06.pdf

Contents

6.1 Circuit Simulation Overview

6.1.1 Hobby Circuit Simulation versus

Circuit Simulation in Industry

6.1.2 Real Circuits versus Simulations: The

Difference

6.2 Computer-Aided Design Examples

6.2.5 Example 5: Simulating the

40-Meter Filter in an RF-Fluent

Simulator

6.2.6 Example 6: SPICE and

Intermodulation Modeling

6.2.7 Circuit CAD in the Radio

Amateur’s Toolbox

6.3 From CAD to CAM: Using Circuit

CAD Tools for Circuit Board

Production

6.3.1 From Pins and Wires to Pads,

Traces and Vias

6.4 References and Further Reading

6.2.1 Example 1: A Two-JFET VFO and

Buffer

6.2.2 Example 2: Modeling a Phase-Lead

RC Oscillator

6.2.3 Example 3: Exploring Issues of

Modeling Gain, Link Coupling and

Q with a 7-MHz Filter

6.2.4 Example 4: Checking Reality with a

10-dB Attenuator

Chapter 6

Computer-Aided

Circuit Design

6.1 Circuit Simulation Overview

That computers are now used wherever information is generated, communicated and used

is everyday knowledge. We expect, or at least can infer if we take a moment to think, that a

computer was used to type and typeset these words, and to compose the pages of this book

and to design its cover. Thinking a bit more deeply, we realize that computers are used in just

about every phase of product design and process design and control, including the electronic

devices we use at work and play. The widespread application of computers is possible because

computers are, at base, generic math machines. The TV set you watch, the radio transceiver

which with you talk to the world, is a math machine — math put to work in the form of a

kinetic sculpture built from copper, carbon, plastic, silicon and steel. You feed radio (or audio)

and ac-line energy — both of which can be readily described numerically — in. The TV or

radio performs mathematical operations on those inputs, and a desired, quantifiable result

comes out: a sound, a moving picture, a Field Day contact, a declaration of rook to queen’s

bishop three from a chess opponent in Spain.

Because radio-electronic circuitry “just does math,” math can predict and analyze the action

of electromagnetic signals and the radio-electronic circuitry we build to produce and process

them. Program an electronic computer — which, at base, is a generic math machine — to do

radio/electronics math in practically applicable ways, and you’re ready to do computer-aided

design (CAD) of radio and electronic circuits. (Program a computer to do radio-electronics

math in real time , and you’re ready to replace radio-electronics hardware with software, as

this book’s DSP and Software Radio Design chapter describes.)

This chapter, by David Newkirk,

W9VES, introduces Amateur Radio

builders and experimenters to fun-

damentals of using computer-aided

design (CAD) tools for RF design.

These tools enable the hobbyist

to harness the circuit-simulation

power employed by professional

electronic and RF engineers work-

ing in large-scale electronic and RF

design and production for industry.

This chapter is about generic

circuit CAD tools capable of

handling arbitrary circuit topologies.

Software to aid design and analysis

in specialized areas, for example

RF filters, switchmode power

supplies, transmission lines and

RF power amplifiers, is covered in

other chapters.

6.1.1 Hobby Circuit Simulation versus Circuit Simulation in

Industry

Professional-grade circuit simulation software exists to facilitate the construction of tightly

packaged, highly integrated, no-tweaking-required modern electronics/RF products. These

products work predictably well even when reproduced by automated processes in large

quantities — quantities that may, with sufficient marketing success and buyer uptake, exceed

millions of units.

Manufacturers of specialized electronic design automation ( EDA ) software serve the engi-

neering needs of this industry. Through comprehensive CAD suites, one may proceed from

graphical component-level circuit and/or IC design ( schematic capture ), through simulation of

circuit and IC behavior (often using a variant of the simulator called SPICE , but increasingly

with non- SPICE simulators more fluent in issues of RF and electromagnetic design), through

design of PC board and IC masks suitable for driving validation, testing and production.

Comprehensive EDA CAD reduces costs and speeds time to market with the help of features

that can automatically modify circuits to achieve specific performance goals ( optimization );

predict effects of component tolerances and temperature on circuit behavior across large

populations of copies ( Monte Carlo analysis ); and generate bills of materials (BOMs) suitable

for driving purchasing and procurement at every step of the way.

Free demonstration versions are available for some EDA CAD products (see Table 6.1 ),

and a subset of these are especially useful for hobby purposes. Although these demoware

tools come to us with a large-scale-production pedigree, they are greatly (and strategically)

Computer-Aided Circuit Design

6.1

Table 6.1

Some Sources of Freeware/Demoware Simulation, Schematic and Layout CAD Software

Source

Address

Resource

Ansoft

www.ansoft.com/

Ansoft Designer SV 2 (schematic, linear RF simulator, planar

electromagnetic simulator, layout [PCB] design), more

Cadence Design Systems

www.cadence.com/

OrCAD 16 (schematic, SPICE simulator, layout [PCB] design)

CadSoft

www.cadsoft.de/

EAGLE schematic and layout design

www.cadsoftusa.com/

gEDA

www.gpleda.org/

GPLed suite of electronic design automation tools

Kicad

iut-tice.ujf-grenoble.fr/kicad/

GPLed full-function schematic and layout design

Linear Technology Corp

www.linear.com/

LTSpice (schematic, SPICE simulator enhanced for power-system

design)

SPICE: What’s in a

Name?

SPICE — Simulation Program with

Integrated-Circuit Emphasis — origi-

nates from the Electrical Engineering

and Computer Sciences Department of

the University of California at Berkeley

and irst appeared under its current

name as SPICE1 in 1972. “SPICE” ,

write the maintainers of the oficial

SPICE homepage at bwrc.eecs.

berkeley.edu/Classes/icbook/SPICE/ ,

“is a general-purpose circuit simulation

program for nonlinear dc, nonlinear tran-

sient, and linear ac analyses. Circuits

may contain resistors, capacitors, induc-

tors, mutual inductors, independent volt-

age and current sources, four types of

dependent sources, lossless and lossy

transmission lines (two separate imple-

mentations), switches, uniform distribut-

ed RC lines, and the ive most common

semiconductor devices: diodes, BJTs,

JFETs, MESFETs, and MOSFETs.”

That SPICE is “general-purpose”

does not mean that its usefulness is

unfocused, but rather that it is well-

established as a circuit-simulation

mainstay of comprehensive power. A

wide, deep SPICE ecosystem exists

as a result of decades of its daily use,

maintenance and enhancement by

industrial, academic and hobby users.

Many excellent commercial versions of

SPICE exist — versions that may be

improved for workhorse use in particular

subdisciplines of electronics, power and

RF design.

Throughout this chapter we will use

a particular commercial variant of the

SPICE simulator: PSpice as embodied

in Cadence Design System’s OrCAD

16.0 EDA CAD demonstration software

suite. With those particulars declared,

we will usually refer to the underlying

simulator merely as SPICE , expecting

and intending that simulations pre-

sented in this chapter will be duplicable

with any modern commercial SPICE

variant. Only details associated with

value-added features, such schematic

and layout editing and simulation-results

reporting, will vary.

CAD Software and Your Computer’s Operating

System

The OrCAD 16.0 and Ansoft Designer SV 2 demoware packages used in this

chapter, and most other CAD products you’re likely to use, are compiled to run under

Microsoft Windows . So what if you want to run RF and electronics CAD software

under Linux or on the Mac?

You’re in luck. EAGLE schematic and layout software is available in native versions

for Windows , Linux , and the Macintosh. The GPLed EDA application suite, gEDA , are

primarily developed on Linux , but are intended to run under, or at least be portable to,

POSIX -compliant systems in general. The GPLed schematic and layout editor Kicad

runs natively under Windows and Linux , and has been tested under FreeBSD and

Solaris . Further, the great strides made in the Wine translation layer ( www.winehq.

com/ ) allow many applications written for Windows to run well under the operating

systems supported by Wine , including Linux and the Macintosh.

In your writer’s experience, MicroSim DesignLab 8 (a widely distributed precursor

to OrCAD 16 that can run all of the SPICE examples described in this chapter) and

Ansoft Serenade SV 8.5 (a precursor to Ansoft Designer SV 2 ) can be run in Wine

under Linux with few artifacts and their expected schematic-capture and simulation

capabilities intact. Cursor handling in OrCAD 16 installs under Wine readily enough.

but cursor-handling artifacts in its schematic editor, at least in the computers tried,

seems to preclude its use under Wine for now. Ansoft Designer SV 2 installs but does

not properly start.

All things considered, however, especially as Wine and CAD applications continue

to strengthen and mature, running your favorite Windows -based applications under

Wine is well worth a try.

feature-limited. Only a relatively few com-

ponents, often representing only a subset of

available component models, may be used per

simulation. Monte Carlo analysis, optimiza-

tion, BOM generation and similar enhance-

ments are usually unavailable. Demoware is

intended to drive software purchasing de-

cisions and serve as college-level learning

aids — learning aids in college study toward

becoming electronics/RF professionals who

will each day work with the unlimited, full

versions of the demoware.

The radio hobbyist’s circuit-simulation

needs are much simpler. Most of us will build

only one copy of a given design — a copy that

may be lovingly tweaked and refined to our

hearts’ content far beyond “good enough.”

Many of us may build as much with the intent

of learning about and exploring the behav-

ior of circuits as achieving practical results

with them. A demoware circuit simulator can

powerfully accelerate such self-driven explo-

ration and education in electronics and RF.

6.1.2 Real Circuits versus

Simulations: The Difference

Having just finished stuffing and soldering

the PC board for a binaural CW receiver, you

find yourself in need of a 7-MHz local oscil-

lator (LO) to drive it. The receiver designer

has provided just such a circuit: a junction

field-effect transistor (JFET) Hartley oscil-

lator driving a JFET amplifier to an output

power of +10 dBm. You obtain the parts,

perhaps substituting components with close-

enough values here and there as your junk box

suggests and your experience allows. Having

completed its construction, you interconnect

it, the main receiver board, a power supply,

headphones and an antenna. You power up

the receiver, hear 40-meter signals, and —

6.2

Chapter 6

perhaps with the aid of a frequency counter

or second receiver — adjust the tuning range

of your new LO to cover your favorite part

of the band. Another project done, another

success achieved — what could be simpler?

Certainly not modeling the same circuit in

circuit-simulation software — precisely be-

cause all actions in the real world are always

maximally and unimaginably complex .

The components we use to build real cir-

cuits always operate to the full robustness of

their intrinsic properties, their simplicity to

our mind’s eye notwithstanding — regardless

of our relative ignorance of what those proper-

ties may be and how and why they operate the

way they do. No real-world component oper-

ates ideally. Nothing — excepting, perhaps,

events in the realm of quantum physics — is

sketched in, so far as science has been able to

determine. So it is that we may set out to build

an amplifier only to discover at power-up that

we have instead built a persistent oscillator. So

it is that we may design, successfully build,

and publish a circuit intended to be an oscil-

lator only to discover that for 3 out of every

100 subsequent reader-builders it does not

oscillate at all!

When we take the “just build it” approach

with circuit simulation software — that is,

when we expect components to “just work”

as they do in real-world circuits — we may

run into trouble rapidly. This can happen be-

cause the electrical and electronic compo-

nents available in circuit simulators are only

mathematical models of real-world compo-

nents — because every modeled behavior of

every modeled component is only sketched

in relative to the behaviors of its real-world

counterpart. Simulated component character-

istics and behaviors approach those of real-

world components only as closely as science

may allow and only as closely as the simula-

tor’s designers, engineers and users need its

simulations to mirror real-world behaviors to

meet their design goals. Once a design works

well enough, reproducibly enough, cheaply

enough, it goes into production, ultimately to

ship in the hundreds, thousands or millions.

Finalizing an Amateur Radio circuit design

is less spectacular in comparison. Even if the

designer of our figurative VFO provided a

quantitative specification for its performance

— +10 dBm output — we very likely will

not ultimately confirm whatever numbers

we have before happily putting the project

into service. We’re hearing signals and mak-

ing radio contacts — the likely object of the

exercise, after all — and so we consider our

construction project successful.

Were this chapter a textbook, or part of a

textbook, on computer-aided circuit design,

we might begin our CAD explorations by re-

viewing the basics of what electronic circuits

are and do, following this with a discussion

of what computerized circuit simulation is

and how it works. An excursion into the ar-

cane world of active-device modeling — the

construction and workings of mathematical

electrical equivalents to the transistors, di-

odes and integrated circuits that await us at

our favorite electronics suppliers and in our

junk boxes — might follow. Finally, we might

systematically proceed through a series of

simulation examples from the basic to the

more complex, progressively building our

store of understood, trustworthy and appli-

cable-to-future-work circuit-CAD concepts

as we go.

But this is a chapter in a handbook, not a

textbook, and following a sequence of abstract

basics to concrete practice very likely does not

reflect the process most of us have followed,

and follow yet, in learning and using what

we know about electronics and radio. More

realistically, our approach is more like this:

We find ourselves in need of a solution to a

problem, identify one, and attempt to apply it.

If it works, we move on, likely having learned

little if we have not had to troubleshoot. If

the solution does not work, we may merely

abandon it and seek another, or — better, if

we are open to learning — we may instead

seek to understand why, with the happily re-

vised aim of understanding what we need to

understand to make the solution work. Even

if we must ultimately abandon the solution

as unworkable in favor of another, we do not

consider our time wasted because we have

further accelerated our deepening intuition by

taking the initiative to understand why.

This chapter introduces circuit CAD in

exactly that spirit: Especially at first, we will

learn about circuit simulation through “just

building,” and simulating with CAD, build-

able circuits we know should work in quanti-

fied and quantifiable ways that we also know

well. If they don’t work as they should, we will

investigate why, fix them and review what we

have learned — just as we do when building

and learning from real-world hardware.

6.2 Computer-Aided Design Examples

6.2.1 Example 1: A Two-JFET

VFO and Buffer

In introducing this chapter, we “just built”

— in our mind’s eye — a receiver LO that “just

worked,” as real-world projects are intended

and expected to do. Building and simulating

the same two-JFET design in the circuit simu-

lator called SPICE transports us right to the

heart of fundamental circuit-modeling prob-

lems and their solutions. If we build through

graphical schematic entry — through sche-

matic capture — its oscillator-buffer circuit

exactly according to its electrical schematic,

it cannot possibly work!

Fig 6.1 shows the circuit for real-world du-

plication. In it, a J310 JFET Hartley oscillator

drives a J310 buffer amplifier, which drives

a 50-Ω load at +10 dBm via a trifilar broad-

band transformer that provides an impedance

step-down ratio of 9:1. Any experimenter ac-

customed to building and using such circuits

needs more no information than that provided

in the Fig 6.1 schematic and its caption to

make the circuit work as expected.

Fig 6.2 shows the same circuit successfully

modeled in OrCAD 16.0 , SPICE based simu-

lator software from Cadence Design Systems.

To understand the differences between Fig 6.1

and Fig 6.2, we’ll examine the simulation’s

components type by type.

in SPICE -speak — differs from what we are

generally accustomed to seeing in electrical

schematics, and that we have multiple options

for specifying values numerically using inte-

ger and decimal floating-point numbers. The

scale factors available in SPICE include:

• F — 1E–9

• G — 1E9

• K — 1E3

• M — 1E–3

• MEG — 1E6

• MIL — 25.4E–6

• N — 1E–9

• P — 1E–12

• T — 1E12

• U — 1E–6

Specifying the value of R1 as 1M would

declare its value as 1 milliohm (0.001 Ω),

short-circuiting the JFET’s gate to common

and breaking our simulation. Specifying the

RESISTORS

The designer’s output power specification

(+10 dBm) assumes that the VFO is connected

to the 50-Ω load afforded by the mixer system

in the receiver it was designed to drive. To

simulate this mixer load, we have added R5.

The value of R1, specified as 1 MΩ in the

original circuit, is now specified as 1E6 — sci-

entific/engineering notation for one million.

We have done this to remind ourselves that

SPICE ’s use of unit suffixes — scale factors

Computer-Aided Circuit Design

6.3

Fig 6.1 — Standard electrical representation of a 7-MHz VFO with buffer ampliier. JFET Q1 operates as a Hartley oscillator; D1, as

a limiter that improves frequency stability by keeping Q1’s gate voltage from going more positive than about 0.6 V; and Q2, as an

ampliier that increases the oscillator output — obtained from the feedback tap on the oscillator inductor by capacitive coupling

(C57) — to +10 dBm. The tapped inductor (L9), is 1.2 µH (22 turns of #28 wire on a T-30-6 toroidal core); the triilar output transformer

(T2), 10 triilar turns of #28 wire on an Amidon FB-43-2401 ferrite bead).

Using Your Computer to Draw Schematics

The art of drawing circuit schematics predates electronic computers; the art of drawing schematics and PC-board layouts

with computers predates the art and science of simulating circuits with computers. What if you only want to draw a circuit’s

schematic and design a PC board without simulating it? What computerized tools are available to you ?

Almost any circuit-simulation program or electronic design automation (EDA) suite that uses schematic circuit capture can,

within functionality limits imposed on the demoware version, serve as a irst-rate schematic editor. Although demoware compo-

nent library limitations usually restrict the types of components you can use — in CAD-speak, place — in a design, part- count

limitations usually operate only at simulation time. Restrictions in physical size and layer count, and in the materials and metal-

lizations available for substrate speciication, will likely apply to whatever layout-design facilities may be available. After all, the

main purpose of demoware is to let students and potential buyers taste the candy without giving away the store.

Excellent simulation-free schematic-capture and layout-design products exist, of course. The schematic style long standard

in ARRL publications comes from the use of Autodesk AutoCAD , a fully professional product with a fully professional price.

Long popular with radio amateurs and professionals alike is CadSoft EAGLE , a schematic-capture and layout-design product

available in freeware and affordable full-version forms. You can even export EAGLE schematics to a SPICE simulator and back

with Beige Bag Software’s B2 Spice . The full-function freeware schematic and PCB-layout application Kicad and the EDA suite

gEDA come to us from the open-source community.

Do-it-yourself schematic CAD can be as close as the basic drawing utility included with your computer’s operating system.

Some hobbyists ind that cutting, copying, moving and pasting components snipped from favorite graphical schematic iles into

new conigurations and adding new wires as graphical lines is enough for what they want to do. The connection between the

world of modern EDA tools and this seemingly primitive approach to schematic creation can be as close as the operating sys-

tem’s clipboard: Some CAD schematic-capture programs represent circuit elements as metaile data during copying, cutting

and pasting operations. Copying a schematic to the clipboard with such a program and then pasting the clipboard contents to

a suitable drawing program creates a picture of the copied schematic — give it a try with Windows Paint and the schematic

editor in the free demoware version of OrCAD 16 .

6.4

Chapter 6

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