HP 8481A Operating And Service Manual page 17

HP 8481A/82 A/83 A
08481-90049
Service
Service instructions consist of principles of operation, troubleshooting,
and repairs. Test equipment which meets or exceeds the critical
specifications in table 3 may be used in place of the recommended
instruments for troubleshooting the Power Sensor.
Note
Check your warranty. Opening the Power Sensor will void warranty.
Principles
Of
Operation
For the following discussion, refer to figure 4 Schematic Diagram and
figure 5 Operational Amplifier. The operational amplifier is made up of
the Power Sensor input amplifier, A2Q1, and the first amplifier stage in
the power meter.
For additional information on thermocouple sensors, refer to HP
Application Note 64-1, "Fundamentals of RF and Microwave Power
Measurements".
The A1 Bulkhead Assembly provides a 50 ohm load (HP 8481A and
8482A) or a 75 ohm load (HP 8483A) to the RF signal applied at the
RF INPUT. The rf signal is coupled through a dc blocking capacitor
and absorbed by the thermocouples which generate a dc voltage
proportional to the rf input power. The dc voltage is routed from the
thermocouples to the input amplifier on gold wires to reduce undesired
thermocouple effects. The gold wires pass through ferrite beads A2E1
and A2E2 which are located in the black plastic block. (See figure 2.)
The ferrite beads increase the self inductance of the gold wires causing
this portion of the wires to provide the properties of an rf choke.
The result is to minimize rf feedthrough to the A2 Input Amplifier
Assembly.
The dc output from the bulkhead assembly is applied to the two field
effect transistors (FETs) in A2U1. These transistors function as a
sampling gate or chopper. The sampling rate is controlled by a 220
Hz square wave supplied by the power meter. The amplitude of the
sampling gate output (at pin 3 of A2TJ1) is a 220 Hz square wave
proportional to the power input. The sampled 220 Hz ac output is
applied to the input amplifier A2Q1 which is the input stage for an
operational amplifier (figure 5). The ac gain of the operational amplifier
is approximately 700.
A dc feedback voltage from the power meter Auto Zero circuit is
coupled to the input of FET A2U1Q1 to set the zero level. The voltage
is developed across the voltage divider consisting of A2R1 and the series
resistance of the thermocouple A1TC1.
When the Power Sensor is used with an HP Power Meter, the short
to ground at Jl-K (Mount Resistor) causes the power meter to
automatically select the proper measurement range of -30 to +20 dBm.
With the HP 435 series Power Meter this short serves no function.
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