Boonton 4240 RF Power Meter - Manual (Page 119)

Boonton 4240 Series RF Power Meter
Step 1 certainty terms has been determined, we can combine them to calculate the worst-
case a
U (±%) K (U×K) 2 ( % 2 )
1. instrument uncertainty 0.115 0.500 0.0033
2. calibrator level uncertainty 2.45 0.577 1.9984
ismatch uncertainty 0.34 0.707 0.0578
ismatch uncertainty 6.68 0.707 22.305
5. sensor shaping error uncertainty 1.00 0.577 0.3333
6. sensor temperature drift uncertainty 0.00 0.577 0.0000
7. sensor noise uncertainty 0.95 0.500 0.2256
8. sensor zero drift uncertainty 1.58 0.577 0.8311
9. sensor calibration factor uncertainty 4.09 0.500 4.1820
___________________________
Total worst case uncertainty: ±17.21%
Total sum of squares: 29.937 % 2
(RSS) : ±5.47 %
rom this example, it can be seen that the two largest contributions to total uncertainty are the source mismatch, and the
nsor calfactor. Also note that the expanded uncertainty is approximately one-half the value of the worst-case uncertainty.
This is not surprising, since the majority of the uncertainty comes from just two sources. If the measurement frequency was
lower, th
wo terms would e worst-case.
Conversely, if one term nd the noise
uncertainty was 30%), the expanded uncertainty value would be expected to approach the worst-case value. The expanded
uncertainty is 0.45 dB.
It should be noted that measurement uncertainty calculation is a very complex process, and the techniques shown here are
somewhat simplified to allow easier calculation. For a more complete information, the following publications may be
consulted:
1. “ISO Guide to the Expression of Uncertainty in Measurement” (1995)
International Organization for Standardization, Geneva, Switzerland
ISBN 92-67- 188-9
2. “U.S. Guide
National Con , Boulder, CO 80301
ANSI/NCSL Z540-2-1996
0: Now that each of the individual un
nd RSS uncertainty values:
3. calibrator m
4. source
m
Combined Standard uncertainty U
C
Expanded Uncertainty U (coverage factor k = 2) : ±10.94 %
F
se
ese t be reduced, and the expanded uncertainty would probably be less than half th
dominated (for example if a very low level measurement was being performed, a
10
to the Expression of Uncertainty in Measurement” (1996)
ference of Standards Laboratories
Application Notes
6-11