Service
Chapter 3 37
Diode Sensors
The A1 Bulkhead Assembly presents a 50 Ohm load to the RF source.
A diode assembly in the bulkhead rectifies the applied RF to produce a dc voltage
that varies with the square of the RF power across the 50 Ohm load. Thus the
voltage varies with the RF power dissipated in the load.
This low level DC voltage is passed on gold wires through ferrite beads A2E1 and
A2E2. 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 Power Sensor Board 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 A2U1) 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.
The A2 Power Sensor Board Assembly also contains various components that
comprise a shaping network. This network brings about a linear change in the
amplitude of the square wave output as RF input power changes. A shaping network
is necessary in diode power sensors to compensate for the characteristics of the
thermocouple type shaping in the power meter, and to make minor diode
corrections. In order to bring about a linear change in amplitude, the components in
this network are factory selected to match variations of the diode assembly in the
bulkhead.
The Auto Zero Feedback circuit is coupled to the power sensor from the power
meter. The dc voltage used to set the zero level is applied to the input of FET A2U1
by using A2R1 and A1TC1 as a voltage divider.
When the Power Sensor is used with a Power Meter, the resistance to ground from
J1-K (Mount Resistor) allows the Power Meter to determine the sensor's dynamic
range.
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