SEC.16] THE SQUARE-LA W DETECTOR
19
frequencies above a few thousand cycles per second because the thermal
time constant limits the rate at which it can respond. Detectors of this
kind are widely used in test equipment, however, because they are easily
procured and have some convenient properties. Because these detectors
are capable of absolute calibration when used as bolometers in bridge
circuits, they are most frequently used in low-level power-measuring
equipment. A discussion of these applications is outside the scope of
this volume, and is to be found in Vol. 11 of this series.
A sensitive detector for microwave power is the very highly developed
microwave version of the familiar crystal detector.
Crystal detectors
were early recognized as being especially suited to microwave circuits
because of their extremely small physical size. A large amount of
research has been devoted to the development of crystal detectors in
fixed adjustment and packaged in small cartridges. A large advance in
the understanding of the mechanism of operation of these devices and
studies of the factors making possible the manufacture of high-quality
crystals have led to mass production of cartridge units that are considera-
bly superior to their earlier prototypes. The principal work on these
devices has been toward the development of rectifiers for use as frequency
converters, but advances in the development of low-level detectors have
also been significant and have benefited considerably from the other
development. Because the development of crystal detectors and units
for mixers is a very large field in itself, it will not be possible to give it more
than a cursory treatment in the next chapter. The reader is referred to
Vol. 15 of this series for a thorough review of the subject. The use of
receivers of the variety under discussion here is not sufficiently wide-
spread or complex to warrant a separate treatment in this volume. In
the following section some of the considerations that affect the figure of
merit for such a receiver will be discussed.
1.6. The Square-law Detector.—Both diodes and crystals function as
detectors because of the nonlinear relationship between the current
induced in them and the magnitude of the voltage impressed. In general,
a smaller current is induced by a voltage of one sign than by a voltage
of the other sign. If the current through a crystal is plotted as the
ordinate on a linear scale, and the impressed voltage is plotted as the
abscissa, a curve of the type shown in Fig. 1.8 is obtained. This plot will
be seen to show considerable curvature or nonlinearity in the region of
the origin, and it is upon this curvature that the action as a low-level
detector depends. If an alternating voltage, such as is shown on the
negative current axis, is impressed across this crystal, the current that is
passed through the unit has the form indicated in the plot on the right-
hand voltage axis. Because there is less current flowing during the
negative half-cycles than during the positive ones, there is a net positive