Boonton 4500B Peak Power Meter - Manual (Page 330)

Boonton 4500B RF Peak Power Analyzer
Application Notes 6-20
The sensor calfactor uncertainty is due to uncertainties encountered while performing this frequency calibration (due
to both standards uncertainty, and measurement uncertainty), and is different for each frequency. Both worst case
and RSS uncertainties are provided for the frequency range covered by each sensor, and are listed on the sensor
datasheet and in the Boonton Electronics Power Sensor Manual.
If the measurement frequency is between sensor calfactor entries, the most conservative approach is to use the
higher of the two corresponding uncertainty figures. It is also be possible to estimate the figure by linear
interpolation.
If the measurement frequency is identical to the AutoCal frequency, a calfactor uncertainty of zero should be used,
since any absolute error in the calfactor cancels out during AutoCal. At frequencies that are close to the AutoCal
frequency, the calfactor uncertainty is only partially cancelled out during AutoCal, so it is generally acceptable to
take the uncertainty for the next closest frequency, and scale it down.
6.5.3 Sample Uncertainty Calculations. The following example shows calculations for a peak power sensor, Model
56518. The figures used in this example are meant to show the general technique, and does not apply to all
applications. Some ―common sense‖ assumptions have been made to illustrate the fact that uncertainty calculation
is not an exact science, and requires some understanding of your specific measurement conditions.
Typical Example: Model 56518 Peak Power Sensor
Model 4500B measurement conditions:
Source Frequency: 900 MHz
Source Power: 13 dBm (20mW)
Source SWR : 1.12 (reflection coefficient = 0.057) at 900 MHz
AutoCal Source: Internal 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 Model 4500B 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 4500B internal 1GHz 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 Ч 0.69%))
= ±3.11%