Gunn Diode Oscillators

- summary of the Gunn diode oscillator circuit and Gunn diode or transferred electron device circuit considerations.

A Gunn diode oscillator or transferred electron device oscillator generally consists of a diode with a DC bias applied and a tuned circuit.

The Gunn diode oscillator circuit or transferred electron oscillator uses the negative resistance over a portion of the V/I curve of the Gunn diode, combined with the timing properties within the device to allow the construction of an RF relaxation oscillator. When a suitable current is passed through the device it will start to oscillator.

The negative resistance created by the V/I characteristic will cancel out any real resistance in the circuit so that any oscillation will build up and will be maintained indefinitely while DC is applied. The amplitude will be limited by the limits of the negative resistance region of the Gunn diode.

Gunn diode characteristic
Gunn diode characteristic

Gunn diode tuning

The frequency of the signal generated by a Gunn diode is chiefly set by the thickness of the active region. However it is possible to alter it somewhat. Often Gunn diodes are mounted in a waveguide and the whole assembly forms a resonant circuit. As a result there are a number of ways in which the resonant frequency of the assembly can be altered. Mechanical adjustments can be made by placing an adjusting screw into the waveguide cavity and these are used to give a crude measure of tuning.

However some form of electrical tuning is normally required as well. It is possible to couple a varactor diode into the Gunn oscillator circuit, but changing the voltage on the varactor, and hence its capacitance, the frequency of the Gunn assembly can be trimmed.

A more effective tuning scheme can be implemented using what is termed a YIG. It gains its name from the fact that it contains a ferromagnetic material called Yttrium Iron Garnet. The Gunn diode is placed into the cavity along with the YIG which has the effect of reducing the effective size of the cavity. This is achieved by placing a coil outside the waveguide. When a current is passed through the coil it has the effect of increasing the magnetic volume of the YIG and hence reducing the electrical size of the cavity. In turn this increases the frequency of operation. This form of tuning, although more expensive, produces much lower levels of phase noise, and the frequency can be varied by a much greater degree.

By Ian Poole

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