Boonton 4530 Peak Power Meter User Manual - Manual (Page 157)

Boonton Electronics Chapter 5
4530 Series RF Power Meter Making Measurements
5-15
The formula for calculating RSS measurement uncertainty from worst-case values and scale factors is:
___________________________________________
U
RSS
=
F(U
1
K
1
) 2 + (U
2
K
2
) 2 + (U
3
K
3
) 2 + (U
4
K
4
) 2 + ... (U
N
K
N
) 2
where U
1
through U
N
represent each of the worst-case uncertainty terms, and K
1
through K
N
represent the
normalizing multipliers for each term based on its distribution shape.
This calculation yields what is commonly referred to as the combined standard uncertainty, or U
C
, with a level
of confidence of approximately 68%. To gain higher levels of confidence an Expanded Uncertainty is often
employed. Using a coverage factor of 2 (U = 2U
C
) will provide an Expanded Uncertainty with a confidence level
of approximately 95%.
5.7.2 Discussion of Uncertainty Terms. Following is a discussion of each term, its definition, and how it is
calculated.
Instrument Uncertainty. This term represents the amplification and digitization uncertainty in the power
meter, as well as internal component temperature drift. In most cases, this is very small, since absolute errors
in the circuitry are calibrated out by the AutoCal process. The instrument uncertainty is 0.20% for the 4530
Series.
Calibrator Level Uncertainty. This term is the uncertainty in the calibrator’s output level for a given setting
for calibrators that are maintained in calibrated condition. The figure is a calibrator specification which
depends upon the output level:
50MHz Calibrator Level Uncertainty:
At 0 dBm:
±0.055 dB (1.27%)
+20 to -39 dBm:
±0.075 dB (1.74%)
-40 to -60 dBm:
±0.105 dB (2.45%)
1GHz Calibrator Level Uncertainty:
± (0.065 dB (1.51%) at 0 dBm + 0.03 dB (0.69%) per 5 dB from 0 dBm)
The value to use for calibration level uncertainty depends upon the sensor calibration technique used. If
AutoCal was performed, the calibrator’s uncertainty at the measurement power level should be used. For
sensors calibrated with FixedCal, the calibrator is only used as a single-level source, and you should use
the calibrator’s uncertainty at the FixedCal level, (0dBm, for most sensors). This may make FixedCal seem
more accurate than AutoCal at some levels, but this is usually more than offset by the reduction in
shaping error afforded by the AutoCal technique.
Calibrator Mismatch Uncertainty. This term is the mismatch error caused by impedance differences between
the calibrator output and the sensor’s termination. It is calculated from the reflection coefficients of the
calibrator (
,
CAL
) and sensor (
,
SNSR
) at the calibration frequency with the following equation:
Calibrator Mismatch Uncertainty = ±2
0 ,
CAL
0 ,
SNSR
0 100 %
The calibrator reflection coefficient is a calibrator specification:
Internal Calibrator Reflection Coefficient (
,
CAL
): 0.024 (at 50MHz)
External 2530 Calibrator Reflection Coefficient (
,
CAL
): 0.091 (at 1GHz)
The sensor reflection coefficient,
,
SNSR
is frequency dependent, and may be looked up in the sensor datasheet
or the Boonton Electronics Power Sensor Manual.