HP 8921A - TX Pwr Zero

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Chapter 4, Adjacent Channel Power Screen
Field Descriptions
TX Pwr Zero
The transmitter power zero function establishes a 0.0000 W reference for
measuring RF power at the RF IN/OUT port.
CAUTION: RF power must not be applied while zeroing. Set the RF Generator screen’s Amplitude
field to off to prevent internal cross-coupling into the power detector while zeroing.
Operating Considerations
When power is applied to the RF IN/OUT connector, the temperature of the
internal circuitry increases. This can cause changes in the TX Power measurement
when low power levels are measured immediately following high power
measurements.
When alternately making high and low power measurements, always zero the
power meter immediately before making the low power measurements; this
provides the best measurement accuracy.