HP 436A - Page 52

•
•
Model 436A
3-77. Known Uncertainties (cont'd)
c. CAL FACTOR switch resolution
±0.5%
or
±0.02
dB.
The
total
uncertainty from these sources is ±
1.
7%
or ±0.07 dB.
3-78 Calculating Mismatch
Uncertainty.
Mismatch
uncertainty
is
the
result
of
the
source mismatch
interacting with
the
Power
Sensor
mismatch. The
magnitude
of
uncertainty is related
to
the
magni-
tudes
of
the
source and Power
Sensor
reflection
coefficients, which can be calculated from
SWR.
Figure 3-9 shows how
the
calculations are
tq
be
made and Figure
3-10
illustrates mismatch
uncer-
tainty and total calculated uncertainty
for
two
cases.
In
the
first case,
the
Power
Sensor's SWR
=
Operation
1.5,
and in the second case,
the
Power
Sensor's
SWR
=
1.26. In both cases
the
source has a
SWR
of
2.0.
The example shows the effect
on
power
measurement accuracy a poorly matched power
sensor will have
as
compared
to
one with low
mismatch.
3-79. A faster, easier way
to
find mismatch
uncer-
tainty
is
to
use the
HP
Mismatch Error (uncer-
tainty) Limits/Reflectometer Calculator. The
calculator may be obtained, on request, from your
nearest Hewlett-Packard office by using HP Part
Number 5952-0448.
3-80. The method
of
calculating measurement
uncertainty from the uncertainty in dB is shown
by
Figure 3-11. This method would
be
used when
the
initial uncertainty calculations were made with
the
Mismatch Error/Reflectometer Calculator.
3-31