Boonton 4540 Peak Power Meter User Manual - Manual (Page 249)

Boonton 4540 Series RF Power Meter
Application Notes 6-21
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 both CW and peak power sensors. The figures used in these examples are
meant to show the general technique, and do not apply to every application. 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 #1: Model 51075 CW Power Sensor
4540 Series measurement conditions:
Source Frequency: 10.3 GHz
Source Power: -55 dBm (3.16 nW)
Source SWR : .50 (reflection coefficient = 0.2) at 10.3 GHz
AutoCal Source: Internal 50MHz Calibrator
AutoCal Temperature: 25C
Current Temperature: 25C
In this example, we will assume that an AutoCal has been performed on the sensor immediately before the measurement.This
will reduce certain uncertainty terms, as discussed below.
Step 1: The Instrument Uncertainty figure for the 4540 Series is ±0.20%. Since a portion of this figure is meant to include
temperature drift of the instrument, and we know an AutoCal has just been performed, we’ll estimate (for lack of more
detailed, published information) that the instrument uncertainty is ±0.10%, or half the published figure.
U
Instrument
= ±0.10%
Step 2: The Calibrator Level Uncertainty for the power meter’s internal, 50MHz calibrator may be read from thecalibrator’s
specification. It is ±0.105dB, or ±2.45% at a level of -55dBm.
U
CalLevel
= ±2.45%
Step 3: The Calibrator Mismatch Uncertainty is calculated using the formula in the previous section, using the internal
50MHz calibrator’s published figure for ρ
CAL
and calculating the value ρ
SNSR
from the SWR specification on the51075’s
datasheet.
ρ
CAL
= 0.024 (internal calibrator’s reflection coefficient at 50MHz)
ρ
SNSR
= (1.15 - 1) / (1.15 + 1) = 0.070 (calculate reflection coefficient of 51075, max SWR = 1.15 at 50MHz)
U
CalMismatch
= ±2 × ρ
CAL
× ρ
SNSR
Ч 100 %
= ±2 Ч 0.024 Ч 0.070 Ч 100 %
= ±0.34%