We will demonstrate a CAD program designed for the lay out

and analysis of microstrip circuits. The program runs on the

IBM PC or AT. Circuit elements are selected from a parts list

and drawn on the screen using the cursor keys. The analysis

may then be performed, directly from the screen drawing. Puff

has been used by microwave students in classes at the California

Institute of Technology and the University of California at Los

Angeles

Compton, Richard C.

Rutledge, David B.

Williams, Wyman L.

English

Caltech Authors - Main

Puff, an Interactive Microwave Computer Aided Design

Program for Personal Computers

Q=42
Puff, an Interactive Microwave Computer Aided Design
Program for Personal Computers
Richard C. Compton, Wyman L. Williams and David B. Rutledge
Division of Engineering and Applied Science
California Institute of Technology
Pasadena, CA 91125.
Abstract
We will demonstrate a CAD program designed for the lay out
and analysis of microstrip circuits. The program runs on the
IBM PC or AT. Circuit elements are selected from a parts list
and drawn on the screen using the cursor keys. The analysis
may then be performed, directly from the screen drawing. Puff
has been used by microwave students in classes at the California
Institute of Technology and the University of California at Los
Angeles.
Introduction
There are several difficulties in using commercially available
microwave CAD software in an educational environment.
These programs are copy protected which makes them hard
to distribute onto a large number of personal computers on
a campus and at the same time prevent unauthorized use
and copying. Second, the source code and algorithms for
these programs are proprietary, so that it is not possible
to determine how the programs operate or how to modify
them for your own purposes. Finally, these are big, pow-
erful programs with thick manuals, which means that a lot
of time is spent learning to use the program rather than
learning about microwave circuits. We have written a pro-
gram, called Puff after the magic dragon in the folk song,
which overcomes many of these problems. Puff uses several
unique tools to provide the user with a fast and easy-to-use
microwave CAD package. The code is written in Turbo
Pascal, a popular inexpensive compiler for IBM personal
computers, and can be easily modified.
Puff’
Figure 1 shows how the screen is organized. There are three
main window blocks that are simultaneously displayed: a
Circuit window, a Plot window, and a Parts list. You can
move from window to window by pushing one of the func-
tion keys (F1. . .F3). Once you are in a particular window,
there are about ten commands that can be used in that
window, and they are shown on a command window in the
top center of the screen. The number of commands in each
window is kept deliberately small. If there are too many
commands, we found that the user starts to freeze up and
retreat to the manual.
Command window,
1n 23 4
+4essage windaw
k
[
PARTS COMtiANDS
De1,Backspace, Ins
l + f & Cupsm. 1
F1 : CIRCUITsetup TDe,ijn J______
F3 : PPRN z=5Bfi fd=S. E GHz
lr
F2 : PLOT
a lumped 15BQ 4mn fd/# 2E
b tline 5LM98” Srnitk radius 1
c tl ine 5Ell 68”
d t 1ine 5fI$l 138”
Part window A Plat window~ g=lJ I %=1
y=, , ~=-,
Figure 1. Pufl Enhanced Graphics Adaptor screen dump,
obtained from a dot-matrix printer.
The Parts list contains the different things that the user
might want to put in the circuit: transmission-line sections,
coupled-line sections, lumped elements, and user-specified
devices like tra&istors. Parts may be added or changed
using a screen editor built into Puff. When you push a
cursor key in the Circuit window, Puff draws a particular
element from the Parts list, oriented in the direction of
that key. We have found it dramatically easier to draw the
circuit this way rather than to describe the circuit with a
text file. The circuit drawing provides information about
the relative size and spacing of the components and helps
with the lay out and positioning of the various elements.
The number of errors is greatly reduced since ,the circuit
on the screen is a scale version of the circuit that will be
made. You cannot get a direction or a connection wrong,
or miss the impedance by a factor of ten if you can see it
on the screen.
707
0149 -645 Xi8710000-0707$01 .00 0 1987 IEEE 1987 IEEE MTT-S Digest
One can push the p key to plot the circuit behaviour at any
time. Pufl displays the s-parameters, at each frequency,
as they are calculated and then joins the points using a
spline interpolation. A plot typically takes 5 seconds on an
IBM AT. The s-parameters are displayed on a Smith chart
and a rectangular dB plot. The plotting variables may be
changed, to vary the plotting scales and s-parameters that
are calculated, by typing directly over existing values. Cir-
cuit performance can be optimized by varying any of the
parameters in the parts list. Each time a part is altered
Puff redraws the circuit to take into account changes in a
part’s dimensions. The redraw is performed by stepping
through a linked-list of key strokes that were originally used
to lay out the circuit. A moveable marker allows you to
read off the calculated values at different frequencies. The
frequency response may then be fast fourier transformed to
provide time-domain information. The time-domain plots
are interesting, because they demonstrate the reflections as-
sociated with impedance discontinuities. When the design
is complete, Pufl can produce a 5:1 circuit mask on a stan-
dard IBM graphics printer. The printer makes two passes,
and achieves a resolution of about 150 dots per inch.
Figure 2. shows a PuB lay out for a simple 4 GHz low noise
f?f3T amplifier circuit. The transistor behaviour is obtained
from a file of s-parameters taken from the manufacturers
data sheet. The stubs at the input provide noise matching
while the output stubs provide a conjugate power match.
Pufi has also been used to design oscillator circuits by using
a feedback loop around the transistor. In addition, Pufl has
been used to lay out and analyze branch-line and rat-race
couplers, low-pass filters, and coupled-line bandpass filters.
1H3 d> a ,. 2b
F1 : CIRCUIT trans
– PLOTCOITL=INDS
1
++t&-*op
p,ctrl-p plot
PgUp,PgDnnarker
Ctrl-s saw f i Ie
Ctrl-a artuwk
t time b-main
Esc exit
- F3 : rmrs z=5812 f d=4 .BEGHZ
a tline 501 72’
b tline SIl 61”
c tline WI 1S”
d device fsclt7 4mn
e
f 1
I
1
28
1s1
dB
-20q
3 f GHz 5
- !2 : PLOT
~ &/d 20
I 7
itk radius 3.6
@
Figure 2. PuB lay out showing an FET amplifier circuit. Each
of the elements in the Circuit drawing (top left) are identified by
a single letter corresponding to a component in the Parts list.
Element d is a I?ujitsu fsc10 transistor. The Smith chart and
rectangular plot show sII , SZI
Algorithms
The s-parameters for each of the circuit elements are com-
bined using the subnetwork growth technique[l,2]. In this
method connectors between two networks are joined and
the s-parameters associated with the neighboring networks
are recalculated using one of two simple formulas depending
of whether the connectors belong to two different networks
or the same network. These formulas may be derived by
applying Mason’s rule [3] to signal flow graph’s for the two
cases (Fig 3.) [4]. When all the connectors are joined the
analysis is complete. The circuit is stored by Puff as a
linked-lists of networks and connections. The dynamic na-
ture of these lists allows them to grow when adding parts
to the circuit and collapse when the analysis takes place.
( (
I I
,,5s”
Ok =b,
(b)(?.)
Figure 3. Block diagrams and the corresponding signal-flow
graphs for the two basic operations in the subnetwork growth
method: joining connectors that belong to (a) different networks
or (b) the same network.
Acknowledgements
The authors wish to acknowledge assistance provided by
Professor N. C. Luhmann Jr. at the University of Cali-
fornia, Los Angeles. This work was supported by the Na-
tional Science Foundation. Mr Williams acknowledges the
support of the Fannie and John Hertz Foundation.
References
[1] G. Filipsson,<(A new general computer algorithm for
s-matrix calculation of Interconnected mult iports ,“ Proc.
llth Euro. Microwave Con f., pp. 700-704, 1981.
[2] K. C. Gupta, R. Garg, and R. Chadha, Computer-aided
design of microwave integrated circuits, Artech House, Ded-
ham, Massachusetts, 1981.
[3] S. J. Mason, “Feedback Theory–Further Properties of
Signal F1OW Graphs,” Proceedings of the IRE, 44, pp. 920-
926, July, 1956.
[4] R. C. Compton and D. B. Rutledge,’’Microwave Com-
puter Aided Design and Education using Personal Com-
puters,” to be submitted to the MTT Transactions Special
Issue on Computer Aided Design.
708


Computer-aided design of microwave integrated circuits, Artech House,

Feedback Theory–Further Properties of Signal

Filipsson,<(A new general computer algorithm for s-matrix calculation of Interconnected mult iports ,“

Rutledge,’’Microwave Computer Aided Design and Education using Personal Computers,”

http://authors.library.caltech.edu/32110/1/COMims87.pdf

Puff, an Interactive Microwave Computer Aided Design Program for Personal Computers

Abstract

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