Introduction:

The solid-state sources used in oscillator-based radars are primarily based on two devices: Gunn oscillators and impact ionization avalanche transit time (IMPATT) diodes.

Overview of Solid State oscillators:

·         Gunn oscillators operate based on the principle of differential negative resistance within a bulk semiconductor material, such as GaAs or indium  phosphide (InP).

·         Gunn diode oscillators are low-noise sources but are capable of only low-output power levels—tens to hundreds of mill watts.

 

 

Figure 8.40 Injection-locked amplifiers.

·         They are useful as radar local oscillators or as the output source of low-power transmitters such as short-range frequency modulated continuous wave (FMCW) radars or altimeters. Gunn diode oscillators are available well into the MMW frequency regime.

·         IMPATT diode oscillators, in contrast with the Gunn diode oscillator, are fairly noisy but are capable of higher output powers, reaching into the tens of watts.

·         They can be power combined for even higher powers and are therefore more common as power sources.

·         They are available up to MMW frequencies.

·         They can also be injection-locked for amplifier operation as illustrated in Figure 8.40. An RF signal is input into one port of a circulator as shown and then enters the output of the diode oscillator circuit.

·         The signal interacts with the diode oscillator, locking the oscillation frequency to that of the input signal. The resulting output is then transferred to the load through the third port of the circulator.

·         A reasonable MMW power amplifier can be constructed from injection-locking a power-combined set of IMPATT diodes.

 

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