Service
Chapter 3 45
FET Balance Adjustment
The FET balance adjustment should be performed if the wires connecting J1 (the
sensor cable connector) to A2 (the power sensor board assembly) have been moved.
If you have replaced A2 assembly or moved the wires during troubleshooting you
need to perform this adjustment.
NOTE You do not need to perform a FET balance adjustment after an A1 bulkhead
assembly replacement, if the wires between J1 and A2 have not been disturbed.
Equipment Required
• Oscilloscope
• BNC cables (2 required)
• Power Meter (Modified as described in Appendix A)
Test Description
This test applies to both thermocouple and diode power sensors. Among the
required equipment is a modified E4418B Power Meter. The High Gain output of
the power meter is connected to Channel 1 of an oscilloscope, and the Chop Output
(220Hz square wave) of the power meter is connected to the oscilloscope trigger -
the High Gain output is the amplified version of the Power Sensor chopped signal.
Ideally, when no RF power is applied to the power sensor, the High Gain output
signal displayed on the oscilloscope is a straight line. If there is a sensor offset, the
offset signal is visible on the oscilloscope as a square wave (chopped signal). Also, a
switching transient (spike) usually occurs at the edge of the chopped signal, due to
the switching of the FET in the Power Sensor. Sensor offset and spike balance are
affected by the relative positions of the wires connected to pins G and H of
connector J1. One wire is black and white; the other is brown and white. Moving the
black and white wire adjusts the amplitude of the switching transient (spike).
Moving the brown and white wire changes the offset. Once positioned, care must be
taken not to displace these wires. To correctly position these wires, perform the
“FET Balance Procedure” on page 46.
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