Boonton Power Sensor Instrukcja Użytkownika Pobierz pdf (Strona 51)

Step 9: The Sensor Calfactor Uncertainty needs to be interpolated from the uncertainty

values given in Table 3-2 (Peak Power Sensor Calibration Factor Uncertainty). At 1 GHz, the

sensor’s calfactor uncertainty is 1.7 %, and at 0.5 GHz it is 1.6 %. Note, however, that

we are performing our AutoCal at a frequency of 1 GHz, which is very close to the

measurement frequency. This means that the calfactor uncertainty cancels to zero at 1 GHz.

We’ll use linear interpolation between 0.5 GHz and 1 GHz to estimate a value. 900 MHz

is only 20% (one fifth) of the way from 1GHz down to 500MHz, so the uncertainty figure at

0.5 GHz can be scaled by one fifth.

CalFacto

= [ ( F - F

) * (( CF

- CF

) / ( F

- F

)) ] + CF

where; F = 0.9

= 0.5 CF

= 1.6

= 1.0 CF

= 0.0

= [ ( 00.9 - 00.5 ) * (( 0.0 - 1.6 ) / ( 1.0 - 0.5 )) ] + 1.6

= [ ( 0.4 ) * (( -1.6 ) / ( 0.5 )) ] + 1.6

= [ ( 0.4 ) * ( -1.6 ) ] + 1.6

= 0.32 %

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 0.2 normal

2 0.10

Calibrator

Level

R 3.11 rectangular

( 3 )

0.5

1.80

Mismatch

1.27 U-shaped

( 2 )

0.5

0.90

Source

Mismatch

0.80 U-shaped

( 2 )

0.5

0.57

Sensor

Shaping

S 2.00 rectangular

( 3 )

0.5

1.15

Temp. Drift

T 1.69 rectangular

( 3 )

0.5

0.98

Noise

N 0.03 normal

2 0.02

Cal Factor

K 0.32 normal

2 0.16

Combined Standard

Uc normal

2.58

Uncertainty

Expanded

U normal

5.17

Uncertainty

(k=2)

From this example, different uncertainty terms dominate. Since the measurement is close t

the calibration frequency, and matching is rather good, the shaping and level errors are the

largest. The Expanded Uncertainty of 5.17 % translates to an uncertainty of about 0.22 dB

in the reading.

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