Software Tools for Teaching High Frequency

Software Tools for Teaching High Frequency Electronics Courses
Andrew Rusek, Barbara Oakley,
Department of Electrical and Systems Engineering
Oakland University, Rochester, Michigan 48309
Abstract
One of the most critical issues in high frequency electronics and communication university courses is the
availability of inexpensive or free software that demonstrates major circuit analysis and design
considerations necessary for laboratory, homework, and projects. The most recent technological
developments in communication circuit design have created a necessity for introduction of new
experiments and associated software to demonstrate applications of very complex methods of high
frequency circuit design that include scattering matrix parameters, stability tests, distributed and lumped
parameters, and circuit nonlinearities. This paper has been prepared and written to share our interesting
experiences using both free and inexpensive, circuit level software, such as PSPICE, PUFF, APPCAD,
SERENADE, and SMITH182, in a course entitled “High Frequency Electronics” (EE626), which is a
gradate level subject at Oakland University in Rochester, Michigan. Examples presented in the paper
illustrate only selected implementations. Some of the simulated circuits have been also designed and tested.
1. Introduction
Academic and non-academic courses related to wireless, radio, and high frequency systems and circuits
have become much more popular over the last few years than before. The major reason for such
proliferation has been the booming interest in high-speed personal and office communication systems,
which incorporate a large number of electronic technologies, especially microwaves, integrated circuits,
electromagnetic compatibility, software embedded systems, and quite a few others. One of the most
difficult problems related to development of such courses is accessibility to inexpensive or free software
support. Oakland University has prepared a large number of software examples based on free
demonstration versions of software, and also on other inexpensive software packages. The high frequency
course offered at Oakland University is structured so as to include the topics listed below:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Component parameter identification based on frequency and time domain responses.
Properties of transmission lines, frequency analysis, time domain analysis with de Bergeron diagrams;
time domain reflectometry.
Analysis and pulse behavior of basic lumped components with their parasitic elements.
Frequency and time domain operation of diodes and transistors.
HF amplifiers; y, s, and ABCD parameters.
HF oscillators (sinusoidal and non-sinusoidal).
HF communication circuits, including mixers; modulators; demodulators.
High speed logic circuits.
HF measurements and basic instruments such as network and spectrum analyzers
The software introduced throughout the course includes PSPICE of Microsim (currently CADENCE),
PUFF of CALTECH, APPCAD of Agilent Technologies, SERENADE of ANSOFT, and SMITH of Berne
Institute of Technology. The package of MATLAB (education version) supports additional calculations and
data processing. The examples presented here constitute only a small fraction of all examples available, and
used throughout the course but they demonstrate the extent of software support and its flexibility. One of
the examples described below is also practically implemented in a larger measurement system. The intent
of the paper is to show how the students can benefit from availability of highly economical software in
processes of learning high frequency electronics.
2. Description of the Circuits
a. PSPICE Examples
The PSPICE is a classical circuit analysis program, which includes nonlinear models of electronic devices
but it does not have a direct link to high frequency parameters such as S-matrix parameters.
Figure 1 shows the circuits assembled to determine the S-parameters for the MRF501 bipolar transistor of
Motorola. The upper circuit extracts s11 and s21, while the lower circuit extracts s22 and s12. The linear Sparameters are determined from Gummel-Poon PSPICE nonlinear model at the DC operating point defined
by the supply current (I = 5mA). The S-parameters dependence upon frequency is shown in Figure 2. The
S-parameters can be later used to design HF amplifiers or generators.
Figure 1: S-parameter extraction circuits.
Figure 2: Magnitude and phase versus frequency of s11, s22, s12, and s21.
Another PSPICE based example of a circuit called balun is shown in Figure 3, and the time domain signals
are demonstrated in Figure 4.
The balun is a device that provides a transition between an unbalanced transmission line and a balanced, or
symmetric transmission line. As shown in Figure 3, a signal is sent from the source through a 70 ohm
unbalanced transmission line (grounded outer connector) to a 300 ohm balanced (symmetric) transmission
line that is connected to a 300 ohm resistor representing a loop antenna. The inputs of the balanced line are
separated by a 70 ohm unbalanced section of half-wave length cable. This introduces 180-degree phase
shift between the signals of its end to drive the balanced line. The same principle is used to connect a
balanced receiving antenna to an unbalanced transmission line that is connected to a receiver.
Figure 3: Schematic model for a balun.
Figure 4: Waveforms affiliated with the balun.
b.
PUFF Examples
The PUFF is a DOS based program that helps analyze the HF circuits whose topologies can be drawn the
way they are finally implemented in practice. The circuit layout shown in Figure 5 is an HF amplifier
whose S-parameters are determined during simulation. The transistor, represented by component a, is
connected to several lumped and distributed components, which shape the frequency response. The center
frequency of operation is set to 6GHz where the amplifier delivers about 8dB of the gain and has very low
reflection coefficients. The transistor S-parameters can be entered directly from the device specifications or
from PSPICE extraction schemes.
Figure 5: HF amplifier analysis using PUFF
The hybrid ring (rat-race) circuit, shown in Figure 6, operates in such a way that if a signal is applied to
port 1, it is split evenly into two signals reaching ports 2 and 3 with small attenuation, while port 4 is
isolated (Figure 6). The structure is designed to operate at 5GHz.
Figure 6: The hybrid ring (rat-race) circuit analyzed by means of the PUFF program.
c.
APPCAD Examples
The APPCAD is an excellent example of free software package that provides educators with a suite of HF
analysis and design tools. Computerized Application Notes support many designs. The most typical
modules allow calculating parameters of various configurations of transmission lines, simple amplifiers,
and some nonlinear circuits, such as mixers and detectors.
The examples shown here include an amplifier designed to satisfy the power gain of 17dB (Figure 7) and
two structures of the transmission lines (Figures 8 and 9). The amplifier software calculates component
values for the bias circuit when the gain and bandwidth are given. as shown in Figure 7. The preloaded data
include parameters of an Agilent MGA-85563 low noise integrated circuit. The transmission line calculator
computes the characteristic impedance of the line when line geometry and dielectric parameters are known
or it calculates dimensions for a given value of the impedance.
Figure 7: Amplifier design for the nominal gain of 17dB and the bandwidth of about 2200MHz.
Figure 8: Determination of the characteristic impedance of the two-wire transmission line from known
geometry.
Figure 9: Design of the 50-ohm microstrip transmission line.
d. SERENADE Examples
The SERENADE SV (student version) 8.5 of ANSOFT Corporation is a limited version of the professional
SERENADE 8.5 package. The student version software is applied here to demonstrate the software
analysis of two HF broadband amplifiers. One amplifier (Figures 10 and 11) includes a discrete transistor,
whose model parameters were arbitrarily chosen to simulate properties of a typical microwave transistor. It
also includes discrete components for coupling and matching and a single transmission line of given
geometry. The results of simulations are plotted in terms of S-parameters versus frequency. The s21
parameter shows the gain of the designed stage. The results of analysis of more advanced amplifier are
shown in Figures 12 and 13. The integrated circuit of RF Micro-Device company is used with external
supply components to construct a broadband HF amplifier with a bandwidth above 2 GHz. The results of
simulations are shown for a single stage. The s21 parameter representing the stage gain is plotted versus
frequency. Similar design was implemented practically. The single stage and the two-stage amplifier were
designed and tested. The results of measurements not only confirmed the simulation data but they appeared
to have slightly higher values. In addition, Figure 14 shows the Smith chart with plots of s11 and stability
circles at 10 MHz for the integrated circuit applied in the simulations and tests.
Figure 10: Diagram of a broadband amplifier with a bandwidth about 2 GHz
Figure11: S-parameters determined as a result of simulation of the amplifier of Figure 10.
Figure 12: The diagram of the single stage of the amplifier implemented practically.
Figure 13: Simulated forward transfer function of the single stage amplifier shown above.
Figure 14: The Smith chart display of s11 and the stability circles. The larger circle is the “source” stability
circle, the smaller circle is the “load” stability circle.
e. The SMITH Examples
The SMITH182 program of Berne Institute of Technology is an interesting tool that can be used to design
HF matching networks composed of lumped and distributed passive circuit elements, including capacitors,
inductors, resistors, and transmission lines. Figures 15 and 16 illustrate the processes of impedance
transformation. The first circuit (Figure 15) shows how an arbitrary impedance (Data Point 1) is transferred
through a transmission line of the relative length of 0.12 of the wavelength, then through an L-C network,
and finally through another transmission line (0.06 of the wavelength). The example presented here is one
the examination problems used to verify manual calculations performed with the aid of the Smith chart.
The second example (Figure 16) involves the matching process. Arbitrary impedance (Data Point1) has to
be transferred to the source terminals to represent a 50-ohm load. The initial transfer involves the
transmission line. Final reactance compensation is achieved using an inductor (Data Point 2 to Data Point
3)
Figure 15: Impedance transformation using the Smith chart program. The load impedance (Data Point 1) is
transformed through a section of the transmission line, inductor, parallel capacitor and another
section of the transmission line (Data Point 5).
Figure 16: Process of the load matching. The load impedance (Data Point 1) is transformed to the source
using a section of a 50-ohm transmission line and inductor to represent 50 ohm input impedance.
3. Conclusions
All demonstrated software can be an extremely effective pedagogical tool applied in teaching high
frequency electronics. The examples shown in the paper are only a small fraction of the material available
to students at Oakland University. Collecting the software to support their courses, the authors of the paper
prepared initially a broad selection of examples limited only to applications of PSPICE and PUFF [3]. Later
in this process, the authors expanded their search thanks to the efforts of various companies, which decided
to make many of their software packages available either on Internet, or as the demonstration samples. The
use of instructor-devised sets of examples for high frequency electronics related courses greatly extends
students’ interest, understanding and design capabilities in studying wireless technologies at an extremely
low cost.
References
[1] Behzad Razavi, RF Microelectronics, Prentice Hall 1997.
[2] S-Parameter Design, HP Application Note 154.
[3] A.Rusek, B. Oakley, Pspice Applications in the Teaching of Wireless and High Frequency Electronics,
Proceedings of the 2001 American Society for Engineering Education Annual Conference &
Exposition.
[4] AppCAD2.0, Agilent Technologies, www.agilent.com/view/rf
[5] Serenade SV 8.5 PC, www.ansoft.com
[6] S. W. Wedge, R. Compton, D. Rutlege, PUFF, Computer Aided Design for Microwave Integrated
Circuits, Version 2.0, CALTEC, 1991