Calibration & Testing
Publication 21555, Rev. E, September 2002 4-7
4.3.3 Calibrator Voltages
To measure the calibrator voltages, first make sure that neither side of the DVM is grounded. The
following measurements should find most of the problems that can occur in the calibrator circuitry.
1. Connect the DVM across the large resistor, A1R174. Measure 0.4 to 0.9volts depending on the
room temperature and how long the unit has been operating. This voltage is proportional to the
current in the thermistor heater transistor which maintains the calibrator thermistor in a 60°C
(140°F) environment. The voltage measured in the next step is dependent on this being correct.
2. Connect the low side of the DVM to A1TP3 and the high side to A1TP1. Measure +7 to
+8.5volts. This is the voltage applied to the thermistor bridge that is used to measure the calibrator
power. This voltage will vary as the calibrator provides different amounts of RF power. This
measurement assumes that the calibrator is OFF. To verify that the calibrator is off, send
OUTP:ROSC OFF.
3. Turn the calibrator ON by sending
OUTP:ROSC ON. Now connect the high side of the DVM to
A1U3, pin 7. Measure +2 to +11volts, which should change less than 2mV per minute. If the
voltage is incorrect or drifts excessively, troubleshoot the sample and hold circuit surrounding
A1U3A.
4.3.4 Calibrator Frequency Check
To measure the frequency of the calibrator:
1. Connect a 50MHz counter to the calibrator output connector.
2. Turn ON the calibrator according to the procedure given in Step 3, above.
3. Measure 49 to 51MHz.
4. Turn OFF the calibrator by sending
OUTP:ROSC OFF.
NOTE: The exact ambient temperature and power-on time of the instrument in Step 1
are not specific factors, but do have some effect on the reading taken across A1R174. If
there is a problem in the circuit, the measured voltage will usually be some amount
outside of the 0.4 to 0.9volts specification (such as 0, +4, or +5volts).
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