Boonton 4540 Peak Power Meter User Manual - Manual (Page 250)

Boonton 4540 Series RF Power Meter
Application Notes 6-22
Step 4: The Source Mismatch Uncertainty is calculated using the formula in the previous section, using the DUT’s
specification for ρ
SRCE
and calculating the value ρ
SNSR
from the SWR specification on the 51075’s datasheet.
ρ
SRCE
= 0.20 (source reflection coefficient at 10.3GHz)
ρ
SNSR
= (1.40 - 1) / (1.40 + 1) = 0.167 (calculate reflection coefficient of 51075, max SWR = 1.40 at 10.3GHz)
U
SourceMismatch
= ±2 × ρ
SRCE
× ρ
SNSR
Ч 100 %
= ±2 Ч 0.20 Ч 0.167 Ч 100 %
= ±6.68%
Step 5: The uncertainty caused by Sensor Shaping Error for a 51075 CW sensor that has been calibrated using theAutoCal
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 the sensor’s datasheet. Noise uncertainty is the ratio of these two figures.
U
Noise Error
= ± Sensor Noise (in watts) / Signal Power (in watts)
= ±30.0e-12 / 3.16e-9 _ 100 %
= ±0.95%
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
Zero Drift
= ± 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 in the Boonton Electronics Power Sensor
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%. These two values are fairly close, so we’ll perform a linear interpolation to estimate the
uncertainty at 10.3GHz:
U
CalFactor
= [ ( F - F1 ) * (( CF2 - CF1 ) / ( F2 - F1 )) ] + CF1
= [ ( 10.3 - 10.0 ) * (( 4.3 - 4.0 ) / ( 11.0 - 10.0 )) ] + 4.0
= 4.09%