Boonton Power Sensor - Manual (Page 46)

If the measurement frequency is identical to the AutoCal frequency, a calfactor uncertainty of
zero should be used, since any absolute error in the calfactor cancels out during AutoCal. At
frequencies that are close to the AutoCal frequency, the calfactor uncertainty is only partially
cancelled out during AutoCal, so it is generally acceptable to take the uncertainty for the next
closest frequency, and scale it down.
6-4 Sample Uncertainty Calculations
The following examples show calculations for two measurement applications - one using a CW
sensor (Model 51075), and the other with a peak power sensor (Model 57518). The figures
used in these examples are meant to show the general techniques, and do not apply to all
applications. Some "common sense" assumptions have been made to illustrate the fact that
uncertainty calculation is not an exact science, and requires some understanding of your
specific measurement conditions.
Typical Example #1: Model 51075 CW Power Sensor
Measurement conditions:
Source Frequency: 10.3 GHz
Source Power: -55 dBm (3.16 nW)
Source SWR : 1.50 (reflection coefficient = 0.2) at 10.3 GHz
AutoCal Source: Internal 50MHz Calibrator
AutoCal Temperature: 25 °C
Current Temperature: 25 °C
In this example, we will assume that an AutoCal has been performed on the sensor
immediately before the measurement. This will reduce certain uncertainty terms, as discussed
below.
Step 1: The Instrument Uncertainty figure for the 4530 Series is ±0.20%. Since a portion of
this figure is meant to include temperature drift of the instrument, and we know an AutoCal has
just been performed, we'll estimate (for lack of more detailed, published information) that the
instrument uncertainty is ± 0.10 %, or half the published figure.
U
Instrument
= ± 0.10%
Step 2: The Calibrator Level Uncertainty for the power meter's internal, 50MHz calibrator
may be read from the calibrator's specification. It is ±0.105dB, or ±2.45% at a level of -55
dBm.
U
CalLevel
= ± 2.45%
Power Sensor Manual 39 41