Electro-Optics - Applied Optics Fundamentals and Device Applications - page 25

if the IMPATT frequency is 10 GHz and the microwave frequency into the
electro-optic modulator is 10 GHz, then fundamental frequency locking is
occurring. However, if the IMPATT frequency is 10 GHz and the microwave
source frequency is some subharmonic of the IMPATT, say 5 GHz, then sub-
harmonic locking is occurring. Subharmonic locking is advantageous over
fundamental locking, especially in higher IMPATT frequency applications,
because lower frequency microwave sources can be used that results in cost
savings.
Experimental results of a 38.82 GHz silicon IMPATT oscillator using indi-
rect subharmonic injection locking will now be discussed. Figure 2.14 shows
the experimental setup. The master oscillator frequency is 3.235 GHz. This
frequency modulates the drive current to the laser that results in the laser,
being intensity modulated at the same frequency. The output power of the
laser is 10 mW and it operates at a wavelength of 830 nm. The modulated
laser output is split and the two laser signals control the PIN photodetectors.
The fourth harmonic of the master oscillator, 12.94 GHz, is generated by the
PIN photodetectors and is amplified by variable gain amplifiers. This fourth
harmonic is electrically injected into the IMPATT and injection locking at
38.82 GHz, the third harmonic of 12.94 GHz, occurs.
Results of the experiment are encouraging. FM noise performance improve-
ment of the injection locked IMPATT versus the free-running IMPATT is sig-
nificant. The free-running IMPATT FM noise was −50 dBc at 100 kHz offset
from carrier. Once the IMPATTs were injection locked, the FM noise was
−55 dBc at 5 kHz offset from carrier. Locking range of the IMPATT was also
evaluated. When the amplifier following the PIN photodetector had a gain
of 22 dB, a locking range of 2 MHz was observed. With an amplifier gain of
45 dB, the locking range was 132 MHz. These results indicate that indirect
injection locking could be used to apply FM to the master oscillator in order
to use the IMPATT in a communication or radar system.
2.2.4 TRAPATT Oscillators
TRAPATT stands for TRA pped P lasma A valanche T Triggered T Transit.
Oscillators using TRAPATT diodes have higher power and high-efficiency
capability [32]. Pulsed outputs of up to 1.2 kW at 1.1 GHz and efficiencies of
75% at 0.6 GHz have been observed. Operating frequencies are limited to the
microwave frequencies and below.
Diode structures can be either n + -p-p + or p + -n-n + . The basic theory of
operation is as follows. Upon application of a current step function to the
diode, an electric field will be established that decreases linearly with dis-
tance from the injected current. The electric field will increase until the
critical field, E m , is reached. This critical field will move across the diode
causing avalanche breakdown. The velocity with which the avalanche zone
sweeps across the diode is very high-higher than the saturation velocity of
the newly created free carriers. This leaves the diode filled with a plasma