Keysight 8478B, Руководство по эксплуатации и обслуживанию

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Keysight 8478B Thermistor Mount 23
Operation
Thermoelectric Effect
Thermoelectric Effect
When using 431 Power Meters, thermoelectric errors must be taken into
consideration. Mount calibration uncertainties given in Table 2-1 on page 15
include inaccuracies caused by thermoelectric effect error. Calibration Factor
uncertainty of ±1.5% and Effective Efficiency uncertainty of ±2.5% can be
maintained on the three lowest power ranges of the Model 431 series Power
Meters by correcting for the measurement error introduced by thermoelectric
effect. An error correction procedure is given in “Thermoelectric Effect Error
Correction for 8478B/431 Combination” .
A mild thermocouple exists at each point of contact where the connecting
wires join to the thermistor elements. Each thermocouple creates a dc voltage.
Thus, two thermocouple voltages of opposite relative polarity are formed, one
at each junction to each thermistor element.
Ideally, each thermocouple voltage would be equal in magnitude so that they
cancel with no resultant effect on the accuracy of power measurement. In
practice, however, each point of contact does not have identical thermocouple
characteristics, and in addition, the temperatures at each junction may not be
the same. These differences cause an incomplete cancellation of the
thermoelectric voltages, resulting in a voltage that causes a thermoelectric
effect error. The magnitude of the error is important when making dc
substitution measurements on the 0.1mW, 0.03mW, and 0.01mW ranges with
one of the Model 431 series Power Meters. On other ranges, the effect is
negligible. Maximum error introduced by thermoelectric effect is about 0.3µW
and is typically 0.1µW on the 0.01mW range. Refer to “Specifications” on
page 15 .
Thermoelectric Effect Error Correction for 8478B/431 Combination
Use the following technique to correct for thermoelectric effect error.
1. Measure power.
2. Connect a HP Model 8402B Power Meter Calibrator to the power meter DC
CALIBRATION AND SUBSTITUTION connector.
3. Zero and null power meter.
4. By dc substitution (refer to procedure in 431 Manual), duplicate power
measurement made in step 1. Calculate and record substituted power as P
1.
5. Reverse connection polarity between the calibrator and power meter.
6. Re-zero and re-null power meter, if necessary.
7. By dc substitution, duplicate lower measurement made in step 1. Calculate
and record substituted power as P2.