Giga-tronics 8650A Series, Руководство по эксплуатации

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Series 8650A Universal Power Meters
2-40 Manual 31470, Rev. E, April 2001
2.3.20 Improving Accuracy
Mismatch uncertainty is the largest source of error in power measurement. The 6 dB pad that is used in
the attenuator calibration procedure above reduces mismatch uncertainty by effectively improving the
return loss (or reducing the SWR) of the source. Mismatch uncertainty is large when a device has a
poor impedance match relative to 50
Ω
.
Poorly matched devices reflect a large proportion of incident signals and create standing waves along
the transmission line. At various points along the transmission line, the standing wave will be at
maximum or minimum amplitude. Mismatch uncertainty is a measure of the deviation between these
amplitude levels.
Inserting an attenuator into the transmission line reduces mismatch uncertainty by reducing the
amplitude of the reflected signal, thereby reducing the difference between a standing wave’s maximum
and minimum levels.
Compared to an attenuator, most microwave sources have poor impedance matching. Using the 6 dB
attenuator during the calibration has the effect of lowering the SWR of the microwave source. The only
compromise is a corresponding 6 dB reduction in the source’s dynamic range when the 6 dB attenuator
is attached.