US2396360A - Method of detecting frequency modulated waves - Google Patents

Description

arch 12, 1946. ZEGLER 2,395,369

METHOD OF DETECTING FREQUENCY MODULATED WAVES Filed July 16, 1942 2 Sheets-Sheet 1 ATTORNEY.

M. ZlEGLER ala'ch 12, 1946.

METHOD OF DETECTING FREQUENCY MODULATED WAVES 2 Sheets-Sheet 2 Filed July 16, 1942 v n BN Patented Mar. 12, i946 UNHTED STATES PATENT @EFECE" METHQD F DETECTING FREQUENCY MODULA'EED WAVES Application July 16, 1942, Serial No. 451,186

7 Claims.

The present invention relates to the detection of frequency modulated waves. Neglecting certain possible constant phase displacements, a frequency modulated wave may be considered as a sinusoidal function of the sum of two angles, one of which is the central angle equal to the product of the constant central frequency by the time, and the other of which is the variable angle, hereinafter called modulation angle, which represents the impressed phase variation correspond ing to the intelligence. For the normal customary case, this variable phase is proportional to the integral of the intelligence. The method of the present invention, however, makes its equally easy to receive more elaborately modulated waves,

the variable phase of which can have, with respect to the intelligence, a wide variety of relationships which can be comprised under the general term of modulation characteristic. Thu-s, such a Wave may be written E sin (wt+ i), with 411:? (s) where \l represents the varia le or modulation angle, 8 is the intelligence, 5 the modulation characteristic, to is the central frequency and of the central angle.

As is well known, classical frequency modulation receivers are based on the selective properties of L. C. circuits which make it possible to change frequency modulation into amplitude modulation. From the amplitude modulated signal thus obtained, the intelligence is then detected according to the classical amplitude modulation detection method. Known frequency modulation receivers normally consist of a radio frequency stage, one or two frequency converter stages, two 'wide band amplifier stages, two limiter stages, a discriminator and detector, followed by the ordinary amplifier and output stages.

Now it can be shown that the highest value of 1 01 that will be found in the practice of high fidelity frequency modulation, is of the order of 2,000 radians, which corresponds, for a modulating frequency of 30 c./s., to a frequency excursion of '75 kc. It will readily be appreciated therefore. that by reason of this large frequency excursion, the classical receiver, in order to pass the whole spectrum of the frequency modulated signal, must be equipped with intermediate frequency ampli-- fiers of large bandwidth, of the order of 200 kc. Consequently, in the classical receivers, it is nec-- essary to use special wide band amplifiers working on relatively high frequency (5 mo.) and with a relatively low amplification per stage (an amplification of the order of 20).

It is the principal object of the present invention to provide a new and novel method of de tecting a frequency modulated wave.

Another object of the invention is to provide a method of demodulating a wave varying as a inusoidal function of a central angl and a modulation angle.

A further object of the invention is to provide a device for detecting a modulated wave varying as a sinusoidal function of the sum of a central angle and a modulation angle in which device the need for special wide band amplifiers is obviated.

These and further objects of the invention will appear as the specification progresses.

In accordance with the invention I obtain accurate detection of a frequency modulated wave with a considerably simplified apparatus, by subjecting the modulating component of the incoming wave to an operation which I have for convenience designated shrinkage, and which is effected by impressing on the wave a certain phase shift angle 11 1' which combines with the modulation angle n to give a difference angle we, which, as hereinafter will be made clear, can be made very small and proportional to the modulation angle, so that said difference angle will finally reproduce the same modulation characteristic with respect to the intelligence as the modulation angle does. The result of producing the small difference angle is to compress or shrink the frequency excursion so that the spectrum does not extend further from the central frequency than a value equal to the highest modulating frequency. This means that the bandwidth required is exactly the same as in ordinary amplitude modulation receivers, and further, that the intermediate frequency itself may be the same as that commonly used in such receivers, for example 450 kc.

The wave treated as bove described by passing it through a phase shifting device and comprising the difference angle in lieu of the modulation angle, is subjected to on or more steps of heterodyne frequency changing to produce an intermediate frequency wave varying as a sinusoidal function of the sum of the intermediate frequency angle and the said difierence angle. The intermediate frequency wave so produced is then impressed on a phase comparison device, where it is compared or mixed with a locally generated oscillation Varying as a sinusoidal function of the intermediate frequency angle. The output of the phase comparison device comprises an electrical quantity which is proportional to the difference angle. The said quantity, after pre-amplification, is impressed on a restoring device, Where the inverse operation of the modulation characteristic is performed, as for example, a differentiation by means of a simple inductance when .a classical frequency modulated wave is being treated, in which the modulation characteristic is an'integration. This restoring device thus converts the impressed quantity into a second quantity which is directly proportional to the intelligence itself. Said second quantity may, for example, be impressed on a loud speaker or the like, whereby the intelligence will be audibly reproduced.

Thus it will be clear that the novel method of detecting frequency modulated waves, according to the present invention,.is basically quite different from the known methods.

As will be more clearly explained below, these results depend on the possibility of maintaining the difference angle proportional to the modulation angle and yet smaller than radian, and these conditions can be satisfied in practice by impressing on the phase shifting device a quantity derived from the phase compar son device and univocallv dependent on the output thereof.

As will hereinbel w be made clear, my novel method permits faithful detection of the inte11i-' gence inscribed in the frequency modulated wave to be achieved independently of the'amn itude of the wave and also irrespective of non-linearity of arts of the circ it.

The invention will be described in greater detail with reference to t e appended draw ngs forming part of the specification and in which Fig. 1 is a diagram illustrating the demodulating effect of a p ase shifting device on a frequency modul ted wave.

Fig. 2 is a diagram illustrating the manner of securin automatic adiustrnent of the operating con itions of the phase s iftin device so that the dem dulating action shall be nearly equal to the?1 modulation inscribed on the incoming wave an Fig. 3 is a circuit diagr m of a receiver including a phase shift ng device and desi ned to operate on the method of detection of the present invention.

Fig. 4 illustrates a phase shift ng device and its arrangement in the receiver circuit of Fig. 3.

From the fore oing explanations it will be evident that a means for impressing on the incomin wave a phase shift of the kind desired is an essential and necessary part of a receiver adapted to operate on my novel method of detection. The specific construction and design of such a means is no part of the present invention, since apparatus capable of giving the desired results is at present available. although it has not hitherto been so employed. However, for the better understanding of the present method. it is thought advisable to explain briefly what is here meant by a phase shifting device.

A phase shifting device, represented diagrammatically by the rectangle T in Figure 1, comprises input terminals A, output terminals B and control terminals C, and has an internal arrangement such that if a tension v1(t) is applied to the terminals A, there will appear at the output terminals B, tr seconds later, a tension p being a proportionality factor and tr being at each instant proportional to an electrical quantity applied to the controls terminals C. Apparatus suitable for use as a phase shifting device in the present invention, has been described as a phase modulation tube in Ed. Labins U. S. patent application Ser. No. 436,374, filed on March 26, 1942, now U. S. Patent No. 2,372,210, issued March 27, 1945, and the use thereof in a circuit arrangement in accordance with the invention is illustrated in Fig. 4, later to be more fullyreferred to.

Now, if as a measure of the time tr, the phase of an oscillatory wave having an angular velocity 0 be taken, the efiect of passage through the phase shifting device will be evidenced as a phase shift 11/: (equal to Qtr) and if a current 1' be applied to' the control terminals C, it can be shown that the practical formula for such a phase shiftins device is Where )4 is the phase shifting constant of the device in radians per milliampere.

As is apparent from the above cited prior application of Ed. Labin, such a device has heretofore been used for the modulation of the frequency of a wave. The present invention provides a method in which such device will be used to perform the inverse action.

Referring again to Figure 1, it is supposed that there is applied to the phase shifting device a Wave E1 sin wt+1('t) Where. 01(1.) represents a variation of phase with time. ample, be of the form gl/io sin Qt.

The control or phase shifting current i applied to the terminals C, is made equal to E2 sin (wt-i-yW-ybr) =E2 sin (wH-ybi-dgbi) E2 sin (wt-Hts) If a=1, the wave has no more phase variations;

if a is slightly less than 1 it has a phase variation which is small compared with the original varia-.

tion. That is to say, a shrinkage of the phase difference has been produced to the value emu-am This means that a shrinkage of the excursion has also been achieved, which reduces the spectrum of the Wave to a relatively narrow frequency band.

The next step will now be so to arrange matters that, in the event that 1/4 varies in an unknown 'manner, 51 shall be automatically maintained at a value nearly equal to it, (11 1), so that as a consequence, the demodulating current 1', equal to rrsL' x x will be proportional to the modulation angle l -z, in order that said current will carry or represent the intelligence, while the shrunk angle its will be relatively small.

It will now be shown how this result can be achieved.

As shown in Fig. 2 the output oscillation E2 sin (wt-H1111) of the phase shifting device T is mixed with a local signal Er. cos wt in a phase comparisondevice M, The mixing of the oscillations in the rind) may, for exdevice M produces in Well-known manner a difference frequency current proportional to The other frequencies produced by the mixing can readily be filtered out as they are of high frequency, and will not be considered in the present description. By analogy with the operation of inverse feedback in an amplifier on the input of which a large signal is applied which is reduced automatically to a small value by subtrac tion of a tension derived from the output, those skilled in the art will appreciate that by thus applying, as in the figure, to the phase shifting device, the output of the phase comparison de vice pro ortional toasinusoidal function of the difference angle is, the desired phase shifting current i proportional to the modulation angle can automatically be obtained. Evidently the output current derived from the phase comparison device and applied to the phase shifting device must be univocally correlated with ts, so that this difference angle must not reach values exceeding This condition can readily be satisfied by so choosing the constants of the arrangement that ts is sumciently small for the sine of the difference angle to be approximately equal to the angle itself.

From the feedback theory it can be deduced that, in the arrangement described, we have 31/1 z ybi (1) and 11d tl/i/A (2) when A is made very much greater than 1 and do is less than /4 radian, A being the number of radians phase shift corresponding to /ezl radian.

Now A can be written equal to xii where i1 is approximately the variation corresponding to an increase he of one radian in the plate current of the mixer tube constituting a phase comparison device M. It is readily possible to make x of the order of hundreds of radians per milliampere and ii of the order of milliamperes, so that a large value of A can easily be obtained in practice and Formulae 1 and-2 applied. That is to say, that the difference angle 111d can be made proportional to the modulation angle 1h. Hence the output of the phase comparison device will also contain a term proportional to the intelligence.

If, byway of example, it be supposed that A=8,000 and that 1/1 has a variation of from 0 to 2,000 radians, the variation of he will be only 2,000/8,00O= radian, so that variations of \//i of the order supposed and even more, can be tolerated. This is an essential property of the novel method. Not only does it allow these phase fluctuations of for example 2,000 radians to be considered admissible, but it enables a large frequency excursion to be reduced to a very low value, which makes superfluous the use of the special wide band amplifiers necessary in classical receivers.

The arrangement shown in Figure 2 has no selective properties. Such properties can be added by including a selective I. F. amplifier which, owing to the shrinkage of the excursion need be of only narrow bandwidth.

It is further desirable to limit the phase shift ing current i so that the range of shrinkage does not extend further than the maximum phase excursion of the incoming modulated wave. In practice this limitation is obtainable by using the natural properties of an amplifying tube.

In Figure 3, I have shown a connection diagram for a receiver capable of operating according to the method of the present invention. In this figure the various elements have been defined, for clarity, by rectangles of broken lines. The circuit is shown as adapted for the reception of a frequency modulated signal E1 sin (wt-Hm) which is impressed on the antenna AE and is transmitted by way of a selective circuit K to the input A of the phase shifting device T described hereinafter. Phase shifting device T also acts as an M ordinary amplifier tube and develops at the output terminal B a nearly demodulated'F; M. wave E2 sin (wi-I-gbd) which is passed through the frequency changer FC to convert the central frequency to 450 kc./sec. The signal then passes through a common selective intermediate frequency amplifier IFA of a bandwidth determined by the fidelity desired. From the amplifier IFA the signal passes to the mixer tube MT operating r as a phase comparison device, and in which the I. signal is mixed with a local oscillation Er. cos wt of also 450 kc./sec. generated by the same tube MT and the associated circuit indicated at L0. The direct and low frequency tension on the plate of the mixer MT are amplified by the direct voltage amplifier PA, the plate current of which varies therefore proportionally to il d and is used as the demodulating current i by applying it to the control terminals C of the phase shifting device T. The said plate current is also applied to a restoring device RZ, in this case consisting of a self-inductance L, on which a voltage that is a voltage proportional to the previously integrated intelligence is created. This voltage is then transmitted to the grid of a power output tube PO and thence, if desired, to a loud-speaker LS.

As above noted a suitable phase shifting device T is described in patent application Ser. No. 436,- 374. Such a device and its embodiment in a circuit arrangement in accordance with the present invention is shown in Fig. i. The device comprises a thermionic tube provided with an indirectly heated cathode to generating an electron beam 4|, 2. control grid @2 connected to input terminal A and controlling the intensity of the a generated electron beam, and an accelerating plate 43 provided with a hole 45 through which the electron beam penetrates into a transit zone between plate 43 and a diaphragm t5 the central hole 56 of which forms the point of origin for the electron beam with respect to a phase shifting zone included between said diaphragm t5 and a collector electrode -ll'. Output terminal B of phase shifter T is connected to the junction point between collector electrode 3'! and an external resistance 38. A load impedance 49 tuned to the central frequency of the wave to be received is connected in shunt with resistor 8 said load impedance being shown in Fig. 3 within the block FC.

The transit zone included between accelerating plate 33 and diaphragm is designed to control the velocity and trajectory of the electron beam before entering the phase shifting zone and may comprise an electronic lens system or any other electron velocity filter of the known type. In

the embodiment of phase shifter T shown in Fig. 4, electron beam 4| describes a helical path controlled by an electric lens system not shown in the drawings. It should be noted, however, that these velocity filtering means are only needed for low velocity electrons. a

As can be observed from Fig. 4, the phase shifting zone includes an assembly of two serially connected coils 5G and 5| which creates a retarding or phase shifting field, so that the current applied to control terminal C determines the phase shift or retardation impressed on the electrical magnitude represented by the intensity modulated electron beam 4 I.

It has, moreover, been shown that when the factor A is sufficiently large, that is to say, when there is sufficient amplification and sufiicientsignal strength, 1, l/ z', and this means that or hence holds even when the amplifier used is not linear,

so that in such cases as well a faithful reproduction of the intelligence is assured. From the negative feedback theory, it also follows that local noises are considerably reduced.

In the foregoing discussion it has been tacitly assumed that it was possible to produce in the local oscillator LO, a local oscillation, the frequency of which would always remain in step with that of the central frequency of the incoming wave or with the frequency converted derivative of the central frequency. Such an'assumption is considered justifiable, since, in my co-pending U. S. application on Method for synchronising a plurality of oscillations, Ser. No. 450,584, filed July 11, 1942, I have disclosed one Way in which the said result may be ensured. Moreover, the method of detection of the present invention is independent in itself of the particular means or process used for securing the synchronism of the local and central frequencies. Similar remarks apply if the incoming wave is of the more general type hereinabove referred to.

Although I have described my novel method of detection with particular reference to certain specific examples and certain theoretical explanations I do not inmnd to be limited thereto, and it is to be understood that I may introduce various modifications in the method without thereby departing from the spirit and scope of the present invention as defined in the accompanying claims.

I claim:

1. A method of detecting a frequency modu lated wave varying as a sinusoidal function of the sum of a central angle and a modulation angle representing intelligence, comprising the steps of impressing on the wave a phase shift proportional to a fraction of said modulation angle approaching unity to produce a treated wave varying as the sinusoidal function of the sum of the central angle and a difference angle substantially small compared with the modulation angle, deriving from the so-treated Wave an electrical potential varying as a sinusoidal function of the sum of a given angle andsaid difference angle, impressing on the said potential a local oscillation varying as a sinusoidal function of said given angle, deriving from the said potential and said local oscillation a resultant electrical potential substantially proportional to the said difierence angle, and converting said resultant electrical potential into an electrical potential directly proportional to the intelligence. 7

2. A method of detecting a frequency modulated Wave varying as a sinusoidal function of a central angle and modulation angle representing intelligence, comprising the steps of shrinking the excursion of said wave by impressing thereon a phase shift proportional to a fraction of the modulation angle approaching unity to produce a wave having a shrunk excursion and varying as a sinusoidal function of the sum of the central angle and a difference angle having a value substantially smaller than V 3. A method of detecting a frequency modu-f lated wave varying as a sinusoidal function of the sum of a central angle and a modulation angle representing intelligence, comprising the steps 'of impressing on the incoming wave a phase shift proportional to a fraction of said modulation angle approaching unity to produce a treated wave varying as a sinusoidal function of the sum of the central angle and a difference angle substantially small compared with the modulation angle, heterodyning the so-treated wave with a first local oscillation to obtain an intermediate frequency oscillation varying as the sinusoidal function of the sum of an intermediate central angle and said difference angle, the intermediate central angle being equal to the difierence between the central angle of the treated wave and a central angle corresponding to said local oscillation, impressing on said intermediate frequency oscillation a second local oscillation varying as the sinusoidal function of saidintermediate central angle, deriving from the said intermediate frequency oscillation and the said second local oscillation a resultant electrical potential proportional to said difference angle, varying the phase shift impressed on the incoming wave proportional to said resultant electrical potential, and converting said resultant electrical potential into an electrical potential directly proportional to the intelligence.

4. In a device for detecting a modulated Wave varying as a sinusoidal function of the sum of a central angle and a modulation angle reprerenting intelligence, means for impressing on the incoming wave a phase shift which is a fraction of the modulation angle approximatingunity to produce a wave varying as a, sinusoidal function aeoaeeo of the sum of the central angle and a difference angle proportional to and small compared with the modulation angle, means for deriving from the so-treated Wave an electrical potential varying as a sinusoidal function of the sum of a given angle and said difference angle, means for generating a local oscillation varying as a sinusoidal function of the said given angle, means for mixing the said potential with said local oscillation to produce a resultant electrical potential substantially proportional to the said difference an.- gle, means to control said phase shifting means in accordance With said resultant electrical potential, and means to convert said electrical potential into an electrical potential directly pro portional to the intelligence.

5. In a device for detecting a modulated wave varying as a sinusoidal function of the sum of a central angle and a modulation angle representing intelligence, means for impressing on the incoming wave a phase shift which is a fraction of the modulation angle approximating unity to produce a Wave varying as a sinusoidal function of the sum of the central angle and a difference angle proportional to and small compared with the modulation angle, means for generating a first local oscillation varying as a sinusoidal function of a heterodyne angle difiering by a predetermined intermediate angle from said central angle, means for mixing said so-treated Wave with said first local oscillation to producean intermediate-frequency wave varying as a sinusoidal function of the sum of said intermediate angle and said difference angle, means for generating a second local oscillation varying as a sinusoidal function of said intermediate angle, means for mixing said intermediate-frequency wave with said second local oscillation to produce an electrical potential substantially proportional to the said difference angle, means to control said phase shifting means in accordance with said resultant electrical potential, and means to convert said electrical potential into an electrical potential directly proportional to the intelligence.

6. A device for detecting a modulated Wave varying as a sinusoidal function of the sum of a central angle and a modulation angle representing intelligence, comprising a thermionic tube having an electron beam generating means, means to vary the intensity of the electron beam, a collector electrode for the electron beam, and control means to vary the time of transit of the electron beam between an effective point of origin of the beam and the collector electrode proportional to and in synchronism with a potential applied thereto, means to apply said modulated wave to said beam intensity varying means to thereby modulate the intensit of the beam in synchronism with the frequency of the modulated wave, means to apply to said control means a resultant potential to control the time of transit of the intensity modulated beam proportional to a fraction of the modulation angle approaching unity to thereby produce at the collector electrode a treated Wave varying as a sinusoidal function of the sum of the central angle and a difference angle proportional to and small compared with the modulation angle, means to derive from the treated Wave a potential varying as a sinusoidal function of the sum of a given angle and said difference angle, a source of a local oscillation varying as a sinusoidal function of the said given angle, means to mix the potential derived from said treated wave and said oscillation and to produce said resultant potential, and means to derive from said resultant potential an electrical potential directly proportional to the intelligence.

'7. A receiver for a frequency modulated wave varying as a sinusoidal function of the sum of a central angle and a modulating angle proportional to a function of the intelligence to be detected, comprising a thermionic tube having electron beam generating means, means to vary the intensity of the electron beam, a collector electrode for the electron beam, and control means to vary the time of transit of the electron beam between an effective point of origin of the beam and the collector electrode proportional to and in synchronism with a potential applied thereto, means to apply said modulated wave to said beam intensity varying means to thereby modulate the intensity of the beam in synchronism with the frequency of the modulated wave, means to apply to said control means a resultant potential to control the time of transit of the intensity modulated beam proportional to a fraction of the modulation angle approaching unity to thereby produce at the collector electrode a treated wave varying as a sinusoidal function of the sum of said central angle and a difference angle proportional to and small compared to said modulating angle, a narrow band intermediate frequency amplifier, a frequency changer stage coupled to said collector electrode and the intermediate frequenc amplifier to produce in said amplifier an intermediate frequency wave varying as a sinusoidal function of the sum of an intermediate central angle and said difierence angle, a source of local oscillations varying as a sinusoidal function of said intermediate central angle, means to combine the output of said amplifier and the oscillations derived from said source to produce said resultant potential, and means for deriving from said resultant potential an electrical potential directly proportional to said intelligence.

MARC ZIEGLER.

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