no current flows in the detector arm of the bridge. If RF power is now applied
The bridge becomes unbalanced and
current flows in the detector arm of the bridge. The unbalanced current can be
calibrated against low frequency power so that a meter placed in the detector
arm of the bridge can have its scale marked to read microwave power directly.
(2) A more precise method is one in which the bridge is always
maintained at balance when measurements are made.
The bridge is initially
balanced with low frequency bias power only, and then the bolometer is exposed
to the RF power so that the bridge becomes unbalanced. Sufficient bias power
is then removed to rebalance the bridge, and the low frequency power is
measured and equated to the incident RF power. The principle advantage of this
technique is that it provides a continuous calibration of the bolometer against
low frequency power at each measurement. It is not as direct as the unbalanced
bridge technique described above but is the more reliable and precise of the
two methods because of the self-calibrating feature and because the bolometer
is always operated at a constant resistance so that the resistance power curve
of the element does not enter into the power determination.
(3) There is a wide variety of commercial bridges available for
microwave bolometric power measurements.
These units all operate as either
balanced or unbalanced bridges although they may have many variations and
refinements. Some of these are explained in detail in the following lessons.
One important variant of the balanced bridge circuit that bears mention at this
time is the self-balancing bridge. This instrument contains a feedback network
between the detector and the source of bias power.
After the initial
calibration adjustment, this type of bridge automatically rebalances itself.
This device has the accuracy of the balanced bridge and the ease of operation
of the unbalanced bridge incorporated into a direct reading power meter.
generally made to permit calibration of the instrument with a DC or 60 Hertz
source and meter.
Depending upon the design of the device, the calibrating
signal is either applied directly to the microwave absorbing element itself, or
an auxiliary meter located near the thermocouple junction.
rise of the load is plotted against the known low frequency calibrating power.
This curve is then used under RF operating conditions to translate temperature
rise into microwave power.
d. Special Equipment for High Power Measurement.
(1) Thermistors, crystals and barretters have relatively low power
handling capacities and are generally suitable only for low power level
However, many attractive features of the low power instruments make it
employed so that the low power devices can be used for high power measurements.