Manual

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
Instrumen
t
= ± 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
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