Giga-tronics 8501A, Operation & Maintenance Manual

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D.4 Detector Maintenance
There is no regular maintenance required for the detectors. It might become necessary to open the
detector mount to correct a fault either in the diode element or in the amplifier/PROM circuit, or to
replace the PROM.
D.5 Detector Troubleshooting
The most common cause of failure is the application of too much power, which may destroy the diode
element. This will most likely result in the detector failing to autocalibrate. Since there are several faults
that can cause the calibration routine of the PPM to fail, it is recommended that you have an
understanding of the calibration process, described in Chapter 2 of the Series 8500A manual.
Understanding the calibration process allows you to narrow the number of fault possibilities if the history
of the detector is unknown. If the detector has been connected to a source of power in excess of +23 dBm
(200 mW), an autocalibration should be performed before further use. If the detector fails this process, it
should be removed from service until a new diode element has been installed and the unit recalibrated.
If a component in the preamplifier/line driver fails, the detector will also fail autocalibration. To verify
that the problem is in the amplifier and not the diodes, a measurement of the voltage can be performed by
first removing the outer cover from the detector (see Disassembly of the Detectors in Section D.5.1).
Refer to the circuit description and Figure D-2 in order to proceed with troubleshooting. It is best to
connect the detector to a source of RF power at a level of about 0 dBm to trace the signal level through
the amplifier. The signal can be pulse modulated if tracing is done with an oscilloscope, or CW if tracing
is being done with a voltmeter. With no signal applied, the voltage at the output of the amplifier should
be 0 V
±
0.1 mV. The output voltage with 0 dBm applied will typically be about 0.5 V. This reading is
taken at the emitter of the output transistor, not at the output end of the 75
Ω
resistor that feeds the coax
cable.
The PROM and address counter circuit is very reliable, but in the event that it does fail, the PPM will be
unable to correctly read the PROM, and an error message to that effect will be displayed. The most likely
failure in that case would be a broken wire in the cable or in the connectors at each end of the cable. If a
PROM read failure occurs, connect the detector to a different channel or to a different PPM. If a failure
occurs again, it will be necessary to troubleshoot the PROM circuit. After verifying that there are no
broken wires in the detector, proceed to check the active components. Since the PROM is mounted in a
socket, the simplest check is to first try a different PROM to determine if the circuit is functioning. It is
necessary to use a programmed PROM to accomplish this check, such as one from working detector. Note
that this will produce incorrect RF power readings, but it is otherwise a valid test.
The other component that can be checked is the thermistor. The thermistor has two purposes. The PPM
checks the voltage at the thermistor to determine if the detector is present. If the voltage is above 10 V
the PPM makes the assumption that there is no detector present. If the voltage is in the normal working
range, the voltage level is converted to a temperature reading which is compared with the temperature of
the detector when it was last calibrated. If the difference in the two readings is more than 5 degrees
Celsius, a message will display showing the difference. Therefore if the thermistor is open, possibly due
to a broken wire, the PPM will not recognize that the detector is connected. If the thermistor has
somehow been shorted, perhaps due to twisted wires, then it will indicate a very large positive change in
the temperature reading from the reading taken during calibration. It will also indicate an impossibly
large temperature on the detector. The resistance of the thermistor should normally measure 10K at 25 °C.
RF Detectors
Manual No. 20790, Rev C, November 1998
D-7