1999 May 17 1
Philips Semiconductors
S-Parameter Definitions General
S-PARAMETER DEFINITIONS
The S-parameter symbols in this section are based on IEC
publication 747-7.
S-parameters (return losses or reflection coefficients) of a
module can be defined as the S11 and the S22 of a two-port
network (see Fig.1).
(1)
(2)
where:
(3)
(4)
From (1) and (2) formulae for the return losses can be
derived:
(5)
(6)
In (5), a2= 0 means output port terminated with Zo
(derived from formula (4)).
In (6), a1= 0 means input port terminated with Zo
(derived from formula (3)).
Fig.1 Two-port network with reflection coefficients
S11 and S22.
D.U.T.
S11 S22
a1
b1
b2
a2
MLB335
b1S11 a1S12 a2
⋅+⋅=
b2S21 a1S22 a2
⋅+⋅=
a11
2Z
o
⋅
------------------ V1Zoi1
⋅+()⋅ signal into port 1==
a
2
1
2Z
o
⋅
------------------ V2Zoi2
⋅+()⋅ signal into port 2==
b
1
1
2Z
o
⋅
------------------ V1Zoi1
⋅+()⋅ signal out port 1==
b
2
1
2Z
o
⋅
------------------ V1Zoi2
⋅+()⋅ signal out port 2==
S
11 b1
a1
------ a20==
S
22 b2
a2
------ a10==
Measurement
The return losses are measured with a network analyzer
after calibration, where the influence of the test jig is
eliminated. The necessary termination of the other port
with Zo is done automatically by the network analyzer.
The network analyser must have a directivity of at least
40 dB to obtain an accuracy of 0.5 dB when measuring
return loss figures of 20 dB. A full two-port correction
method can be used to improve the accuracy.
