Boonton Power Sensor - Manual (Page 50)

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 found in Section 2.
D
SRCE
= 0.057 (source reflection coefficient at 900 MHz)
D
SNSR
= (1.15 - 1) / (1.15 + 1) )
= 0.070
(calculated reflection coefficient of 57518, max SWR = 1.15 at 0.9 GHz)
U
SourceMismatch
= ± 2 * D
SRCE
* D
SNSR
* 100 %
= ± 2 * 0.057 * 0.070 * 100 %
= ± 0.80%
Step 5: The uncertainty caused by Sensor Shaping Error for a 57518 peak sensor is 4% at
all levels (from table 2-2). But since we’re measuring at 900MHz, which is very close to the
1GHz AutoCal frequency, we’ll assume that the frequency-dependent portion of the shaping
error becomes very small, and we’ll estimate that 2% remains.
U
ShapingError
= ± 2.0 %
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 our
case, we are using a temperature compensated sensor, and the temperature has drifted by 11
degrees C (49C - 38C) from the AutoCal temperature. We will use the equation in the
previous section to calculate sensor temperature drift uncertainty.
U
SnsrTempDrift
= ± ( 0.93% + 0.069% / °C)
= ± ( 0.93 + ( 0.069 * 11.0 ) ) %
= ± 1.69 %
Step 7: This is a relatively high-level measurement, so the noise contribution of the sensor is
probably negligible, but we’ll calculate it anyway. We’ll assume modulate mode with default
filtering. The signal level is 13dBm, or 20mW. The “noise and drift” specification for the
57518 sensor is 50nW, from Table 2-2 (Peak Power Sensor Characteristics).
Noise uncertainty is the ratio of these two figures.
U
Noise&Drift
= ± Sensor Noise (in watts) / Signal Power (in watts)
= ± 50.0e-9 / 20.0e-3 * 100 %
= ± 0.0003 %
Step 8: A separate Sensor Zero Drift calculation does not need to be performed for peak
sensors, since “noise and drift” are combined into one specification, so we’ll just skip this
step.
Power Sensor Manual 43 45