Advertisement
Model 8902A
Adjustments
Adjustment
1
9-Power
Reference (Using DC Substitution)
Reference
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 n o 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
Ajkr adjirslmcnr, thc? accwracy
oj'
fhc Mcasirring Rcwivcr's powcr rcjiwncc
will bc
M.
7%. A simplcr,
hiif
kss accirratt' mcvhodjor adjiisling [he powcr rtfi
erenct' can he joiind
in
Adjustment 18-Power Reference (Using Reference
Comparison).
Equipment
Digital Voltmeter
................................................................
HP 3455A
Power Meter..
...................................................................
H P 432A
Thermistor Mount
........................................................
H P 478A Opt H75
Procedure
1. On the Measuring Receiver, connect the CALIBRATION RF
POWER
OUTPUT to the IN-
PUT. (If a sensor module is being used, connect its input to the CALIBRATION R F POWER
OUTPUT.)
2. On the Measuring Receiver, 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 R F power
calibrator. Press TRACK MODE. The Measuring Receiver'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 A32L I
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.)
5-3
1
Advertisement
Chapters
