3-21
loop 2. Both bridges are excited by a common 10-kHz source. The 10-kHz amplifier-oscillator supplies
10-kHz power to bias the thermistor in feedback loop 1 to produce the resistance required to balance the
rf bridge. An equal amount of 10-kHz power is supplied by the same oscillator to the second thermistor in
feedback loop 2 through two series-connected transformers. Feedback loop 2 balances the meter bridge.
When rf is applied to the thermistor in the detection bridge (but not to the compensation and metering
bridge), an amount of 10-kHz power is present, equal to the rf power being removed from the detection
bridge by the self-balancing action of the bridge. Since the rf power replaced the 10-kHz power, the
detection bridge is in balance; however, the metering bridge must be balanced by its separate feedback
loop. Sufficient dc power to equal the 10-kHz power lost by the metering bridge is automatically
replaced, balancing this loop. Hence the dc power applied to the metering bridge thermistor is equal to the
microwave power applied to the detection bridge. The meter circuit senses the magnitude of the feedback
current. The resultant meter current passes through a differential amplifier to the indicating meter. The
two thermistors are matched with respect to their temperature characteristics; therefore, there is only a
very small amount of drift of the zero point with ambient temperature changes. When there is a change in
temperature, there is a change in the electrical power needed by the thermistors to maintain constant
operating resistances. This change is automatically performed by feedback loop 1, which changes the
amount of 10-kHz power for both thermistors by the proper amount. The dc power in feedback loop 2 is
not changed; and since it is this dc power that is metered, the temperature change has not affected the
meter indication.
CALORIMETERS
The calorimeters are the most accurate of all instruments for measuring high power. Calorimeters
depend on the complete conversion of the input electromagnetic energy into heat. Direct heating requires
the measurement of the heating effect on the medium, or load, terminating the line. Indirect heating
requires the measurement of the heating effect on a medium or body other than the original power-
absorbing material. Power measurement with true calorimeter methods is based solely on temperature,
mass, and time. Substitution methods use a known, low-frequency power to produce the same physical
effect as an unknown rf power being measured. Calorimeters are classified as STATIC (nonflow) types
and CIRCULATING (flow) types.
Q-15.
Power measurements performed with calorimeters are based on what three variables?
Static Calorimeters
The static calorimeter uses a thermally shielded body. Since an isolated body loses little heat to a
surrounding medium, the temperature increase of the body is in direct proportion to the time of applied
power. The product of the rate of temperature rise in the calorimetric body and its heat capacity equals
applied power. Figure 3-18 illustrates a static-type calorimeter.
