User Manual

9
Mismatch Uncertainty
Another factor of measurement accuracy is mismatch
uncertainty. When a source and a load have different
impedances, some signal will be reflected back to the
source. This uncertainty depends on both the VSWR of
the TPS and the VSWR of the rest of the system. For a
system VSWR of 1.0, the mismatch uncertainty would be
0. For a VSWR of 5.0, the mismatch uncertainty would
be 12.5%. Given the VSWR of the TPS and the source,
the mismatch uncertainty can be calculated as follows.
Mismatch uncertainty (MU) is related to the reflection
coefficient (
ρ) by the formula:
 Note: where ρ
s
= reflection coefficient of the
source, and ρ
l
= reflection coefficient of the load
(the TPS)
The reflection coefficients can be calculated from the VSWR
by the formula:
Example - If a source with a 1.50:1 VSWR with
the Terminating Power Sensor was used, which
has a max VSWR of 1.20:1, the mismatch uncer-
tainty would be calculated as follows:
MU percent()1 ρ
s
ρ
1
+()
2
1–[]100×=
ρ VSWR 1–()VSWR 1+()⁄=
ρ
s
1.50 1–()1.50 1+()⁄ 0.200==
ρ
1
1.20 1–()1.20 1+()⁄ 0.091==
MU 1 0.200 0.091×+()
2
1–[]100 3.67±=×=