Boonton Power Sensor - Manual (Page 49)

From the previous example, it can be seen that the two largest contributions to the combined
standard uncertainty are the source mismatch, and the sensor calfactor.
Typical Example #2: Model 57518 Peak Power Sensor
Measurement conditions:
Source Frequency: 900 MHz
Source Power: 13 dBm (20mW)
Source SWR : 1.12 (reflection coefficient = 0.057) at 900 MHz
AutoCal Source: External 2530 1GHz Calibrator
AutoCal Temperature: 38C
Current Temperature: 49C
In this example, we will assume that an AutoCal was performed on the sensor earlier in the
day, so time and temperature drift may play a role in the uncertainty.
Step 1: The Instrument Uncertainty figure for the 4530 Series is ±0.20%. Since it has been a
while since AutoCal, we’ll use the published figure.
U
Instrument
= ± 0.20%
Step 2: The Calibrator Level Uncertainty for the Model 2530 1GHz external calibrator may
be calculated from the calibrator’s specification. The 0dBm uncertainty is 0.065dB, or 1.51%.
To this figure, we must add 0.03dB or 0.69% per 5dB step from 0dBm. 13dBm is 2.6 5dB
steps (13/5) away from 0dBm. Any fraction must always be rounded to the next highest whole
number, so we’re 3 steps away.
U
CalLevel
= ± ( 1.51% + ( 3 * 0.69% ))
= ± 3.11%
Step 3: The Calibrator Mismatch Uncertainty is calculated using the formula in the previous
section, using the 2530 calibrator’s published figure for D
CAL
and calculating the value
D
SNSR
from the SWR specification outlined in Section 2 of this manual.
D
CAL
= 0.091 (external 2530 calibrator’s reflection coefficient at 1GHz)
D
SNSR
= (1.15 - 1) / (1.15 + 1) = 0.070
(calculated reflection coefficient of 57518, max SWR = 1.15 at 1 GHz)
U
CalMismatch
= ± 2 * D
CAL
* D
SNSR
* 100 %
= ± 2 * 0.091 * 0.070 * 100 %
= ± 1.27%
42 Power Sensor Manual 44