HP 8901B Operation And Calibration Manual page 366

Performance Tests
Model 8901B
Characteristic
POWER REFERENCE
Power Output
Power Output Accuracy
Performance Test 7
Performance Limits Conditions
1 mW
*l.2%
worst case; for
1
year;
0
to 55OC
POWER REFERENCE TEST (USING DC SUBSTITUTION)
Specification
Description
The power from the power reference oscillator is measured by a method known as dc substitution. In
this method a power-sensing thermistor is placed in one arm of a self-balancing, dc resistance bridge.
With no power applied, the dc voltage supplying the top of the bridge is automatically adjusted until
the thermistor heats to the temperature that produces the resistance required to bring the bridge
into balance. When
RF
power is applied to the thermistor, the thermistor heats up and momentarily
unbalances the bridge. The self-balancing circuits then readjust the supply voltage to the bridge to
re-balance it. A second bridge, which senses only the ambient temperature, is employed to compensate
for measurement
drift.
Careful measurement of the bridge dc voltages provides a way to determine the
amount of
RF
power applied
to
the thermistor to a higher degree of accuracy than can be obtained by
direct reading of the power meter.
NOTE
The power level can be measured to an accuracy of *1.2%
by
this procedure.
A simpler, but less accurate method for measuring the power reference can
be found in the preceeding Performance Test 6-Power Reference (Using
Reference Comparison).
Equipment
Digital Voltmeter
.................................................................
H P 3455A
Power Meter
......................................................................
H P 432A
Thermistor Mount
........................................................
H P 478A Opt
H 7 5
Procedure
1. With the HP 432A Power Meter switched off and the thermistor interconnect cable connected
to
it but not to the thermistor mount, measure the resistance between the center conductor of the
rear-panel VRF connector and pin 1 (the first pin to the right of the key slot) on the thermistor
end of the interconnect cable. Record the value of the mount resistance for future reference. (It
should be approximately 2000.)
Mount Resistance (R):
0
2.
Connect the equipment as shown in Figure 4-6. The input of the digital voltmeter should be
floating. The positive input of the voltmeter should be connected to the rear-panel VCOMP
4-26
Performance Test 7
reu.1 OJUL87