Boonton 4530 Peak Power Meter User Manual - Manual (Page 160)

Chapter 5 Boonton Electronics
Making Measurements 4530 Series RF Power Meter
5-18
5.7.3 Sample Uncertainty Calculations. The following examples show calculations for two measurement appli-
cations - one using a CW sensor (Model 51075), and the other with a peak power sensor (Model 57518). The
figures used in these examples are meant to show the general techniques, and do not apply to all applications.
Some “common sense” assumptions have been made to illustrate the fact that uncertainty calculation is not
an exact science, and requires some understanding of your specific measurement conditions.
Typical Example #1: Model 51075 CW Power Sensor
Measurement conditions:
Source Frequency: 10.3 GHz
Source Power: -55 dBm (3.16 nW)
Source SWR : 1.50 (reflection coefficient = 0.2) at 10.3 GHz
AutoCal Source: Internal 50MHz Calibrator
AutoCal Temperature: 25C
Current Temperature: 25C
In this example, we will assume that an AutoCal has been performed on the sensor immediately before the measurement.
This will reduce certain uncertainty terms, as discussed below.
Step 1: The Instrument Uncertainty figure for the 4530 Series is ±0.20%. Since a portion of this figure is meant to
include temperature drift of the instrument, and we know an AutoCal has just been performed, we’ll estimate (for lack
of more detailed, published information) that the instrument uncertainty is ±0.10%, or half the published figure.
U
Instrument
= ±0.10%
Step 2: The Calibrator Level Uncertainty for the power meter’s internal, 50MHz calibrator may be read from the
calibrator’s specification. It is ±0.105dB, or ±2.45% at a level of -55dBm.
U
CalLevel
= ±2.45%
Step 3: The Calibrator Mismatch Uncertainty is calculated using the formula in the previous section, using the internal
50MHz calibrator’s published figure for
,
CAL
and calculating the value
,
SNSR
from the SWR specification on the
51075’s datasheet.
,
CAL
= 0.024 (internal calibrator’s reflection coefficient at 50MHz)
,
SNSR
= (1.15 - 1) / (1.15 + 1) = 0.070 (calculate reflection coefficient of 51075, max SWR = 1.15 at 50MHz)
U
CalMismatch
= ±2
0 ,
CAL
0 ,
SNSR
0 100 %
= ±2
0 0.024 0 0.070 0 100 %
= ±0.34%
Step 4: The Source Mismatch Uncertainty is calculated using the formula in the previous section, using the DUT’s
specification for
,
SRCE
and calculating the value
,
SNSR
from the SWR specification on the 51075’s datasheet.
,
SRCE
= 0.20 (source reflection coefficient at 10.3GHz)
,
SNSR
= (1.40 - 1) / (1.40 + 1) = 0.167 (calculate reflection coefficient of 51075, max SWR = 1.40 at 10.3GHz)
U
SourceMismatch
= ±2
0 ,
SRCE
0 ,
SNSR
0 100 %
= ±2
0 0.20 0 0.167 0 100 %
= ±6.68%
Step 5: The uncertainty caused by Sensor Shaping Error for a 51075 CW sensor that has been calibrated using the
AutoCal method can be assumed to be 1.0%, as per the discussion in the previous section.
U
ShapingError
= ±1.0 %
Step 6: The Sensor Temperature Drift Error depends on how far the temperature has drifted from the sensor calibration
temperature, and the temperature coefficient of the sensor. In this example, an AutoCal has just been performed on the