Synchronised YIG Oscillators as Intermediate Frequency Sources

In a spectrometer, the spectral source would normally be locked to a frequency 

The important point is that the beat frequency carries the phase relationship between the synthesiser and the YIG oscillator outputs. The function of the synchroniser is to reduce that phase difference to zero by sending a control signal to the YIG oscillator to adjust its frequency until that zero phase difference is achieved. At that point the YIG oscillator will be phase-locked to the synthesiser and thus have its characteristic stability and resettability, viz. 25 X 0.1 Hz resolution. 

To achieve this there is a hi-stab crystal controlled 20 MHz reference oscillator in the synchroniser circuit that is summed phase-coherently in the mixer with the step recovery diode output containing the 20MHz beat to produce the correction signal. 

That correction signal has an amplitude proportional to the difference in phase between the reference 20 MHz signal and the 20 MHz beat frequency. This phase error signal is then amplified and fed back to control the YIG oscillator output fi-equency. The sign of the phase error signal is such as to force the YIG oscillator frequency towards the synthesiser harmonic frequency. When they become equal the beat frequency from the step recovery diode will be identical to the reference oscillator 20 MHz and the two sources will be locked to zero phase difference. 

The next stage is to lock the Gunn oscillator that is the actual spectral source for the measurement at, let us suppose, 62.520 GHz. This is achieved in the same way the YIG oscillator output is used to drive a varactor multiplier diode held in a MMW structure that couples a fraction of the signal from the Gunn spectral source. The beat frequency at around 20 MHz, between the 5th harmonic of the YIG oscillator at 62.500 GHz and the Gunn oscillator at around 62.520 GHz, is passed to a second synchroniser. 

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