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3
Service
Service
Service instructions consist of principles of operation, troubleshooting, and
repairs.
Principles of operation
The power sensor 'bulkhead' assembly converts input RF to a low frequency
voltage signal representing the RF power envelope. The input is AC coupled to a
3 dB attenuator followed by a 50 ohm load resistor. Two diodes are connected to
the load resistor, forming a pair of half-wave detectors with opposite polarity and
complementary voltage output. The detected signal passes through a low-pass
load filter. The cutoff frequency of the filter is 300 kHz, 1.5 MHz, or 5 MHz,
depending on the model/ bandwidth specification of the sensor.
The detected signal can now follow one of two paths. The average-only signal
path is optimized for high sensitivity and low drift at the expense of detector video
bandwidth. This path chops the signal to a carrier frequency around 440 Hz to
remove sensitivity to DC offsets, then amplifies the AC signal. Amplification and
chopping parameters are much the same as in previous Keysight diode sensors,
with typical dynamic power range of –65 to +20 dBm.
The chopper is a switch that connects the two balanced signals to the two inputs
of a differential amplifier. Thus, the small DC signal from the detector is converted
to an AC signal. The output of the differential amplifier is connected to a switched
gain preamplifier.
The dynamic range of the sensor is greater than 80 dB in this mode, so the sensor
has two power ranges. On the high power range the signal is attenuated before
further amplification. The bandwidth of the chopped signal is limited to less than
half the chop rate. So, this method cannot be used for wide (~5 MHz) bandwidth
measurements.
The normal path is used to detect the instantaneous power of an RF signal and is
optimized for a bandwidth of up to 5 MHz. The peak path trade off includes
reduced dynamic range and increased temperature sensitivity.
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Keysight E9320 Operating and Service Guide
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