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Published on: 24 July, 2024

What is Gunn diode oscillator? What are different modes of operation of Gunn diode? Mention its applications.

A Gunn diode oscillator circuit consists of a resonant cavity. An arrangement for coupling diode to the cavity, a circuit for biasing the diode and a mechanism to couple the R.F power the diode and a mechanism to couple the R.F power from the cavity to the external circuit or load. A coaxial cavity or rectangular waveguide cavities are commonly used and these are shown in Fig. 12(a) and (b).

The circuit using coaxial cavity has the Gunn diode mounted at one end of the cavity and is in continuation with the central conductor of the coaxial line. The output is taken using an inductively or capacitive coupled probe. The length of the cavity determines the frequency of oscillation. The location of the coupling loop or probe within the resonator, determine the load impedance presented to the Gunn diode.

Heat sink conductor away the heat due to power dissipation of the device. This circuit has an advantage that it can be easily fabricated but low Q of coaxial resonator and oscillations at harmonic of the desired frequency are the disadvantage.

Fig. 12(a) Gunn diode oscillator using co – axial cavity

Fig. 12(b) Gunn diode oscillator using waveguide

The circuit using waveguide cavity is more popular, consisting a simple waveguide section separated from the output waveguide by an iris. The Gunn diode is mounted in a post across the narrow dimension in the centre of the waveguide. The rectangular cavity operates in the TE101 mode. The diode post acts as a large inductive susceptance and the iris is also inductive. Hence the resonance frequency is lower than that for which the length l is $$\lambda_g/2$$. The dielectric tuning rod is used to adjust the frequency mechanically. Sapphire dielectric rod is commonly employed.

Applications:

1. Gun diode is frequently used as low and medium power oscillator in microwave receives and instruments
2. As, it has noise figure much less than that of IMPATT, so Gunn diode are used as pump source for most of the parametric amplifiers.
3. High power Gunn diodes are used in CW Doppler Radar.

Mode of operation of Gunn diode:

A Gunn diode oscillator can be made to oscillate in any of the following frequency modes.

1. Transit Time Mode: this mode does not require any external circuit for it operation. The device length is such that here, fL=107 cm/sec=vs . As, here the oscillation period is equal to transit time, i,e., T=τ0 hence it is known as transit time mode. It is low power, low efficiency mode and required that operating frequency be leaser than 30 GHz. In order to achieve more higher frequencies the device length will be ridiculously small.
2. Delayed Mode or inhibited Mode: Here the oscillation period is greater than transit time i.e. T>τ and thus the charge domain will take more time to move from cathode to anode, hence the operating frequency is less than that of transit time mode. This delay inhabited mode has an efficiency of nearly 20%.
3. Quenched Mode: Here the oscillation period is lesser than transit time i.e., T<τ0 and thus the charge domain will take less time to cross the length of the device, hence operating frequencies is higher than the transit time mode. The efficiency in this mode is only 13%.
4. Limited space charge accumulation (LSA): This mode of operation gives high power rise with higher efficiency (16 – 23 %). In this mode the operating frequency is 0.5 – 50 times more than that of transit time mode, hence the Gunn diode can be operated in the LSA mode up to a frequency of 100 GHz.
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