MM0474
k. Thermoelectric Effect Error.
The 432 Power Meter is not affected by thermoelectric effect error.
However, the 431 Power Meter, which applies AC bias to the thermistors, is
significantly affected: the magnitude of this error can be as high as 0.3 W
(0.1 W typical).
Thermoelectric effect error is important on the most
sensitive ranges (.01, .03 and 0.1 mW) and when making DC substitution
measurements. Refer to the 431 Power Meter manual for a procedure to measure
the error and correct for it.
(1) The Thermistors.
The thermistors have a negative temperature coefficient; that is,
when a thermistor's temperature increases, its resistance decreases. The 486A
uses two thermistors: a detection thermistor that samples RF power and a
compensation thermistor that compensates for ambient temperature changes. When
the 486A is attached to a 432 Power Meter the detection thermistor (labeled D
in Figure 34) is part of the RF bridge in the power meter; the compensation
thermistor (labeled C in Figure 34) is part of the compensation bridge, and the
bridges supply sufficient DC bias to drive the thermistors to their operating
resistances (100 or 200 ohms).
(2) Detection Thermistor.
The detection thermistor is mounted within the mount's shorted
waveguide section and is located so that it samples the E field and provides an
optimum match across the band. As it absorbs RF power, its temperature (and
therefore its resistance) attempts to change. However, the RF bridge circuit
is self-balancing and reduces thermistor bias current to maintain the
thermistor's proper operating resistance. Since the amount of bias change is
proportional to incoming RF power, it is measured and used to in indicate the
RF Power absorbed by the mount.
(3) Compensation Thermistor.
To prevent ambient temperature changes from affecting power
readings, a compensation thermistor is mounted so that it shares the detection
thermistor's thermal environment. Any change in ambient temperature causes a
bias change in both thermistor bridges. This means that the total bias change
in the detection thermistor is caused by incoming RF power plus ambient
temperature. The power meter subtracts compensation bias from detection bias
to get stable, error-free power readings.
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