RADIO
`ENGINEERING
`
`BY FREDERICK EMMONS TERMAN, Sc.D.
`
`PROFESSOR OF ELECTRICAL ENGINEERING, AND
`DEAN OF THE SCHOOL OF ENGINEERING
`STANFORD UNIVERSITY
`Past President
`Institute of Radio Engineers
`
`THIRD EDITION
`
`NEW YORK AND LONDON
`
`McGRAW-HILL BOOK COMPANY, INC.
`
`1947
`
`

`

`RADIO ENGINEERING
`
`COPYRIGHT, 1932, 1937, 1947, BY THE
`McGRAW- HILL BooK CoMPANY, INc.
`
`PRINTED IN T HE UNITED STATES OF AMERICA
`
`All rights reserved. This book, or
`parts thereof, may not be reproduced
`in any for mwithout permission of
`the publishers
`
`THE MAPLE PRESS COMPANY, YORK, PA.
`
`The pn
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`
`

`

`776
`
`RADIO ENGINEERING
`
`[CHAP. 15
`
`Methods of Transmitting Intelligence by Pulses.-The most widely used
`method of employing pulses to transmit intelligence is by pulse position
`modulation, also sometimes called pulse time modulation. Pulse position
`modulation utilizes short pulses of constant amplitude that have a repeti(cid:173)
`tion frequency at least several times the highest frequency contained in
`the intelligence to be transmitted. The time of occurrence of these pulses
`
`· "7
`(a) Signal to be
`transmitted ~
`PosiHon of'unmodulofed pulses
`I
`I
`I
`I
`I
`"",- 1<o".>.J
`I
`t
`I
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`1
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`,~ n rn rn rn a m :n :n : n in :n ~
`
`I
`I
`
`t
`I
`
`(b) Pulse position modulation
`
`(c) Pulse width modulation
`
`(d) Pulse amplitude modulation
`
`n 0 ~
`
`(e) Mo_dulation by spacing of
`twin pulses
`Frn. 15-30.- V arious methods of pulse modulation.
`
`is then varied in accordance with this signal. 1 Such pulses are illustrated
`in Fig. 9-24, which also shows one method of producing pulses that are
`modulated in position. These pulses are used to modulate or key a
`microwave oscillator such as a magnetron, klystron, or reflex klystron,
`giving a transmitted signal such as shown in Fig. 15-30b. At the receiver,
`the intelligence thus transmitted may be recovered from the pulses in a
`number of ways. One simple way is to rectify the incoming signal as it
`
`1 Throughout this discussion, the term signal denotes the intelligence t hat is to br
`transmitted.
`
`SEc. 15-IIJ
`
`comes out of
`pulses similar
`can be conside
`frequency wh
`'
`.
`m1tted, togeth
`similarly mod
`frequency-mod
`circuits, passed
`
`(a) Tri
`
`AM
`
`B
`
`,
`
`(b) Sigr
`
`"JLQ
`(c) Puls.
`Fro. 15-31.-Methc
`f~·equency deviatic
`tion receiver.
`Pulse position
`~ay be used to tra
`Width of the pulses
`pulse width module
`Pr?duced by super
`usmg for the transn
`properly chosen am
`p~ssibility is to moc
`with the signal to
`
`

`

`[CHAP. 15
`
`1st widely used
`· pulse position
`Pulse position
`[; have a repeti(cid:173)
`~y contained in
`~ of these pulses
`
`SEC. 15-11]
`
`RADIO TRANSMITTERS
`
`777
`
`comes out of the intermediate-frequency amplifier, thereby obtaining
`pulses similar to those modulated upon the transmitter. These pulses
`can be considered as representing a carrier wave of the pulse repetition
`frequency, which is phase-modulated by the intelligence being trans(cid:173)
`mitted, together with harmonics of this pulse repetition frequency
`similarly modulated. · The fu:q.damental-frequency carrier and
`its
`frequency-modulation side bands may be separated by appropriate filter
`circuits, passed through a chain of harmonic generators to increase the
`
`~ ~ ~ A A ~ A ~ ~ ~ ~ ~ ~·
`
`(a) Triangular pulses
`
`A
`B
`
`(b) Signal plus triangular pulses
`
`· 00 00 000 n n n non
`(c) Pulses obtained by selecting amplitude range AB
`Fro. 15-31.-Method of modulating the width of pulses in accordance with a signal.
`frequency deviation, and then applied to an ordinary frequency-modula(cid:173)
`tion receiver.
`Pulse position modulation is not the only means by which pulses
`may be used to transmit intelligence. Another possibility is to vary the
`width of the pulses in accordance with the signal to be transmitted, giving
`pulse width modulation as shown in Fig. 15-30c. Such a wave can be
`produced by superimposing triangular pulses upon the signal, and then
`using for the transmitted pulse the portion of the wave lying between two
`properly chosen amplitude levels, such as A and B, Fig. 15-31. Another
`possibility is to modulate the amplitude of successive pulses in accordance
`with the signal to be transmitted, giving the result illustrated in Fig.'
`
`,ti on.
`mlses are illustrated
`cing pulses that are
`modulate or key a
`t, or reflex klystron,
`:Ob. Atthereceiver,
`from the pulses in a
`incoming signal as it
`
`intelligence th.at is to be
`
`

`

`778
`
`RADIO ENGINEERING
`
`[CHAP. i5
`
`15-30d. Finally, one can use a combination of pulses to transmit the.
`signal. An example of this is illustrated Fig. 15-30e, in which the pulses
`appear in pairs, with the spacing between the pulses in each pair being
`varied proportionally to the instantaneous amplitude of the signal being
`transmitted.
`·
`Time Multiplex.- The minimum permissible spacing between pulses
`is about 0.4 times the time corresponding to a single cycle of the highest
`frequency contained in the signal to be transmitted. Thus, a 100-µsec
`pulse spacing is quite adequate to transmit 3500 cycles. Furthermore, the
`pulses th'tmselves can be quite short, such as 1 µsec, and in pulse position
`modulation it is not necessary to shift the pulses by over ± 5 µsec at the
`peak amplitude of the signal in order to obtain satisfactory pulse position
`modulation. Thus, only about 10 µsec of each 100-µsec interval is actually
`required for transmitting the intelligence. The remaining 90 µsec can be
`used by other pulses transmitting other signals.
`This leads to what is termed time multiplex, in which successive
`intervals of time are assigned to different channels. Thus, in the case cited
`above, one could simultaneously transmit eight signals assigned succes(cid:173)
`sive 12-µsec time intervals (10 µsec for the pulse, plus an additional 2 µsec
`to provide protection against adjacent channels), and send an extra-long
`synchronizing pulse during the remaining 4 µsec of each 100-µsec period.
`Such a time multiplex signal can be obtained by generating the pulses
`for each individual channel just as though this channel were acting alone,
`except that the pulses for the successive channels have a progressive time
`difference, which in the case indicated above would be 12 µsec. The mix(cid:173)
`ture of pulses obtained in this way from the various channels is then
`modulated on the microwave oscillator in any convenient manner. At
`the receiving end of the system, the receiver output will deliver pulses
`that are identical with those that were modulated on the transmitter.
`This output is then applied to a system consisting of a series of gates,
`one for each channel, with inputs connected in parallel. These gates are
`controlled by the synchronizing pulse, which can be distinguished from
`the other pulses by its greater width. The control is such that the gate
`associated with a given channel is open during the particular 12-µsec part
`of the 100-µsec period in which the pulses of that channel are transmitted.
`Signal-to-noise Ratio .-A noise-suppressing action occurs in pulse
`systems that is analogous to that encountered in frequency modulation.
`In particular, if the signal amplitude is appreciably greater than the noise,
`then it is possible to derive pulses from the receiver output that are almost
`completely free of noise. This can be understood by considering the
`ideal case of a pulse having vertical sides and an amplitude moderately
`If now one arranges
`greater than the noise, as illustrated in Fig. 15-32.
`matters so that amplitude levels less than A of the combined noise and
`
`signal are r
`by biasing
`is required
`amplitudei
`one then c
`whatsoeve1
`the signal
`However i
`of 6 db t~ 1
`
`Fm. It
`
`in this wa3
`the band,,
`15-12.
`techniques
`bility of ei
`wire lines.
`times as n
`
`1 For fur1
`lowing articl
`vol. 7, p. 36i
`the Radio C<
`S. Marks, Jr
`Systems int
`Knox and C
`RCA Rev., v
`Proc. I.R.E ..
`Communicat
`
`