QEX_2002_06.pdf

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Forum for Communications Experimenters

November / December 2002

Inside! Ulrich Rohde

begins a series on receiver

performance and measurements

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About the Cover

Receiver measurements

defined: The article

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Features

3 Theory of Intermodulation and Reciprocal Mixing:

Practice, Definitions and Measurements in Devices

and Systems, Part 1

By Ulrich L. Rohde, KA2WEU/DJ2LR/HB9AWE

16 A 700-W Switch-Mode Transmitter for 137 kHz

By Andy Talbot, G4JNT

27 A Software Defined Radio for the Masses, Part 3

By Gerald Youngblood, AC5OG

37 Linrad: New Possibilities for the Communications

Experimenter, Part 1

By Leif Åsbrink, SM5BSZ

42 A Guy-Wire-Interaction Case Study

By Dick Weber, K5IU

49 Using the HP Z3801A GPS Frequency Standard

By Bill Jones, K8CU

Columns

54 Tech Notes

56 RF By Zack Lau, W1VT

60 2002 Index

62 Letters to the Editor

63 Next issue in QEX

Nov/Dec 2002 QEX Advertising Index

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Nov/Dec 2002 1

THE AMERICAN RADIO

RELAY LEAGUE

The American Radio Relay League, Inc, is a

noncommercial association of radio amateurs,

organized for the promotion of interests in Amateur

Radio communication and experimentation, for

the establishment of networks to provide

communications in the event of disasters or other

emergencies, for the advancement of radio art

and of the public welfare, for the representation

of the radio amateur in legislative matters, and

for the maintenance of fraternalism and a high

standard of conduct.

ARRL is an incorporated association without

capital stock chartered under the laws of the

state of Connecticut, and is an exempt organiza-

tion under Section 501(c)(3) of the Internal

Revenue Code of 1986. Its affairs are governed

by a Board of Directors, whose voting members

are elected every two years by the general

membership. The officers are elected or

appointed by the Directors. The League is

noncommercial, and no one who could gain

financially from the shaping of its affairs is

eligible for membership on its Board.

“Of, by, and for the radio amateur, ”ARRL

numbers within its ranks the vast majority of

active amateurs in the nation and has a proud

history of achievement as the standard-bearer in

amateur affairs.

A bona fide interest in Amateur Radio is the

only essential qualification of membership; an

Amateur Radio license is not a prerequisite,

although full voting membership is granted only

to licensed amateurs in the US.

Membership inquiries and general corres-

pondence should be addressed to the

administrative headquarters at 225 Main Street,

Newington, CT 06111 USA.

Empirical Outlook

New Technology Presents

New Challenges

Amateur Radio systems are now be-

ing fielded that provide Internet gate-

ways, store-and-forward functions,

remote control and other fascinating

possibilities. Additionally, new high-

speed HF digital modes sporting up-

ward of 3 kbps are coming that allow

simultaneous voice, data and even

video. It will not be long before they’re

here. Several HF systems having those

features are slated for release in 2003.

We like to think we’re allowed un-

limited experimentation with those

possibilities, but the current US Rules

don’t encourage high-speed data

modes on HF. Symbol rates are limited

to 300 bauds.

At their July 2002 meeting, the

ARRL Board voted to petition for allo-

cation of Amateur Radio sub-bands

based on occupied bandwidth rather

than on mode. Success would open the

door to high-speed, voice-bandwidth

digital simultaneous voice and data on

HF. Note that other countries already

have that, and it’s been done in other

services for many years now. It’s a pro-

posal that seems to make sense and we

endorse it. The Board also authorized

President Haynie to form a working

group to investigate and report on

such HF high-speed data modes, HF-

Internet gateways and HF automatic

control with a final report due by

January 2003.

Because of its implications for our

service, this important initiative de-

serves wide discussion. Its pros and

cons must be carefully evaluated and

details worked out. The ability to send

a compressed 20-kbyte file in less than

a minute and to construct high-speed

networks on HF is state-of-the-art per-

formance that hams should have.

Networking greatly amplifies the

power of personal computing. In-

creased connectivity enhances the

capacity and reliability of communica-

tions networks. Remote and automatic

control, message store-and-forward

systems and frequency diversity are

among many fertile fields of experi-

mentation in Amateur Radio. We have

a chance to further our capabilities in

high-speed data, digital voice and soft-

ware radio. This may be part of the

Amateur Radio resurgence we antici-

pated exactly three years or 18 issues

ago. High-speed, bandwidth-conserva-

tive data modes tend to use what is

called orthogonal frequency-division

multiplexing, or OFDM. It has gotten

a lot of attention since high-definition

digital television (HDTV) came onto

the scene. OFDM is a way to increase

throughput by using many sub-carri-

ers, each of which bears a fraction of

the total data rate. OFDM tends to

produce high peak-to-average ratios

that limit average power and place se-

vere linearity demands. Subcarriers

spaced at constant frequency intervals

produce IMD that falls within adja-

cent sub-channels. Phase noise from

nearby subcarriers may limit signal-

to-noise ratios because of reciprocal

mixing.

Designers are using predistortion in

DSP and other adaptive-compensation

techniques to combat deleterious ef-

fects. They are the subjects of ongoing

research. Tie-ins among digital voice,

software radio and high-speed multi-

media networking are evident. Write

us or visit www.arrl.org/tis/info/

sdr.html for updates.

In This Issue

We are delighted to welcome back

Ulrich Rohde, KA2WEU . He investi-

gates receiver dynamic range and

IMD measurements in the first seg-

ment of his two-part series. We’re

confident it is of interest to those se-

rious about comparing the large-sig-

nal performance of receivers. Andy

Talbot, G4JNT , of digital-voice fame

(see May/June 2000) shows us how he

built his LF transmitter. Be among

the first on the air when we get

136 kHz!

Gerald Youngblood, AC5OG , con-

tinues his popular series on software-

defined radio (SDR). Gerald’s fourth

segment will come in the Mar/Apr

2003 issue. For you in the Linux

world, Leif Åsbrink, SM5BSZ , begins

a presentation of his Linrad system.

When coupled with minimal hard-

ware, this shareware can get you into

the SDR game very quickly.

Dick Weber, K5IU , donates his case

study on guy-wire/antenna interac-

tion. Modeling and measurement rec-

onciled—hoorah! Bill Jones, K8CU ,

describes the use of surplus Hewlett-

Packard GPS receivers as frequency

standards for weak-signal work. In

Tech Notes , Tom Cefalo, W1EX, de-

scribes a way to fashion custom meter

faces with computer technology. In

RF , Zack Lau, W1VT, analyzes opera-

tion of

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KF6DX, kf6dx@arrl.org

2 Nov/Dec 2002

Theory of Intermodulation and

Definitions and Measurements

in Devices and Systems, Part 1

A man of renown in the communications field

begins an explanation of receiver performance.

By Ulrich L. Rohde, KA2WEU/DJ2LR/HB9AWE

digital receivers and gives a treatment of intermod-

ulation distortion. Starting from a mathematical—

but simple—derivation, it looks at gain compression,

intermodulation distortion and dynamic range. Further-

more, a new dynamic measure is introduced that allows

one to differentiate between different receive systems much

better. Other important characteristics are triple-beat dis-

tortion and cross modulation. To describe the performance

of broadband systems, noise-power ratio is also considered.

Large-signal effects which go beyond the linear range,

such as AM-to-PM conversion, spectral re-growth and ad-

jacent-channel power ratio are considered, as well as phase-

response differential group delay and reciprocal mixing.

This tutorial is a good prerequisite in preparation for look-

ing into high-performance components such as high-IP 3

mixers and modern receiver architectures.

Amplitude Linearity Issues and Figures of Merit 1

A network’s amplitude nonlinearity can be character-

ized by the expansion:

(

)

[

(

)]

[

(

)]

higher

order term

(Eq 1)

where y represents the output, the coefficients k n repre-

sent complex quantities whose values can be determined

by an analysis of the output waveforms, and f ( x ) repre-

sents the input. Although all practical networks exhibit

amplitude nonlinearity, we can (and often do) refer to many

networks as “linear.” We say this of networks that are suf-

ficiently amplitude-linear for our purposes— for example:

weakly nonlinear networks in which small-signal opera-

Synergy Microwave Corporation

201 McLean Blvd

Paterson, NJ 07504

ulrohde@aol.com

1 Notes appear on page 15 .

Nov/Dec 2002 3

Reciprocal Mixing: Practice,

T his two-part paper deals with both analog and

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