Keysight Technologies 8480 Series, Operating And Service Manual

Page: 65 / 99

Service 3
Keysight 8480 Series Operating and Service Manual 65
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.
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 66.
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.