US2095998A - Demodulating circuit and method - Google Patents

Description

Oct. 19, 1937. J. c. MCNARY DEMODULATING CIRCUIT AND METHOD Filed March l, 1934 2 ShQets-Sheet l FI G .2

Oct. 19, 1937. J. c. MCNARY 2,095,998

` DEMODULATING CIRCUIT AND METHOD Filed March 1, 1934 2 Sheets-Sheet 2 o Y B H475 ya 7,465 S14/:Puf

Pafenfe oct. 19, 1937 UNITED STATES PATENT oFFicE DEMODULATING CIRCUIT AND METHOD VJames C. McNary, Washington,l D. C. Applicata@ March 1, 1934, serial No. 713,579

isolants. (Cl.- 250g-21) .l My intveritionrelates to f vthe demodulation of t amplitude 4modulated carrier waves,- withparticuplar but not exclusive reference tothe carriers used in radio telegraphy and telephony. l 5 "-The invention will be-illustratedin terms of its application to modulated waves in ,radio recepftion;` As is well known, the received carrier `wave y `is4 normally characterized by a "constant amplitude,A and becomes modulated as signals are impressed on it. The portions "of: Athe carrier which carry no signals, for example, those portions transmitted before a broadcast begins, or continuing during the momentary` pauses commonto-all spoken or musical programs, are received as a wave ofunmodul'ated amplitudem In many cases reception lof the unc modulated wave is not objectionablebutin other cases serious difculty results from the-,beating yof the Wave another-*1 wav-e of a ydifieren-tial frequency. In the latter caseslthe advantage of removing from the receivingtcircuit the unmodulated portions. of' the carrier is" obvious. Y

fnobjec't of` the present invention is to provide ,a method anda circuit for removing thev carrier wave when 'it' becomes unniodulated and for restoring-it as modulations appear. A primary .advantage of the invention is an improvement of the ratio of signal to noise. Broadly considered,` the'foregoing object is attained by providing a plurality of circuit branches for dividing the wave', sor acting on the1 components of the wave in one of the branches that their passage through the branch is delayed, and l then recombining these delayed components with 35- the components which traversed the other .;:branch.` 4Preferably:.thedelay, is suchwas-ito deliver the affected divisiio'nv ofi the Vwaveztozthe `junc tiorr of4 thegdelay branch vand what maybe called the umetard'edLbranch:at` a' relative displacement f, 40' ozjus't the number of electrical degrees suiiici'ent to' cause-'prior and subsequent amplitude'values fof` the waves. .towoppose each other.

When the divisions'of `the wave are. recombined in opposite phase the resultant wave is the vector sum of the divisions.` Consequently a wave of constant amplitude is completely removed, and a wave ofvarying amplitude is delivered from my novel circuit as a wave having` ,anu amplitude equal to the difference `between lobes of the origi- `nal wave spaced apart a distance equal to the wave length valuepfthe=delaying means, which, as has been indicated, may ,conveniently be one Wave-.lengthor one half wave 1ength,faccording to the circuit arrangement. It is evident that the -plwave resulting fromV a modulated carrier modified by practice of the invention will have its amplitude reduced, but this is readily corrected, if correction is necessary, by increasing the amplication in the audiocircuit. Most radio receiving sets have abundant available amplication, so that this apparent difculty presents no problem whatever.

With the foregoing and other objects in view, the invention comprises certain novel arrangements of -circuit elements and certain novel sequences of method steps which will now be briefly described. However, it is to be understood that the following specific description is by way of illustration only and not limitation, the invention being of wide application and capable of being embodied in other and differently modied. forms, all of which are considered to be within the spirit of the invention to the extent that they embody the principles thereof as pointed out by the appended claims.

In the accompanying drawings, which illus` trate certain preferred embodiments of the ini vention,

Fig. 2 shows the same wave on a larger scale Vfor clarity of illustration after modification by practice of my invention;

Fig. 3 is a block diagram of a typical circuit embodying certain principles of the present inven tion;

Fig, 4 is a diagram showing a type of circuit including one rectifier;

Figs. 5 and 6 are diagrams showing two types of circuits for full wave rectification; and

Fig. '7 is a diagram showing an elementary super-heterodyne circuit to which the invention has been applied.

Referring now to the drawings, it is evident that the modulated radio-frequency wave of Fig. l" comprises successive lobes of varying amplitude, such as those indicated at I, 2, 3, 4, where I is larger than the next succeeding lobe 2, and 3 is smaller than the next succeeding lobe 4, all of the lobes dening an audio-frequency envelopeLJ 5. Each lobe differs in intensity from its adjacent lobes by a small amount. Let it be assumed that the bottom half of the wave be remo-vedby rectication, as' indicated by the broken lines. It will be apparent that the remainder, the rectied full line wave, will be transformed into the wave of Fig. 2 if Veach lobe of Fig. 1 be subtracted from the next succeeding lobe, e. g., if lobe l be subtracted from 2, lobe 3 from 4, etc. Where suc-H loV The conductor may be provided with taps of any cessive lobes are of equal value, i. e., where the carrier is unmodulated, as indicated at 6, the wave will be completely removed.V

An object of the present invention is to transform the wave of Fig. 1 into the wave of Fig. 2, and I accomplish this purpose by dividing the wave into a plurality of divisions, say twoy in number, subjecting one of the divisions of the wave to a time delay capable of retarding the affected portion of the wave until the unretarded division has made a relative advance of the required amount, say one wavelength, and then recombining the divisions in such a way (as for example in opposite phase) that the recombining divisions oppose each other Vand the resultant is the vector sum of the divisions.

It may be noted that the resultant Awave of Fig. 2 may be smoothed out by use of by-pass condensers according to well understood practice, so that the final resultant will be an approximation ofthe envelope' at the modulatingfrequency.

Figs. 3, 4, 5, and 6 illustrate practical embodiments of the invention, and Fig. 7 shows the invention applied to a conventional super-hetero dyne circuit.

' In these figures 'I designates a source of amplitude modulated carrier waves. A half-wave rec tier lof any convenient type is shown at 8 in Figs. 3 and 4, and the output circuit of this recti- .fier contains an unretarded branch 9 and a time delay branch I0, which join at II, conveniently the mid-point of a load resistance I2, to which the grid I3 of an audio amplifier tube is connected. In some cases itis desirable to interpose in the unretardedbranch 9 a compensating impedance I4 to balance the retarding element I5 of the time delay branch.

The retarding element I5 may consist of an appropriate length of Wire or other conductor folded back and forth on a card for economy of space.

sort engaged by a suitable contactor, or made variable in any well understood manner if desired, but of course for use in a heterodyne receiver a V'time delay of fixed value is sufficient.

Optionally I may use a time delay element comyprising a combination of impedances arranged in equal to the vector sum of the divisions, effecting complete removal of the wave when the received carrier is unmodulated.

In Fig. 3 double connections 25 indicate the input terminals and double connections 2B the out-Y put terminals of a generalized compensating im- Y pedance I4. Similarly the double connections 2'1 indicate the input terminalsand the connections '28 the output terminals of a generalized retarding element I5.

In Fig. 6 two rectiers I6 are shown, each in a branch of the coupling transformer output circuit, the rectiers being arranged to pass impulses of different polarity. One rectifier branch, designated 9, is unretarded, and the other, shown at I0, contains a time delay element I'I preferably of a value substantially equal to one-half wave length or any odd multiple thereof, such as 11A?, 2%, 31/2, etc. Due to the arrangement of the rectiiiers waves of opposite phase and relatively displaced one-half wave length are produced in the branches 9 and I0. The time delay element I1 in branch IU retards the wave in this branchsufliciently to cause it to coincide at the junction II with the portion of a succeeding wave (the next succeeding wave if the value of the delay be one-half wave length) which passed through branch 9.

It will be observed that the rectiiiers I6 are shown as of the copper oxide type, While in Figs. 4 and A5 vacuum tubes are used. Y'I'he invention in its various Vembodiments may be' practiced equally Well with any conventional rectifying means.

Fig. 7 shows a typical super-heterodyne circuit in which a demodulating circuit embodying the principles of the present invention is incorporated. Oscillations from the source I8 enter branch circuits containing a time delay element I9 and, if desired, a compensating impedance 20, respectively, and are recombined at the junction 2| in the load impedance 22. The output is passed through the audio frequency and power amplifier circuits according to conventional practice. A by-pass condenser 23 is provided in each of the branches merely for the purpose of isolating the demodulating circuits from theV plate vpotential of the associated vacuum tube.

The invention may be mathematically explained and proved as follows:

The voltage applied to the linear rectifier, which may be, for example, a signal received from a broadcasting station, represents a carrier wave and its two' second-order side-bands. ItV may be expressed as V=P cos pt 1+lc cos qt) '(1) where V=instantaneous voltage P=carrier amplitude p=21r carrier frequency k=factor representing depth of modulation q=21r modulating frequency. 'I'he linear rectifier suppresses the negative lobes of the wave. To determine the wave com ponents of the output of the linear rectifier, it is clear that such an objective lsfattained if the expression for` V, above,'is multiplied bya function which is unity when cos pt is positive, and zero when cos pt is negative. 'I'his is equivalent to replacing cos pt by the result of'linear rectication of cos pt. This function isl well known, by Fourier analysis, to be 1V 1 2 f(t)- lr-I- 2 cos pt-I-r cos 2pt- Y 2 d n 15T cos 4pt+ .Y The Vrectified wave may then be Written as V,=f(f)[1+1 rcos qt] This may be expanded to read A The out'put# circuit of the' linear rectifierM |`may be divided into two sections. If'av l time `delay of At be applied to the currents lflowingiino'ne of the sections, itmay be represented by When the currents flowing inthe twosections are-of equal amplitude vandfcombined in an outof-phase relation, the resulting potential is pro,- portional to the difference ofthe two currents whose wave components are represented in Equa- Now it so happens that the demodulating system is intended to reproduce the components of the q frequency from the modulated carrier wave. This is usually relatively low frequency so that such components may be easily separated from the components of carrier frequency, or of harmonics and modulation products thereof. Interest is centered, therefore, on the components of q frequency in Equation (7) as all the higher frequency components may be removed by the familiar use of by-pass condensers and other devices at appropriate places in the circuit. The demodulation product of interest is which represents the useful voltage appearing across the output terminals of the demodulating system.

It will be observed that the output voltage amplitude of q frequency, as represented by Equation (9) varies directly as the lfactor 1c, the depth of modulation, and more or less inversely as the ratio of the q and p frequencies. Therefore, to obtain a flat response-frequency characteristic for the demodulating system, an equalizer must be used to increase the relative amplitudes of the lower q frequencies.

It is to be noted that carrier frequency components have disappeared entirely from Equation L('7); Hence, an interfering signal has nothingto lieterodyne `with when no modulation-is present Aon the -desiredcarrier wavefand the result isa substantial Vreduction of noise. i

' The foregoingfrepresents ani` analysis of the particularcase wherein a .360 ltime delay, Lwith opposite-phase recombination, is used.` It is obviousthat asimilar analysis'is applicable to other embodiments 'of the invention, `wherein other choices'of circuit elementsl are-used. f f

`Iclaim:

. 1. The method of removing intelligence from a received modulated carrier wave which comprises dividing the wave, retarding one of the divisions thereof Yuntil another. of the divisions has advanced substantially an integral multiple of one wave length at theY frequency 'of the carrier wave over the first, and recombiningf the divisions while preserving` substantially. the4 original relative frequency response of thedivisions. f

2. 'Ihemethod of removing intelligence from a received modulated carrier wave which comprises rectifying the wave, dividing the rectied wave into atleast two divisions, delaying the flow of the wave in one of the divisions untilanother of the divisions has. advanced an integral multiple of one wave length at the` frequency of the carrier wave over the first, and recombining the divisions in out of phase relation while preserving substantially the original relative frequency response of the divisions.

3. The methodof demoduianng an amplitude modulated carrier wave which comprisesseparately rectifying the positive and negative impulses, retarding the impulses of one polarity until the impulses of the other polarity have advanced substantially an odd multiple of one-half wave length over the impulses of the first named polarity, and recombining the impulses of both polarities.

4. A circuit for removing intelligence from a received modulated carrier wave comprising an unretarded branch and a branch including means for delaying the wave flowing therein substantially an integral multiple of one wave length, at the frequency of the carrier Wave, with relation to the wave flowing in the unretarded branch, said branches having substantially identical frequency characteristics, and a junction of the branches.

5. A circuit for removing intelligence from a received modulated carrier wave comprising a rectifier, a rectifier output circuit including an unretarded branch and a branch including means for delaying the wave fiowing therein substantially an integral multiple of one wave length, at the frequency of the carrier wave, with relation to the wave flowing in the unretarded branch, said branches having substantially identical frequency characteristics, and a junction of the branches.

46. A circuit for demodulating an amplitude modulated carrier wave comprising means for receiving modulated waves, a plurality of branches connected to said means, a rectier in each branch, means in one of the branches for delaying the wave flowing therein substantially an odd number multiple of one-half wave length with relation to the wave owing in another branch, and a junction of the branches.

'7. The method of removing intelligence from a received modulated carrier wave which comprises rectifying the wave, dividing the rectified components, retarding one of the divisions thereof, and, while preserving substantially the origivnal relative frequency' response of the divisions, recombining said divisions in such phase relation that the amplituder of the recombined carrier components varies in proportion to modulation.

8. The method of removing intelligence from a received modulated carrier Wave which comprises dividing the Wave, retarding one ofthe divisions thereof, and, while preserving substantially the original relative frequency response of the divisions, recombining said divisions in such phase relation that the amplitude of the recombined carrier components varies in proportion to modulation..

9. The method of removing intelligence from a received modulated carrier wave which comprises rectifyingA the wave, dividing the rectied components into at least two divisions, delaying the flow of theV Wave in one of the divisions, and, while preserving substantially the original relative frequency response of the divisions, recombining said divisions in such phase relation that the amplitude of the recombined carrier components varies in proportion to modulation.

- 10. The method of removing intelligence from a received modulated carrier wave which comprises separately rectifying the positive and nega- 'tive impulses, relatively delaying the impulses of one polarity, and, while preserving substantially the original relative frequency response of the divisions, recombining said divisions in such phase relation that the amplitude of the recombined carrier components varies in proportion to modulation.

11. The method of removing intelligence from a received modulated carrier wave which comprises separately rectifying the positive and negative impulses, retarding the impulses of one polarity until the impulses of the other polarity have advanced substantially one-half wave length over the impulses of the rst named polarity, and, while preserving substantially the original relative frequency response of the divisions, recombining said divisions in such phase relation that the amplitude of the recombined carrier components varies in proportion to modulation.

12. A circuit for removing intelligence from a received modulated carrier wave comprising an unretarded branch and a branch including a time delay, said branches having substantially identical frequency characteristics, and a junction of the branches for recombining the waves flowing through the branches in such phase relation that the amplitude of the recombined carrier components varies in proportion to modulation.

13. A circuit for removing intelligence from a received modulated carrier Wave comprising means for receiving modulated Waves, a plurality of branches connected to said means, a rectier in each branch, a time delay means in one of the branches, said branches having substantially identical frequency characteristics, and a junction of the branches for recombining the waves ilowing through the branches in such phase relation that the amplitude of the recombined carrier components varies in proportion to modulation.

' JAMES C. MCNARY.

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