observed in half the local oscillator bands over a range of output frequencies up to 50 GHz wide. The oscillation at the resonant bias network frequency results in AM side bands on the carrier. After several subsequent multiplier stages one or more sidebands can exceed the carrier signal. This effect can not be detected in a Y-factor measurement. It is also extremely hard to detect in the pumped SIS mixer IV curve due to the relatively small frequency offset of the extra LO frequencies, so determination of the mixer response usually requires spectroscopic measurements. These oscillations have been observed in half of the HIFI local oscillator bands. We describe a method for detecting and suppressing of parametric oscillations by reviewing the pumped IV curves of the multiplier as function of input power and bias. This has successfully been used to mitigate the oscillations in HIFI band 1 and 5. Further studies need to be done to provide design criteria to prevent parametric oscillations in varactor multipliers.