MM0474
0.5 volt will equal full scale deflection. This loading of the RECORDER output
has no effect on the accuracy of the 432A panel meter.
(6) A digital voltmeter can be connected to the rear panel RECORDER
output for more resolution of power meter readings.
When a voltmeter with
input impedance greater than 1 megohm is connected to the RECORDER output, 1
volt equals full scale deflection.
(7) The 432A has two calibration jacks (VRF and VCOMP) on the rear panel
that can be used for precision power measurements.
Instrument error can be
reduced from 1% (0.2% of reading + 5 W) of reading, depending on the care
taken in measurement and on the accuracy of auxiliary equipment.
e. Microwave Power Measurement Accuracy.
(1) A number of factors affect the overall accuracy of power
measurement.
The major sources of error are mismatch error, RF losses, and
instrumentation error.
(2) Mismatch Error.
In a practical measurement situation, both the
source and thermistor mount have SWR, and the source is seldom matched to the
thermistor mount unless a tuner is used. The amount of mismatch loss in any
measurement depends on the total SWR present. The impedance that the source
sees is determined by the actual thermistor mount impedance, the electrical
length of the line, and the characteristic impedance of the line, Zo.
(3) In general, neither the source nor the thermistor mount has Zo
impedance, and the actual impedances are known only as reflection coefficients,
mismatch losses, or SWR.
The power delivered to the thermistor mount, and
hence the mismatch loss, can only be described as being somewhere between two
limits.
The uncertainty of power measurement due so mismatch loss increases
with SWR. Limits of mismatch loss are generally determined by means of a chart
such as the Mismatch Loss Limits charts in Figures 6 and 7. The total mismatch
loss uncertainty in power measurement is determined by algebraically adding the
thermistor mount losses to the uncertainty caused by source and thermistor
mount Zo match.
(4) RF Losses. RF losses account for the power entering the thermistor
mount but not dissipated in the detection thermistor element. Such losses may
be in the walls of a waveguide mount, the center conductor of a coaxial mount,
capacitor dielectric, poor connections within the mount, or due to radiation.
(5) Instrumentation Error. The degree of inability of the instrument to
measure the substitution power supplied to the thermistor mount is called power
meter accuracy or instrumentation error.
Instrumentation error of the Model
432A is 1% of full scale, 0C to +55C.
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