Adjustment 19 - HP 8901B Operation And Calibration Manual

Adjustments Model 8901B
Adjustment 19
POWER REFERENCE (USING DC SUBSTITUTION)
Service Sheet 3
Description
The frequency of the internal power reference is checked and adjusted, if necessary, to
50 MHz. 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
After adjustment, the accuracy of the Modulation Analyzer's power refer-
ence will be &0.7%. A simpler, but less accurate method for adjusting the
power reference can be found in Adjustment 18
-
Power Reference (Using
Reference Comparison).
Equipment
Digital Voltmeter
.................................................................
HP 3455A
Power Meter
......................................................................
H P 432A
Thermistor Mount
........................................................
HP 478A
Opt
H75
Procedure
1. On the Modulation Analyzer, connect the CALIBRATION RF POWER OUTPUT
to
the INPUT.
(If a sensor module
is being used, connect its input
to
the CALIBRATION
RF
POWER
OUTPUT.)
2. On the Modulation Analyzer, press the blue key then press INSTR PRESET (the AUTOMATIC
OPERATION key) to preset the instrument. Key in 45.16 SPCL
to
turn on the
RF
power
calibrator. Press TRACK MODE. The Modulation Analyzer's display should read between 49.5
and
50.5
MHz.
3.
If
the frequency of the RF power calibrator is not within the proper limits, remove the thumb
screw retaining the A32 Power Reference Oscillator and slide the assembly out. Adjust A32L1 for
a frequency reading between 49.5 and
50.5
MHz. (The adjustment is accessed through a hole on
the back side of the assembly.)
After removing the RF power calibrator assembly, take care that the power
supply terminals do not short against the metal chassis parts.
5-30 Adjustment 19