350-GHz phase/frequency locked loop for use with a carcinotron backward wave oscillator

Abstract

A phase/frequency locked loop for use in a millimeter-wave radio-astronomy receiver is described for locking a carcinotron backward wave oscillator (BWO) operating in the 330–370-GHz range to a fixed 150-MHz reference source. The appropriate 29th–31st harmonic of a stable and tunable frequency source around 11.75 GHz acts as the local oscillator to down convert the high BWO frequency to 150 MHz prior to phase/frequency comparison. A double loop is required, whereby the low-frequency part of the spectrum of the loop error voltage is applied to the line voltage circuit of the carcinotron power supply over more than 30 m distance. The high end of the spectrum is coupled to this voltage via a voltage transformer located next to the carcinotron. Under these conditions a 135-kHz loop bandwidth is easily obtained. The locked signal shows excellent stability, and the measured performance level in terms of the Allan variance to characterize fractional frequency stability is about 10−16 for integration times in the range 100–1000 s. The performance in general satisfies all applications where a high level of phase/frequency stability is required, such as in plasma physics and in presently planned and future uses in radio astronomy.