MM0474
(1) RF Power Turned Off. There is minimum zero drift when zero is set
with the RF system connected to the thermistor mount and RF power turned off at
the generator or shorted out by a shorting switch. After allowing time for the
mount to stabilize thermally, follow the steps for zero-set described in the
power meter manual, and then turn on the RF power for measurement.
(2) Use of High Attenuation with RF Source On. When it is inconvenient
to turn off the RF source for zero-set, connect a waveguide variable
attenuator, such as the Model 382A, between the RF system and the thermistor
mount.
Attenuate the RF power at least 30 dB for zero-set, and reduce
attenuation to zero during the measurement.
(3) Disconnecting the Mount.
When it is inconvenient to turn off or
attenuate RF power, simply remove the mount from the source and, using COARSE
and FINE ZERO, zero the power meter.
h. Power Measurement.
(1) The thermistor has a long thermal time constant, which causes it to
respond to average microwave power whether CW or modulated (pulse, sine wave,
or square wave).
(2) In pulse modulation, response is proportional to the amplitude and
the duty cycle of the pulse. The power level of an individual pulse can be
determined by dividing the average power reading by the duty cycle of the
pulse. Accurate measurements can be made with pulse repetition rates as low as
50 Hz.
(3) To measure microwave power in excess of 10 mW insert a calibrated
attenuator, such as one of the 382A, or 375A series, or a directional coupler,
such as one of the 752 series, between the mount and the RF source to be
measured.
CAUTION
Do not exceed maximum
power
rating
of
attenuator
or
directional coupler.
i. Mount Calibration Data.
(1) The calibration points imprinted on the label of each 486A allows
power measurements to be made with increased accuracy. Values of Calibration
Factor and Effective Efficiency are given at specific frequencies and the
mounts are tested on a swept-frequency basis to assure accurate interpolation
between calibration points. Calibration Factor and Effective Efficiency values
are traceable to the National Bureau of Standards to the extent allowed by the
Bureau's calibration facilities.
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