YIG oscillator

The amplified rf power is fed to a step recovery diode housed in a microwave structure with output in an intermediate frequency range, typically low GHz. That range is not critical and can be chosen to be compatible with existing or low cost equipment used in other fields, e.g. radars and satellite downlinks. The present authors, for example, chose the band 11-15 GHz as it permitted the use of low cost coaxial components, synchroniser and YIG oscillator for the locking steps. 

Figure 3.7 Schematic diagram of a phase-sensitive detector at 20 MHz. The transformer is replaced by an active circuit in the HP8709A synchroniser, and probably most high precision configurations. The phase error voltage output is amplified and used in the spectrometer to control the YIG oscillator magnetic field and hence lock the YIG source frequency to the synthesiser frequency. An identical device locks the Gunn MMW source to the YIG frequency (Adapted from Connor )...

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. 

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. 

At frequencies up to -150-200 GHz, solid-state sources such as YIG-tuned oscillators or Guim diode oscillators are now available with power outputs of up to 100 mW. The harmonic generation of such millimetre-wave sources is relatively efficient for doubling and tripling (>10-15%), but for higher harmonics the power drops rapidly ( (1 THz)< 0.1-10 pW). Nevertheless, harmonic generation was used as early as the 1950s to record the submillimetre wave spectra of stable molecules [33]- Harmonics from optimized solid-state millimetre-wave sources are now used to drive astronomical heterodyne receivers up to 900-1100 GHz... 

Figure 3.6 Schematic drawing of a YIG tuned oscillator. When the DC magnetic field is applied, energy is coupled from one loop to the other at the resonant frequency of the YIG sphere, which is itself defined by the magnetic field and can therefore be swept, or held at one frequency...

---