Boonton Power Sensor - Manual (Page 40)

6
Calculating Measurement Uncertainty
6-1 Introduction
This Section has been extracted from the 4530 manual since it provides examples using CW
and Peak Power sensors. As such, in calculating Power Measurement Uncertainty ,
specifications for the 4530 are used. If one of Boonton's other Power Meters are in use,
refer to its Instruction Manual for Instrument Uncertainty and Calibrator Uncertainty.
The 4530 Series includes a precision internal RF reference calibrator that is traceable to the
National Institute for Standards and Technology (NIST). When the instrument is maintained
according to the factory recommended one year calibration cycle, the calibrator enables you
to make highly precise measurements of CW and modulated signals. The error analyses in
this chapter assumes that the power meter is being maintained correctly and is within its valid
calibration period.
Measurement uncertainties are attributable to the instrument, calibrator, sensor, and impedance
mismatch between the sensor and the device under test (DUT). Individual independent
contributions from each of these sources are combined mathematically to quantify the upper
error bound and probable error. The probable error is obtained by combining the linear
(percent) sources on a root-sum-of-squares (RSS) basis.
Note that uncertainty figures for individual components may be provided given in either
percent or dB. The following formulas may be used to convert between the two units:
U
%
= (10(U
dB
/10) - 1) * 100 and U
dB
= 10 * Log10(1 + (U
%
/ 100))
Section 6-2 outlines all the parameters that contribute to the power measurement uncertainty
followed by a discussion on the method and calculations used to express the uncertainty.
Section 6-3 continues discussing each of the uncertainty terms in more detail while presenting
some of their values.
Section 6-4 provides Power Measurement Uncertainty calculation examples for both CW and
Peak Power sensors with complete Uncertainty Budgets.
References used in the Power Measurement Uncertainty analysis are:
1. “ISO Guide to the Expression of Uncertainty in Measurement,”
Organization for Standardization, Geneva, Switzerland,
ISBN 92-67-10188-9, 1995.
2. “U.S. Guide to the Expression of Uncertainty in Measurement",
National Conference of Standards Laboratories, Boulder, CO 80301, 1996.
ANSI/NCSL Z540-2-1996,
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