Boonton Power Sensor - Manual (Page 47)

Step 3: The Calibrator Mismatch Uncertainty is calculated using the formula in the previous
section, using the internal 50MHz calibrator's published figure for D
CAL
and calculating the
value D
SNSR
from the SWR specification on the 51075's datasheet.
D
CAL
= 0.024 (internal calibrator's reflection coefficient at 50MHz)
D
SNSR
= (1.15 - 1) / (1.15 + 1) = 0.070
(calculated reflection coefficient of 51075, max SWR = 1.15 at 50MHz)
U
CalMismatch
= ± 2 * D
CAL
* D
SNSR
* 100 %
= ± 2 * 0.024 * 0.070 * 100 %
= ± 0.34%
Step 4: The Source Mismatch Uncertainty is calculated using the formula in the previous
section, using the DUT's specification for D
SRCE
and calculating the value D
SNSR
from the
SWR specification on the 51075's datasheet.
D
SRCE
= 0.20 (source reflection coefficient at 10.3GHz)
D
SNSR
= (1.40 - 1) / (1.40 + 1) ) = 0.167
(calculated reflection coefficient of 51075, max SWR = 1.40 at 10.3GHz)
U
SourceMismatch
= ± 2 * D
SRCE
* D
SNSR
* 100 %
= ± 2 * 0.20 * 0.167 * 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 sensor, and the temperature has not
drifted at all, so we can assume a value of zero for sensor temperature drift uncertainty.
U
SnsrTempDrift
= ± 0.0 %
Step 7: This is a relatively low-level measurement, so the noise contribution of the sensor
must be included in the uncertainty calculations. We'll assume default filtering. The signal
level is -55dBm, or 3.16nW. The RMS noise specification for the 51075 sensor is 30pW,
from Section 2. Noise uncertainty is the ratio of these two figures.
U
NoiseError
= ± Sensor Noise (in watts) / Signal Power (in watts)
= ± 30.0e-12 / 3.16e-9 * 100 %
= ± 0.95%
40 Power Sensor Manual 42