Boonton 4240 RF Power Meter - Manual (Page 117)

Boonton 4240 Series RF Power Meter
6.2.3 Sample Uncertainty Calculations.
The following example shows calculations for a CW power sensors. The figures used in these examples are meant to show
technique, and do not apply to ion se” assumptions have been made to
illus calculation exa e, a some understanding of your specific
mea
Typ Powe
424
dBm (3. )
ti fficient t 10.3
Cal Source: Internal 50M
re: 25C
In th performed on th iately before the measurement.This
will
CalLevel
= ± 2.45%
tep 3: The Calibrator Mismatch Uncertainty is calculated using the formula in the previous section, using the 50MHz
= (1.15 - 1) / (1.15 + 1) = 0.070 (calculate reflection coefficient of 51075, max SWR = 1.15 at 50MHz)
0.34%
Step 4: e he previous section, using the DUT’s
specifica on on the 51075’s datasheet.
SRCE
cient at 10.3GHz)
ρ
SNSR
= (1.40 - 1) / (1.40 + 1) = 0.167 (calculate reflection coefficient of 51075, max SWR = 1.40 at 10.3GHz)
U
SourceMismatch
= ± 2 × ρ
SRCE
× ρ
SNSR
Ч 100 %
= ± 2 Ч 0.20 Ч 0.167 Ч 100 %
= ± 6.68%
tep 5: The uncertainty caused by Sensor Shaping Error for a 51075 CW sensor that has been calibrated using the AutoCal
method can be assumed to be 1.0%, as per the discussion in the previous section.
ShapingError
= ± 1.0 %
the general every applicat . Some “common sen
trate the fact that uncertainty is not an ct scienc nd requires
surement conditions.
ical Example #1: Model 51075 CW r Sensor
0 Series measurement conditions:
Source Frequency: 10.3 GHz
Source Power: -55 16 nW
Source SWR : .50 (reflec on coe = 0.2) a GHz
Auto Hz Calibrator
AutoCal Temperature: 25C
Current Temperatu
is example, we will assume that an A oCal has been e sensor immed
reduce certain uncertainty terms, as discussed below.
ut
Step 1: The Instrument Uncertainty figure for the 4240 Series is ± 0.23%. 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 ± 0.115%, or half the published figure.
U
Instrument
= ± 0.115%
Step 2: The Calibrator Level Uncertainty for the power meter’s 50MHz calibrator may be read from the calibrator’s
specification. It is ± 0.105dB, or ± 2.45% at a level of -55dBm.
U
S
calibrator’s published figure for ρ
CAL
and calculating the value ρ
SNSR
from the SWR specification on the 51075’s datasheet.
ρ
CAL
= 0.024 (calibrator’s reflection coefficient at 50MHz)
ρ
SNSR
U
CalMismatch
= ± 2 × ρ
CAL
× ρ
SNSR
Ч 100 %
= ± 2 Ч 0.024 Ч 0.070 Ч 100 %
= ±
Th Source Mismatch Uncertainty is calculated using the formula in t
ti for ρ and calculating the value ρ
SNSR
from the SWR specification
SRCE
ρ = 0.20 (source reflection coeffi
S
U
Application Notes
6-9