Boonton Power Sensor - Manual (Page 48)

Step 8: The Sensor Zero Drift calculation is very similar to the noise calculation. For sensor
zero drift, the datasheet specification for the 51075 sensor is 100pW, so we'll take the liberty
of cutting this in half to 50pW, since we just performed an AutoCal, and it's likely that the
sensor hasn't drifted much.
U
ZeroDrift
= ± Sensor Zero Drift (in watts) / Signal Power (in watts)
= ± 50.0e-12 / 3.16e-9 * 100 %
= ± 1.58%
Step 9: The Sensor Calfactor Uncertainty is calculated from the uncertainty values specified
in Section 3 of this manual. There is no entry for 10.3GHz, so we'll have to look at the two
closest entries. At 10GHz, the calfactor uncertainty is 4.0 % and at 11GHz it is 4.3 %.
A linear interpolation must be done to determine the Calfactor at 10.3 GHz. The uncertainty is
then;
U
CalFactor
= [ ( F - F
1
) * (( CF
2
- CF
1
) / ( F
2
- F
1
)) ] + CF
1
where; F = 10.3
F
1
= 10 CF
1
= 4.0
F
2
= 11 CF
2
= 4.3
= [ ( 10.3 - 10.0 ) * (( 4.3 - 4.0 ) / ( 11.0 - 10.0 )) ] + 4.0
= [ ( 0.3 ) * (( 0.3 ) / ( 1.0 )) ] + 4.0
= [ ( 0.3 ) * ( 0.3 ) ] + 4.0
= 4.09 %
Step 10: Now that each of the individual uncertainty terms has been determined, we can
create an uncertainty budget and calculate the combined standard uncertainty (Uc) .
Source of Symbol Value Probabilty Divisor Ustd
Uncertainty (+/- %) Distribution (+/- %)
Instrument I 0.10 normal 2 0.05
Calibrator
Level R 2.45 rectangular ( 3 )
0.5 1.41
Mismatch
M
C
0.34 U-shaped ( 2 )
0.5 0.24
Source
Mismatch
M
S
6.68 U-shaped ( 2 )
0.5 4.72
Sensor
Shaping S 1.00 rectangular ( 3 )
0.5 0.58
Temp. Drift T 0 rectangular ( 3 )
0.5 0.00
Noise N 0.95 normal 2 0.48
Zero drift Z 1.58 rectangular ( 3 )
0.5 0.91
Cal Factor K 4.09 normal 2 2.05
Combined Standard Uc normal 5.47
Uncertainty
Expanded U normal 10.94
Uncertainty (k=2)
Power Sensor Manual 41 43