MM0474
specifically designed to measure low levels of power. High-power measurements
are made, both with special high-power measuring devices and with calibrated
attenuators designed to reduce the power by a known amount, to a level that can
be measured by low level type devices.
Medium-power levels are usually
measured by low power meters used in conjunction with calibrated attenuators.
(2) Most power-measuring devices respond only to average power, that is,
if the power source consists of some form of low frequency amplitude modulation
of an RF signal, the power meter will read the average of the power over a
number of cycles. In many applications, microwave power flows as a constant-
amplitude (CW) wave which is amplitude modulated by a square wave or sine wave,
or pulse modulated by pulses whose pulse repetition frequencies are in the
audio range.
Illustrated in Figure 1 are some typical microwave power
In all these cases the power meters commonly used to measure
microwave power read the average power over several cycles of pulses of the
modulation frequency. For pulsed power it is frequently necessary to know the
peak power. This can be found from the average power by the formula:
where:
Po = peak pulse power in watts
P = average power in watts
T = pulse length in seconds
fr = pulse repetition frequency in pulses/sec.
The quantity Tfr describes the nature of the pulses power.
It is equal to
P/Po and is called the "duty cycle" of the pulsed wave.
b. Nature of power-sensitive elements.
(1) The response of a device to the temperature rise caused by the
dissipation of electrical power is the fundamental basis of microwave power
measurements.
Thermal devices for power measurements include three general
types, thermocouples, bolometers, and calorimeters.
(2) A thermal junction occurs at the point where two dissimilar metal
wires are connected in series.
A temperature difference between junctions
produces an emf, that is proportional to the difference, which can be metered
by a DC instrument. A thermocouple may be used to measure the temperature rise
of a load which dissipates microwave power. Appropriate calibration converts
this temperature rise into an indication of microwave power.
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