MM0474
(3) Bolometers are temperature sensitive resistors.
Any component
having a large temperature coefficient of resistance can be used as a
bolometer. However, certain elements have more desirable features than others
in power measuring amplifications.
A bolometer for RF power measurements
should have, in addition to a high temperature coefficient of resistance, a
resistance of around 200 ohms and a physical size which is small compared to
the wavelengths, also a diameter less than the skin depth of the measured
signal. When a bolometer is used to measure microwave power the temperature
rise due to the power dissipated in the element changes the resistance of the
element. This resistance change can either be measured and calibrated in terms
of power, or else an equal resistance change can be effected by DC or audio
frequency power.
The latter power is then measured by conventional low
frequency methods, and its value is equal to the RF power. Bolometer devices
include beads of semi-conducting material (thermistors), thin platinum wires
(barretters) and metallic or carbon films deposited on glass or mica.
A
description of the characteristics of these elements, and the manner in which
they are used is presented in later lessons.
(4) In the calorimetric technique, power is dissipated on a load and the
temperature rise of the load, or of a cooling fluid flowing in or around the
load, is measured.
This temperature rise can be used to compute the power
dissipation in the load to a fair degree of accuracy if sufficient information
concerning the heat capacity, heat losses, and other factors are available.
However, generally it is easier to calibrate a calorimeter with DC or low
frequency power. The temperature rise of the calorimeter can be measured to a
high degree of accuracy with thermocouples or bolometers. Therefore, it is not
unusual to find calorimetric power meters that incorporate many of the features
of the other types of microwave power meters.
(5) The devices previously described all measured power in terms of
thermal response.
Silenium or germanium crystal rectifiers may be used at
microwave frequencies to rectify the RF signal and produce a low frequency or
DC output.
These microwave crystals may be calibrated with low frequency
meters to measure RF power, but as crystals are quite non-linear and extremely
sensitive to temperature, humidity, age, and previous operating history
indicates that they are not too reliable and frequently require recalibration.
However, crystals are useful when only nominal accuracy is needed or when an
indication of relative power level rather than an absolute power measurement is
desired, for they are rugged and involve a minimum of auxiliary equipment.
c. Methods of Measurement.
(1) In measuring the change in resistance of a bolometric device it is
generally practical to place the bolometer in one arm of a Wheatstone bridge.
A DC or low frequency bias supply is adjusted to charge the low frequency power
in the bolometer. By this means the resistance of the bolometer is varied and
the bridge is balanced until
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