Biomedical Engineering Reference
In-Depth Information
The IMPATT diode is the most powerful solid state device in the micro-
wave and millimeter wave regions. Pulsed outputs of >30 W have been
achieved at 10 GHz. Output power approaching 10 W has been seen at a fre-
quency of >100 GHz. The efficiency of the IMPATT diode has reached as high
as 37% in a pulsed operational mode. The aforementioned IMPATT diode
characteristics make it an important device for the future and a logical choice
for optical control.
2.2.3.2 Illumination Effect on IMPATT Diode Operation
Many effects on IMPATT diode behavior due to optical control have been
investigated [26]. This section will cover four of the most important effects
on performance. These are frequency tuning or shifting, noise reduction,
AM/FM modulation of the oscillating frequency, and injection locking of the
IMPATT, which is a “cleaner” signal.
Frequency tuning is a very important capability for oscillators. Not only
can the frequency be shifted by illuminating the IMPATT diode, but the
shifting can be accomplished much more quickly than by conventional con-
trol techniques. Noise reduction is important in any frequency-generating
system because the lower the noise, the more sensitive a system will be to
small offsets of frequency such as in a Doppler radar or a coherent receiver
system. Also, noise reduction is important since present frequency spec-
trums are already overloaded. By reducing the noise output of an oscillator,
the interference with other systems operating on nearby frequencies will be
reduced. AM/FM modulation of the IMPATT is very useful when using the
IMPATT as an oscillator in a communications system, because now the diode
can be modulated by a light source in order to transmit information from
one point to another. Also, because the light source is usually a laser, the
modulation frequency can be expected to go high in the future due to the
constant improvements in laser technology. Finally, injection locking of
the IMPATT diode using optical techniques is very promising because of the
capability to control the frequency of one or more frequency sources. This is
very beneficial when utilizing IMPATTs in a system where coherent sources
are necessary as in Doppler radars.
2.2.3.3 Experimental Results on IMPATT Diodes: Optical Tuning
Optical tuning is achieved by illuminating the IMPATT diode by a laser
source. Photogeneration of carriers occurs that alters the timing of the ava-
lanche cycle. This alteration in avalanche injection phase produces a change
in the oscillation frequency of the diode. These changes in frequency can
occur more quickly than by conventional means of control.
Data from one particular experiment will now be discussed [27]. This
experiment is singled out because the IMPATT diode had an operating
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