JAMES F. GIBBONS
`
`Department of Electrical Engineering
`
`Stanford University
`
`SEMICONDUCTOR ELECTRONICS
`
`McGRAW-HILL BOOK COMPANY
`
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`
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`
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`RPX-Farmwald Ex. 1045, p 1
`
`

`

`SEMICONDUCTOR ELECTRONICS
`
`Copyright© 1966 by McGraw-Hill, Inc.
`All rights reserved. Printed in the United
`States of America. This book, or parts
`thereof, may not be reproduced in any farm
`without permission of the publishers. Library
`of Congress catalog card number 66-16049.
`
`1234567890 HIJ 721069876
`
`23162
`
`RPX-Farmwald Ex. 1045, p 2
`
`

`

`Carrier-frequency
`oscillator, frequency f <
`
`I +
`I
`I
`I
`I
`I
`I
`I
`L _______ J
`
`eL
`
`-
`
`(a) Diode bridge modulator
`
`I<
`- - - -
`
`kORL
`I ts+RL } ''""" oom"""'"'/,,.0<) d,o"..,,
`(b) Load waveform for (a)
`
`L
`
`'
`
`... -
`
`ec
`
`(c) Fourier component at fc with amplitude varia(cid:173)
`tion at frequency w,. Actually Fourier representa(cid:173)
`tion of ec(t) requires three frequencies:
`We, We + w,, We - w,.
`
`FIG. 7 • 22 A DIODE BRIDGE MODULATOR AND WAVEFORMS
`
`Such a waveform is shown in Fig. 7 · 22c. It is described by saying that the
`amplitude of the carrier has been modulated by the voltage v8 sin w8t.
`Using trigonometric identities, we can rewrite Eq. (7 · 27) as
`
`eo(t) =Ac cos wet+ A~ms sin (we - w8)t +_ A~ms sin (we+ w8)t (7 • 28)
`
`In Eq. (7 • 28) we see that the time-variable Fourier amplitude given in
`Eq. (7 • 27) has been resolved into three new Fourier components with
`constant amplitude. The frequencies of these components are w0 , w0 + ws,
`and We - Ws.
`.
`The voltage represented by Eq. (7 · 'P) or (7 · 28) is typical of the signals
`which are broadcast by an AM (amplitude modulation) radio station. The
`amplitude of the carrier varies in accordance with the information being
`broadcast. In the simple case of a sine-wave tone, the amplitude-modulated
`carrier can be decomposed into a Fourier series consisting of constant(cid:173)
`amplitude signals at the carrier frequency fc and two side-band frequencies
`ifc - fs), ifc + fs). The side-band frequencies really contain the informa(cid:173)
`tion being sent.
`We will return to a further consideration of modulating systems and their
`
`RPX-Farmwald Ex. 1045, p 3
`
`

`

`properties in Chap. 16. For the moment we re-emphasize that the modu(cid:173)
`lator is a frequency-shifting, and therefore nonlinear, system. A simple
`modulator can be made with a diode bridge. Such a modulator is not used
`for commercial radio purposes, though it is extensively used in carrier
`telephone systems.
`·
`
`7 · 6 · 1 Detection of an amplitude-modulated wave. A radio receiver which is
`tuned to the frequency fo will receive a signal voltage of the form given by
`Eq. (7 · 28), but a very small amplitude (perhaps only a few microvolts
`peak amplitude). This signal must be amplified in the receiver to the point
`where a usefully large audio signal can be recovered from it. The process by
`which the audio signal is recovered is called envelope detection. A simple
`type of envelope detector consists of nothing more than a half-wave
`rectifier with a capacitor filter and a resistive load (Fig. 7 • 23a).
`For the purpose of detecting the audio information, we want to select a
`CrRL combination which will allow the half-wave rectifier to follow the
`peak amplitude as nearly as possible (see Fig. 7 • 23). If the time constant
`is too large, the filter will respond only to the peaks of the modulated wave,
`as shown in Fig. 7 · 23c. If the time constant is too short, very little audio
`voltage will be obtained. The best value of CrRL represents a compromise
`between these alternatives. In Prob. 7 · 17 it is shown that a reasonable
`value for the CtRL product is
`
`yfl - ms2
`msWs
`where ms is the modulation index and w8 is the audio frequency. A maximum
`value of ms equal to 80 percent is customarily employed, and the maximum
`
`L r-
`RC
`
`(a) Diode enuelope detector
`
`(b) eLfor normal operation
`
`(c) eL when RC product is too
`large
`FIG. 7 '23 DIODE ENVELOPE DETECTOR AND WAVEFORMS FOR VARIOUS OPERATING
`CONDITIONS
`
`(d) eL when RC product is too small
`
`RPX-Farmwald Ex. 1045, p 4
`
`

`

`a nonlinear element.1 When such an element is driven with two sine
`waves, its output contains not only the two sine waves but, in principle, all
`harmonics of each sine wave and sine waves at all the possible difference
`frequencies.
`This suggests that passing the received signal through a diode will
`accomplish the required frequency shifting, and we can then use a low(cid:173)
`pass filter to pass only the audio-frequency components of the diode
`output. This scheme can be simply put into practice in the elementary
`receiver shown in Fig. 16 • 12. Here, a receiving antenna (a few feet
`of wire in simple cases) is simply transformer-coupled to a diode. A tuning
`capacitor at the input is used to select the desired station, and a parallel
`CR network provides the filtering f~ction. An AM detector which uses
`the nonlinearity inherent in a class B amplifier is the subject of Prob. 16 · 17.
`Gain and selectivity. While detection can be regarded as the basic signal(cid:173)
`processing step in an AM receiver, the elementary receiver shown in
`Fig. 16 · 12 needs to be surrounded by other signal-processing blocks if
`the receiver is to be more than a toy. In particular, the wide usage of radio
`communications equipment is based on the fact that considerable gain
`and selectivity can be obtained in a radio receiver.
`Gain is required because the direct detection of a received signal provides
`far too little power to drive a loudspeaker or even an efficient pair of ear(cid:173)
`phones, if the receiver is far from the transmitter. Generally we want to
`provide a voltage of at least 0.5 to 1 volt to the voice coil of an 8-ohm
`speaker, whereas the received signal may be only a microvolt or so. This
`means that we need a voltage gain of roughly 105 to 10s in the receiver.
`
`1 A linear time-varying element (or amplifier) can also perform the frequency-shifting
`function.
`
`_,--Antenna
`
`r-----1
`
`r-----~
`
`R
`
`____ J
`
`Nonlinear clement
`
`L-----..1
`Filter
`
`Ll_
`
`I
`I,
`Frequcnc y spectrum
`of received signal
`
`L
`
`Frequency spectrum
`of detected signal
`
`FIG. 16 • 12 AN ELEMENTARY AM RECEIVER ILLUSTRATING THE BASIC DETEcnON
`PROCESS
`
`RPX-Farmwald Ex. 1045, p 5
`
`

`

`ii l
`
`~
`~
`.,, .t 8.
`
`~~ ~
`
`~··~"'-~ .......... J~,,..___..,,~l~,~••~l'h-1___...11~l-,~-"'~ll~!~~~•m•l,~b!Lr~··_·__, ..
`
`I 550 600
`
`680 730
`
`800
`
`875
`
`I ,000
`
`f, kcps
`
`(a) Signals are received by the antenna from all broadcasting stations
`
`(b) Transmission characteristic of filter adjusted to select 680 kcps carrier
`and sidebands
`FIG. 16 • 13 AM BROADCAST SIGNALS AND CORRESPONDING FILTERING CllARACTERISTICS
`REQUIRED IN AN AM RECEIVER
`
`We would of course like more, though noise voltages will set a limit on
`sensitivity at some point.
`Selectivity is required to insure that signals from adjacent stations are
`not received simultaneously. In order to obtain selectivity, we have to
`incorporate some type of filtering into the receiver which will allow the
`desired signal to pass and will reject all others. In Fig. 16 · 13b the trans(cid:173)
`mission characteristic of a desirable filter is plotted as a ·function of
`frequency. The filter is positioned to pass the Fourier components of
`signals arriving at the 680-kcps carrier frequency and reject all other signals.
`We can build a reasonable approximation to such a filter by a repeated
`use of the idea that the output voltage of an LC tank circuit will have a
`band-pass characteristic when it is driven from a current source. However,
`as suggested in Fig. 16 · 14a, the filter characteristic will not have the
`required shape.
`To improve the shape, we can cascade a number of these networks,
`using transistors between stages in the manner shown in Fig. 16 · 14b.
`By proper adjustment of the resonance peaks in the individual circuits, we
`can obtain the required selectivity and in the process obtain some of the
`gain required in the receiver.
`Unfortunately, since we wish to receive any one of several stations, we
`have the problem of tuning the filter so that its center frequency can be
`placed in any desired position of the broadcast band. To do this, we have
`to tune each resonant circuit, making sure that their relative positions on
`the frequency scale are always correctly maintained. It is possible to
`do this, and some receivers employ only this type of filtering prior to
`detection. These receivers are called tuned radio frequency receivers.
`However, a more widely used solution to the problem of making a
`tunable filter (with gain) is to use the superheterodyne principle invented
`by E. H. Armstrong, shown in the block diagram of Fig. 16 • 15a. Here we
`
`RPX-Farmwald Ex. 1045, p 6
`
`