Gunn oscillator

G. Thimpt, F. Benmakroha, A. Leontakianakos, J. F. Alder, Analytical microwave spectrometer employing a Gunn oscillator locked to the rotational absorption hne, J. Phys E Sci. Instrum., 19, 823-830 (1986). 

A 76.5 GHz Gunn-oscillator on the radar transceiver delivers the output power for the three beams and the same number of mixers for the demodulation of the received waves. The modulation is controlled by a frequency-locked loop using a... 

The source of microwave radiation at Arizona State University is a set of three phase-locked Gunn oscillators (Fig. 3), which operate in the 65-140-GHz region at power levels of about 50 mW. Higher frequencies are obtained by doubling, tripling, or quadrupling the fundamental frequency in a nonlinear Schottky diode multiplier. The millimeter wave radiation is frequency modulated at 25 kHz by adjusting the reference frequency used in the lock circuit. 

Figure 3. Block diagram of the millimeter wave spectrometer used by the Ziurys group. The instrument uses a phase locked Gunn oscillator as a source of radiation and an InSb detector. [Reprinted with permission from ref. 37. Copyright 1994 American Institute of Physics.]...

This complex relationship means that if the cavity is held at resonance and the spectral line is swept, e.g. by Stark or Zeeman modulation, although other modulation schemes are possible, the cavity impedance changes and the reflected power incident on the Gunn device changes in sympathy with the spectral scan. This causes a current to flow in the Gunn oscillator circuit related to the spectral absorption profile, and therefore to its amplitude and area. That current can be readily transformer-coupled out of the Gunn bias circuit and detected S)mchro-nously with the modulation frequency. 

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. 

The need for transportability coupled with operator- and intrinsic safety considerations dictates the use of solid state sources operating at low voltage. Of these, Gunn oscillators are the least susceptible to noise and, if locked to a high stability source, are easy to stabilise and to scan in frequency. At lower frequencies they are both inexpensive and widely available as components of intruder alarm systems, but their cost rises rapidly with increasing frequency, and becomes uncomfortable beyond about 100 GHz. 

The Gunn oscillator frequency-lock became progressively more unstable as its slewing rate increased, whether caused by rapid frequency stepping or by the application of FM. We found ourselves limited in practice to a stepping time of 50-100 ms and an FM deviation of 10 MHz at a rate of 1 kHz. This was considered reasonably satisfactory in that a complete scan could be completed in a few seconds and that our signal detection rate 2 kHz was close to the frequency at which manufacturers, e.g. Millitech, specified their detector performance. The same FM rate was used whether our detector was a Schottky barrier mixer diode or the helium-cooled bolometer. 

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