Table Of ContentOTHER TITLES PUBLISHED IN THE SERIES
(FORMERLY PERGAMON SCIENCE SERIES ELECTRONICS AND WAVES)
Vol. 1 Signal Noise and Resolution in Nuclear Counter Amplifiers
by A. B. GILLESPIE
Vol. 2 Scintillation Counters
by J. B. BIRKS
Vol. 3 Probability and Information Theory, with Applications to Radar
by P. M. WOODWARD
Vol. 4 Physics and Applications of Secondary Electron Emission
by H. BRUINING
Vol. 5 Millimicrosecond Pulse Techniques (2nd edition)
by I. A. D. LEWIS and F. H. WELLS
Vol. 6 Introduction to Electronic Analogue Computers
by C. A. A. WASS
Vol. 7 Scattering and Diffraction of Radio Waves
by J. R. MENTZER
Vol. 8 Space-charge Waves and Slow Electromagnetic Waves
by A. H. W. BECK
Vol. 9 Statistical Theory of Signal Detection
by CARL W. HELSTROM
Vol. 10 Laplace Transforms for Electronic Engineers
by J. G. HoLBROOK
FREQUENCY
MODULATON THEORY
APPLICATION TO MICROWAVE
LINKS
by
Jacques FAGOT
Ingenieur E.S.E., Licencié és-Sciences
Technical Manager at the Compagnie Genérale de
Telegraphic Sans Fil
Lecturer at E.S.E., Radio
and
Philippe MAGNE
Ingenieur E.S.E., Radio
Laboratory Head at the Compagnie Genérale de
Telegraphic Sans Fil
Professor at I.S.E.P.
Foreword by
Roger AUBERT
Ingenieur E.S.E.
General Technical Manager at the Compagnie Genérale de
Telegraphic Sans Fil
Past President of the Société des Radioélectriciens
P E R G A M ON P R E SS
NEW YORK · OXFORD · LONDON
S O C I E TE F R A N C A I SE
DE D O C U M E N T A T I ON E L E C T R O N I Q UE
PARIS
Al
PERGAMON PRESS INC.
122East55th Street, New York 22, N.Y.
Statler Center640, 900 Wilshire Boulevard
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PERGAMON PRESS LTD.
Headington HillHall, Oxford
4 and5 Fitzroy Square, London, W.1
SOCIETE FRANCAISE
DE DOCUMENTATION ELECTRONIQUE
101 Boulevard Murat, Paris ]6e
Copyright
©
1961
PERGAMON PRESS LTD. and SOFRADEL (FRANCE)
First published in English 1961
Library ofCongress Card Number 60-14939
Set in Times New Roman IO/12pt
andPrintedin GreatBritainby
CHORLEY &PICKERSGILLLTD
LeedsandLondon
FOREWORD
OUR ancestors signalled their victories or important news by means of fires lit
on elevated ground. But the amount of information which could be transmitted
in this way was small. So, in the Middle Ages, when communication routes
became safer and better means of travel were available, this mode of com
munication was abandoned and substituted by the despatch of messengers
bearing a much greater amount of information. Hill-tops were then abandoned.
Yet, though there was a gain in the quantity of information, there was a sub
stantial loss in speed of transmission. Hence, in the eighteenth century there
was an endeavour to increase that speed, while retaining the possibility of
transmitting a large amount of information.
It was Chappe who, with his semaphore telegraph, solved the problem.
His Une from Paris to Lille was opened in March 1794. Fifty years later, the
French network consisted of 500 stations, which, once more, used the hill-tops.
It should be noted that speed of transmission was higher than might be
supposed, since only 2 min were required to transmit an elementary signal from
Paris to Lille, and only 20 min for the transmission of the same signal from
Paris to Toulon, through 116 relay stations.
However, in the nineteenth century the Chappe telegraph was dropped for
the Morse telegraph, the group propagation velocity and transmission capacity
of which were substantially greater.
Hill-tops were once more abandoned and, according to the perpetual evolu
tion of technique, it was only about a century later that the introduction of
microwave links once more drew attention to these high points for the installa
tion of the various relay stations which, as in the case of the Chappe telegraph,
must in practice possess an optical path.
The French microwave link system counts at the present day over 3000
miles of Une and close on one hundred relay stations. It is still under develop
ment, while in the French Union, and more particularly in the Sahara, tropo
spheric and direct Une-of-sight beams are beginning to be developed in order to
exploit these regions.
In accordance with a technique which has become quite general, all these
beams use frequency modulation and transmit either television signals, or
numerous telephone channels, multiplexed on the carrier-current technique.
These beams are a useful complement to the national and international
systems, in association with which they are to be considered more as comple
mentary than as competitive. Also, their transmission quality has been con
stantly improving and they now satisfy the same criteria as the international
cable telephony circuits.
FOREWORD
MM. Fagot and Magne's book first sets out the theoretical principles on
which such beams are based: propagation ofthe waves used, frequency modula
tion, noise and sundry distortions hable to arise either in the course of propaga
tion or in the equipment, and in the methods used for the transmission of the
information; it then goes on to describe the various devices used: modulators,
amphñers, mixers, regulators, aerials, multiplexing circuits for the various
channels, and measuring and maintenance gear. None were better suited than
the authors to write about this subject, since both of them for more than 10
years have effectively designed or supervised the design of much radio hnk
equipment buiU by the group of the Compagnie Genérale de Telegraphic Sans
Fil.
Their book, particularly noteworthy for its clear exposition, has the advantage
of containing not only the theoretical principles, but also many practical details
relative to equipment in service. At every point the reader will be happy to
find current numerical values, difficulties usually met with, and means apphed
for overcoming them.
This book is written not only for the student and the young engineer who
wishes to become famihar with microwave beam technique, but also for the
specialized engineer who will find within its pages a large amount of valuable
information. It is sure to secure a favourable reception from the pubhc, which
it fully deserves.
ROGER AUBERT
vni
INTRODUCTION
THE first applications of frequency modulation arose in the field of broadcasting
and of short range communication. The chief merits of the new system were
its anti-interference and anti-janaming properties. Metric waves, whose
development coincided with the introduction of the new process, had been
chosen for this purpose. Their use under conditions of direct hne-of-sight or
of diffracted propagation provided the large bandwidth, free of distortion,
which had been found necessary. It was in order to cover these apphcations
that the first general pubhcations treating frequency modulation were produced.
Since then the field of frequency modulation has shown a considerable
development. On the one hand, the range of usable waves has considerably
increased towards the higher frequencies, and on the other, new and important
apphcations have been envisaged. These apphcations have arisen through an
increasing demand for long-distance telephone hnks, and for the transmission
of programmes to television broadcasting stations. All these needs have been
covered by a new communication process which has become known as trans
mission by "radio links".
Terminal Repeater Repeater TArminni
FIG. 1.0
The structure of a radio hnk is found to be very similar to that of long
distance buried links using coaxial cables. Repeater stations located on high
ground ensure transmission by means of microwave sections with a direct
optical path. Power loss along the propagation path is compensated by the
repeater gain, in the same way as in "repeater" stations in buried cable telephony
systems, for the compensation of loss in cable sections. The beams, in the case
of telephony transmissions, carry a spectrum of transposed channels in accord
ance with cable technique (carrier current multiplex). They have a bilateral
structure; conveying the "multiplex" signals both in the 0-E (origin-extremity)
direction and in the E-O (extremity-origin) direction. The high-frequency
energy is concentrated by the aerials within highly directional beams. In the
case of television, the "video" signal takes the place of the telephone multiplex
spectrum.
INTRODUCTION
It is proper to stress the merits of frequency modulation for these new
applications. The foremost is undoubtedly the conservation of a constant
"transmission equivalent" on the modulation signal. The level obtained after
demodulation depends only on the "frequency deviation" obtained. It is
completely independent of the amplitude levels developed on the radio-frequency
along the connexion. It was in fact difficult to imagine a transmission with a
large number of link sections if level fluctuations due to fading were to have
repercussions on the characteristics of the signal transmitted. This would have
made it impossible, in particular, to couple the radio trunks to existing telephone
networks.
Other qualities are worthwhile. The great amplitude linearity obtained
on the modulation, and its independence in respect of the characteristics of
the amplifier tubes, for instance, makes it possible to secure correct conditions
of intermodulation between the various telephone channels ("cross-talk"
clauses).
But other factors intervene in this field; in particular, variations of time of
propagation in the useful waveband.
Lastly, the anti-interference properties of the system are of course most
valuable. All this makes it possible to envisage obtaining high quality in the
transmission characteristics, corresponding to the standards laid down by the
International Telephony Consultative Committee (CCIF) and by the Inter
national Radio Consultative Committee (CCIR).
This book deals with frequency modulation, with emphasis on its
applications to the new techniques considered above. The non-specialized
reader will find in it an exposition of all the general problems related to this
modulation process. The microwave link specialist will find the development
of special questions related to this mode of transmission. Below are a few details
on the composition of the book.
Part I examines the transfer of the radio-frequency energy over a given path;
that is to say the properties of propagation of radio waves: metric, decimetric
and centimetric, through the troposphere. This transmission is eflected by
direct path, or by diffraction, or again by the new process of tropospheric
scattering.
Part 2 concerns all the general problems of frequency modulation: principle,
band covered, distortion and improvement of the signal-to-noise ratio, compared
with amplitude modulation. It is shown that the curve of variation of propaga
tion time in the amplifiers used is an important factor as regards distortion.
Part 3 deals with propagation distortion: that which is apparent in a
variable-velocity guided transmission channel, as a function of the frequency
(dispersion velocity); that which appears on an echoing path either in free
space, or over a badly matched feeder.
Part 4 is concerned with the complete problem of telephony and television
transmissions over radio links. It considers in detail the requisite conditions
for meeting the international standards (CCIF and CCIR).
INTRODUCTON
Lastly, Part 5 deals with all the apphed techniques. It is more particularly
concerned with radio hnk equipment but, because of this, it deals with a large
number of general problems. The following may be quoted: obtaining a low
noise figure; highly hnear frequency modulation (modulators and dis
criminators); amphtude limiting. A large amount of space is also given to the
correction of propagation time distortion, and to quahty measurements.
References have been given at the end of each section, going, as far as
possible, back to original pubhcations. We have endeavoured to allow French
bibhography the space to which it is entitled.
Thanks are due to the Directors of the Compagnie Genérale de Télégraphie
Sans Fil, and more particularly to Monsieur Maurice Ponte who has kindly
given his permission for the publication of this book; to his engineers and to
those of the French PTT Administration (CNET). Their theoretical work
and their designs, of some importance in this field, are a valuable source of
information of which we have widely availed ourselves.
1
PROPAGATION—ENERGY TRANSFER
1.1. DEFINITION OF THE TRANSMISSION EQUIVALENT
THE notion of transmission equivalent is often used in telecommunications
technique. It was introduced in order to facilitate the establishment of the
energy balance in signals propagated along a transmission system. In particular
it makes it possible to draw the diagram of levels, essential to the user for
providing junctions of all kinds in a complex network. By means of this notion
it is possible to establish diagrammatically a microwave link from a trans
mission "model" in which the propagation spaces are represented by equivalent
attenuations.
9
— 1 — —1
FIG. 1.1
For example we shall define the transmission equivalent between two
successive aerials separated by a propagation space. Fig. 1 is a diagrammatic
representation of this space between planes A and B. Let us assume^ that the
energy is conveyed up to these planes by feeders of any type whatsoever (e.g.
waveguides). Let Ze and Zr represent the terminating impedances matching
these feeders. If Pe is the power through plane A in the direction of arrow (1),
and Pr that through plane Β in the direction of arrow (1) the transmission
equivalent is defined by the expression:
r= 10 logg (1.1)
It is expressed in decibels (this value being always negative).
The transmission model shown in Fig. 1.1 obeys the reciprocity theorem^.
Thus, when the roles of the transmitter and of the receiver are interchanged,
the following properties can be set down:
The propagation time between A and Β (arrows 1) is the same as the
propagation time between Β and A (arrows 2).
1
2 FREQUENCY MODULATION THEORY
The transmission equivalent between A and Β (arrows 1) is the same as
the transmission equivalent between Β and A (arrows 2).
These propositions are valid even if there are obstacles between planes
A and B, when the hypothesis of linearity of the Maxwell equations is verified
(ε and μ independent of the electric and magnetic fields). This condition
always obtains in the case of ultra-short waves propagated in the troposphere.
Propagation in the ionosphere does not obey this theorem, but such a mode
of propagation (usual in transmissions on medium and short waves) is not
used in microwave links.
1.2. PROPAGATION OF THE CARRIER WAVE IN FREE SPACE —
CALCULATION OF THE TRANSMISSION EQUIVALENT
BETWEEN AERIALS
A. Fundamental formulae
Aerials intended to radiate radio signals are connected to the transmitting
and receiving equipment by feeders whose characteristic impedance generally
corresponds to the terminating impedances equivalent to these aerials. Thus
we are dealing with the case of the matching required, in order to define the
transmission equivalent.
We have to calculate the ratio Pr/Pe in order to know T. Pe is the power
radiated by the transmitting aerial and Pr is the power absorbed by the receiving
aerial.
Since the transmitting and receiving stations of two successive repeaters
are exclusively designed to communicate one with the other, it is advantageous
to radiate energy only in the favoured direction connecting these two stations.
The energy will therefore be propagated along a beam directed in a line parallel
to that joining the aerials of two successive repeaters. Aerial directivity is
obtained by means of radiating structures whose dimensions are large compared
to the wavelength.
In the case of centimetric waves, the aerial is usually a metal envelope. This
generally constitutes a reñector which, when excited at its focus, radiates
directionally by a mechanism somewhat analogous to that of focusing mirrors
in optics. A horn structure can also be used, with the addition of
electromagnetic lenses to improve the focus. In the case of metric waves the
radiating surface more usually consists of arrays of dipoles suitably laid out
and excited, or of helical aerials, or of parabolic mirrors. In all cases the aerial
has an aperture of area S which in a way constitutes a radiating surface. Every
endeavour is made to make this surface area equivalent to a set of sources of
identical phase and constant amplitude, or variable amplitude decreasing from
the centre to the edges. These variations are arranged with the object of
reducing side lobe radiation.
A great many books deal with the problem of aerials, starting from the
notion of gain. Gain is defined ^«^^ a given direction as the ratio of the
power radiated in an infinitely small solid angle, around this direction, to the
