R&S ZNC3, Руководство пользователя

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Concepts and Features
R&S
®
ZNC
56 User Manual 1173.9557.02 ─ 13
In general the imbalance is a quantity with two numeric indices numbering the logical
output port and the logical input port of the DUT during the measurement (Imb
<out><in>
).
3.3.5.4 Reference Impedances
Changing the reference impedances of the analyzer ports is often referred to as renorm-
alization of port impedances. Renormalization means that the measurement results mea-
sured at 50 Ω (75 Ω) are converted into results at arbitrary port impedance.
●
Renormalization of the physical port impedances affects e.g. S-parameters and wave
quantities in "Power" representation.
●
Renormalization of the balanced port impedances affects all measured quantities
("Trace > Measure") that the analyzer provides for balanced ports.
The default reference impedance of a physical port is equal to the reference impedance
of the connector type assigned to the port (50 Ω or 75 Ω). It can be defined as a complex
value. For balanced ports it is possible to define separate complex reference impedances
for differential and for common mode.
The default values for the balanced port reference impedances are derived from the
default reference impedance of the physical analyzer ports (Z
0
= 50 Ω):
●
The default value for the differential mode is Z
0d
= 100 Ω = 2*Z
0
.
●
The default value for the common mode is Z
0c
= 25 Ω = Z
0
/2
Renormalization can be based on two alternative waveguide circuit theories whose con-
version formulas may yield different results if the reference impedance of at least one
test port has a non-zero imaginary part.
Conversion formula for wave quantities and S-parameters
Renormalization transforms the "raw" S-matrix S
0
for the default reference impedances
Z
0i
(with physical port number index i = 1,2,...,n) into a "renormalized" S-matrix S
1
for the
modified reference impedances Z
1i
. In terms of raw and renormalized wave quantities
a
0i
, b
0i
and a
1i
, b
1i
, S
0
and S
1
are defined as follows:
.
... ...
;
... ...
1
12
11
1
1
12
11
0
02
01
0
0
02
01












 
























 












n n n n
a
a
a
S
b
b
b
a
a
a
S
b
b
b
The renormalized wave quantities (a
1
and b
1
) and the S-matrix S
1
can be calculated from
S
0
and the reference impedances Z
0i
, Z
1i
according to two alternative waveguide circuit
theories.
1. Travelling waves
In the model of Marks and Williams ("A General Waveguide Circuit Theory"), the wave
quantities a and b are transformed as follows:
Measurement Results