Anritsu MS2024B - 3-4 Calculating and Displaying S-Parameters

3-4 Calculating and Displaying S-Parameters VNA Fundamentals
3-6 PN: 10580-00289 Rev. K Vector Network Analyzer MG
3-4 Calculating and Displaying S-Parameters
S-parameters are a measure of the ratio of two complex voltage levels, one measured by the
port receiver, and one measured by the reference receiver. S-parameters therefore consist of
unitless complex numbers.
Depending on the application, S-parameters can be displayed in many ways and can be used
to calculate other parameters. S-parameters consist of real and imaginary numbers. More
typically, however, they are represented as magnitude and phase. In most cases, the
magnitude is displayed in dB (this term is often called log magnitude). We can display phase
as “linear phase”. With phase, we cannot tell the difference between one cycle and the next.
After going through 360 degrees, we are back to where we began. We can display the
measurement from –180 degrees to +180 degrees, which keeps the display discontinuity
removed from the important 0 degrees area used as the phase reference.
The VNA Master supports the following display types. Each type is associated with a
particular S-parameter, S
xy
= S
Real
+ jS
Imaginary
(where j is the square root of –1).
Table 3-1. Log Magnitude
Application Notes
To measure return loss at Port 1 (or Port 2), use the Log Mag display with S
11
(or S
22
).
To measure the gain or loss in a DUT that is connected between Port 1 and Port 2, use the
Log Mag display with S
21
or S
12
.
Table 3-2. Log Magnitude / 2
Application Note
For measuring 1-port cable loss, use S
11
or S
22
with the Log Mag/2 display type to account
for the round trip signal path through the cable. When using reflection data to measure
cable loss, the end of the cable must be shorted or must be a perfect open.
Table 3-3. Real and Imaginary
SReal = Real S-parameter
SImaginary = Imaginary S-parameter
LogMagnitude dB() 20Log
10
S
xy
=
LogMagnitude
2
-----------------------------------------
dB() 0.5 20× Log
10
S
xy
=
Phase degrees()Tan
1–
=
S
Imaginary
S
Real
--------------------------
180
π
---------
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