Obtaining S-Parameter Data from
Probe Window in PSpice
Product Version SPB 17.4-2019
December 2019
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Obtaining S-Parameter Data from Probe Window in PSpice
Contents
Purpose ........................................................................................................................4
Audience ......................................................................................................................4
Database ......................................................................................................................4
S-Parameter Theory ......................................................................................................5
Measuring S-Parameter Data from the PSpice Probe ......................................................7
Defining the Sub-Schematics .........................................................................................8
Using the Sub-Schematics ........................................................................................... 12
Support....................................................................................................................... 16
Feedback.................................................................................................................... 16
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Obtaining S-Parameter Data from Probe Window in PSpice
Purpose
This document explains the steps for measuring S-parameters in PSpice from the probe
window. Obtaining S-parameters for a passive circuit (a band pass filter) and an active
circuit (transistor circuit) are covered.
Audience
This document is intended for Cadence® PSpice® users who want to measure Sparameter data using PSpice probe.
Database
RAK database and references can be found at ‘Attachments’ and ‘Related Solutions’
sections below the PDF. This RAK pdf can be searched with the document title on
https://support.cadence.com
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Obtaining S-Parameter Data from Probe Window in PSpice
S-Parameter Theory
S-parameters measure the ratio of incident versus reflected signals. The incident
signals are defined as a1 and a2, while the reflected signals are defined as b1 and b2.
The incident and reflected signals are related to voltages and currents at ports 1 and 2
as:
a1 =
b1 =
a2 =
𝑉𝑉1+𝑍𝑍0𝐼𝐼1
2 √𝑍𝑍
𝑉𝑉1−𝑍𝑍𝑍𝑍𝐼𝐼1
2 √𝑍𝑍
𝑉𝑉2+𝑍𝑍𝑍𝑍𝐼𝐼2
2 √𝑍𝑍
𝑉𝑉2−𝑍𝑍𝑍𝑍𝐼𝐼2
b2 =
2 √𝑍𝑍
(1)
(2)
(3)
(4)
The scattered or reflected waves are related to the incident waves by the following set
of linear equations:
b1 = S11a1 + S12a2
(5)
b2 = S21a1 + S22a2
(6)
Or, in matrix form as:
𝑏𝑏1
S11 𝑆𝑆12
𝑎𝑎1
� � =�
�� �
𝑏𝑏2
𝑆𝑆21 𝑆𝑆22 𝑎𝑎2
(7)
The Sij coefficients are dimensionless ratios. For most applications, the characteristic
impedance of the system, Z0, is 50 ohms. S11 is the input reflection ratio and is
defined as the ratio of the input port reflected wave to the input port incident wave. If the
incident wave at the output, a2, is set to 0, the equations reduce to b1 = S11a1 and
b2 = S21a1. Using the defining equations, this reduces to
𝑏𝑏1
S11
=
𝑎𝑎1
𝑉𝑉1− 𝑍𝑍0𝐼𝐼1
=
𝑉𝑉1+𝑍𝑍𝑍𝑍𝐼𝐼1
𝑍𝑍
= 2 (𝑍𝑍+𝑍𝑍𝑍𝑍)
- 1 (8)
Where
•
V1/I1 is the input impedance Z.
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Obtaining S-Parameter Data from Probe Window in PSpice
•
S21 is the forward transmission ratio and is defined as b2/a1.
If the input and output load impedances of the circuit are the same, S21 is the voltage
measured at the output multiplied by 2.
If the incident wave at the input is set to zero, then the equations reduce to:
b1 = S12a2
b2 = S22a2
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Obtaining S-Parameter Data from Probe Window in PSpice
Measuring S-Parameter Data from the PSpice Probe
Sub-schematics created in Allegro® Design Entry CIS (Capture CIS), shown in Figure 1
and Figure 2, are required to make the necessary measurements.
Figure 1- XMITS circuit
Figure 2 – REFLECTS circuit
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Obtaining S-Parameter Data from Probe Window in PSpice
Defining the Sub-Schematics
The design (extract_s_parameter.zip) with this note has Parts XMITS (Figure 3),
REFLECTS(Figure 5), and the corresponding Model Libraries.
Sub-schematic ‘XMITS’ for Transmission Coefficients Measurement
Figure 3 – XMITS circuit and its hierarchical part
The XMITS circuit shown in Figure 3 is used to measure:
•
Forward transmission coefficient - S21
•
Reverse transmission coefficient - S12
Transmission coefficients are the output voltages multiplied by 2, since the output load
matches the input load. The E device, E1, has a gain of 2.
The port CKT is used to connect to an external circuit.
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Obtaining S-Parameter Data from Probe Window in PSpice
The port STR is a hidden pin as shown in Figure 4. Capture CIS will generate a unique
net if this pin is left unconnected in a schematic. Alternatively, a specific net can be
named for the connection by editing the PSpiceDefaultNet property (Figure 4).STR pin
will have a known label S21, when analyzing simulation results in the Probe window.
Figure 4 – Pin Properties for the XMITS part showing hidden STR pin
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Obtaining S-Parameter Data from Probe Window in PSpice
Sub-Schematic ‘REFLECTS’ for Transmission Coefficients Measurement
Figure 5 - REFLECTS circuit and its hierarchical part
The REFLECTS circuit shown in Figure 5 is used to measure:
•
Input reflection coefficient - S11
•
Output reflection coefficient - S22
Reflection coefficients are the input voltage multiplied by 2 minus AC unity. The E
device, E1, has a gain of 2. As in the transmission coefficients measurement circuit, the
interface pin labeled CKT is used to connect to the external circuit.
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Obtaining S-Parameter Data from Probe Window in PSpice
The pin, SRE, is a hidden pin (Figure 6), like STR described above. The REFLECTS
symbol also has a DCbias property.
In active circuits, the DC level can be set to voltage source, V1, by changing the DCbias
property for the REFLECTS part instance in Capture CIS. By default, this property is set
to zero.
Figure 6: Pin Properties for the Reflects symbol showing hidden S_RE pin
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Obtaining S-Parameter Data from Probe Window in PSpice
Using the Sub-Schematics
Sub-schematics can be used for both passive and active circuits. You can also use the
XMITS and REFLECTS symbols, available in the sparam.olb library included in the
design for this application note.
Passive Circuit Implementation
The circuit shown in Figure 7 is a fourth-order Butterworth bandpass filter with a center
frequency of 250 MHz.
The first circuit measures S11 and S21. The second circuit measures S12 and S22.
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Obtaining S-Parameter Data from Probe Window in PSpice
AC analysis is performed on the design with Start Frequency of 200 MHz and End
Frequency of 350 MHZ. The Simulation results are shown in Figure 8.
Figure 7 - Bandpass filter example
Figure 8 - S11 and S21 for Filter Example
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Obtaining S-Parameter Data from Probe Window in PSpice
Active Circuit Implementation
It is also possible to measure the s-parameters of an active circuit.
The design with this note measures the s-parameters of RF transistor MRF5711/MC.
Figure 9 shows the circuit for this example.
Figure 9 - Transistor example
The Transistor is biased for a VCE of 6.0 volts and an IC of 5.0 mA. The current is
set by the current source I2 at the Emitter of the Transistor. DC bias is set by V1 in the
circuit that measures S11 and S21.The DCBIAS property specifies the DC bias in
reflection measurement sub-circuit that measures S12 and S22.
AC analysis is performed on the Design with Start Frequency of 200 MHz and End
Frequency of 2 GHz.
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Obtaining S-Parameter Data from Probe Window in PSpice
Figures 10 through 12 show the magnitude and phase measurements after Simulation.
This is typically the way most manufacturer’s data sheets show the S-Parameters.
Figure 10 - S11 and S22 magnitude and phase
Figure 11 – S21 Magnitude and Phase
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Obtaining S-Parameter Data from Probe Window in PSpice
Figure 12 – S12 Magnitude and Phase
Support
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Feedback
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