Boonton 4500B Peak Power Meter - Manual (Page 332)

Boonton 4500B RF Peak Power Analyzer
Application Notes 6-22
Step 9: The Sensor Calfactor Uncertainty needs to be interpolated from the uncertainty values in the Boonton Electronics
Power Sensor Manual. At 1 GHz, the sensor‘s calfactor uncertainty is 2.23%, and at 0.5GHz it is 1.99%. Note, however,
that we are performing our AutoCal at a frequency of 1GHz, which is very close to the measurement frequency. This means
that the calfactor uncertainty cancels to zero at 1GHz, as discussed in the previous section. We‘ll use linear interpolation
between 0.5GHz and 1GHz to estimate a value. 900MHz is only 20% (one fifth) of the way from 1GHz down to 500MHz,
so the uncertainty figure at 0.5GHz can be scaled by one fifth.
U
CalFactor
= 1.99 Ч (900 - 1000) / (500 - 1000)
= 1.99 Ч 0.2
= ±0.40%
Step 10: Now that each of the individual uncertainty terms has been determined, we can combine them to calculate the
worst-case and RSS uncertainty values:
U (±%) K (U×K) 2 ( % 2 )
1. instrument uncertainty 0.20 0.500 0.0025
2. calibrator level uncertainty 3.11 0.577 3.2201
3. calibrator mismatch uncertainty 1.27 0.707 0.8062
4. source mismatch uncertainty 0.80 0.707 0.3199
5. sensor shaping error uncertainty 2.00 0.577 1.3333
6. sensor temperature drift uncertainty 1.69 0.577 0.9509
7. sensor noise & drift uncertainty 0.00 0.500 0.0000
8. sensor calibration factor uncertainty 0.40 0.500 0.0400
___________________________
Total worst case uncertainty: ±18.43%
Total sum of squares: 6.6729 % 2
Combined Standard uncertainty U
C
(RSS) : ±2.58 %
Expanded Uncertainty U (coverage factor k = 2) : ±5.17 %
From this example, different error terms dominate. Since the measurement is close to the calibration frequency, and
matching is rather good, the shaping and level errors are the largest. Expanded uncertainty of 5.16% translates to an
uncertainty of about 0.22dB in the reading.
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 92-67-10188-9
2. ―U.S. Guide to the Expression of Uncertainty in Measurement‖ (1996)
National Conference of Standards Laboratories, Boulder, CO 80301
ANSI/NCSL Z540-2-1996