US2583138A

US2583138A - Frequency modulator

Info

Publication number: US2583138A
Application number: US718498A
Authority: US
Inventors: John M Carter; John R Boykin
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1946-12-26
Filing date: 1946-12-26
Publication date: 1952-01-22
Anticipated expiration: 1969-01-22
Also published as: BE478223A

Description

Jan. 22, 1952 J. M. CARTER ETAL FREQUENCY MODULATOR Filed Dec. 26, 1946 Ila/19,502?! i p 7 INVENTORS JED/217M Carla! and WITNESSES:

John ,9. Eayk/n.

ATTORNEY imw J Patented Jan. 22, 1952 Md, assignors to Westinghouse Electric (1911-.v poration, East Pittsburgh, Pa, a corporation of fennsirl nia Application DecemberZG, 1946, Serial No. 11,13,43

(01. ss2-2s) 16. Claims 1 u i ent on. relat to, electric is ha app s. d it has. rticular re ation. to an: paratus for producing frequency-modulated oscillations.

A par us ons ruc e ac ordance; with he teachings of the prior art, of which we,- are aware, is exemplified by the Fichandler Patent,2,000,584. Fichandler discloses. a system for producing freen myo u a e o ci at o s compr s ng a. fr quency-determining network, across which a variable resistance and ar actancere. conne te The co po en s. at he, ne we h and the. es anee and, r etanee a e. so s tthat vari tion o he t nc er a ma l range pr duc sa ti n i the eactance i he Whole.- net or Without accomp ny n variation of resist nc handler sho s t e: var able. res stance a a three-electrode tube, the. grid potential 0i which is varied to; vary the plate resistance.

W have found that a transmitter including the Fichandler system. has severa undes ra le features. Th output a tra sm tt s distorted. It is. marredby hum such asarises from undesired pulsations of a, (id-cycle, power supply and by other sounds notintrodimed by the modulating source, Amplitude modulations are also at times present in the outp t;

It is, accordingly, an object of ourinvention to provide a frequency-modulation system which shall produce signals substantially free; of dis-. tortion.

Another object of our invention is to provide a f qu cy-modulation. system which shall pro duce s na substantially ree oi alternati s urren hum.

A fur her obj ct f our invention is to prov de a irequen y-modu ati n s in wh ch ex raneous ounds, are substantially suppr ss d.

St ll. anoth r objec r our invention is to provide a frequency-modulation syst m which. sha pr duce si als substantially fr e oi amp tu e modul tions. I

Mor generally stated, it; is an. object o our v nti n t pr vide. a, irequeney nodulation transmitter of, simple struc ure, which e traneous signals shall be suppressed.

Our inv ntion rises from. our r alization that he sensitive point in a ir queney-modu atiqn transmitter is the network at; which the modes lations are introduced. Beyond. this. point. the.

amplified by the.v multiplication effect and ma.- terially deteriorate the. output,

The interelectrode capacity of Fichandlers resistance tube is substantial, as is, also the capacity to ground of the, electrodes. These capacities introduce distortion into, the modulations impressed on the frequency-determining network. The amplitude of the distortions is small, but the distortionsare highly amplified by the frequency multiplication and deteriorate the out,-

nu o e r n mit eh.

hahd ers re i tan e tube reou es a l e upply ich s. customarily o t e r t alternating-current. type. Although this supply is filtered, the filament current retains pulsa-,

. tions of low magnitude The pulsations highly modulating network. The variable resistance tube (3.4., Fig, 5), haying substantial anode grid capacity, p ovides a r at y ow mped c f e a nathe enerative e f c p u d by the feedback amplified by multiplication also dete o ates th outpu I c r anc i h. r n n ion we pro: e a i eu n y-moduiatisn network c mp sing a crystal of thetype usedcrystal rectifiers. Such crystals are commonly composed of germanium, silicon, galena or similar substance. As the current through such crystals varies, the resistance or the crystal varies, This variation i resi ance is ntroduced n a i e uen v eerm ning netw rk, o roduce fre ue y uation Feedback; oI energy from the oscillation gen erator to he m du ting etwork is upp ess y a crys al re tifie n erposed n h ,1 0dulation net orkhis rect fierhas l w a aci y an efiectivelyblocks he n w of feedb ck o la ioh- The novel features that we consider characeristie Qi inven ion are set forth with partieulari y inflict appended laims. The invention itself, however, both as to its origination and its method of operation together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment when read in connection with the accompanying drawing, in which:

Figure 1 is a diagrammatic view showing a frequency-modulation system in accordance with our invention,

Fig. 2 is a view in section showing a crystal assembly used in the preferred practice of our invention,

Fig. 3 is a diagrammatic view showing a modification of our invention,

Fig. 4 is a diagrammatic view showing a further modification of our invention,

Fig. 5 is a diagrammatic view showing still another modification of our invention, and

Fig. 6 is a diagrammatic view showing a still further modification of our invention.

The apparatus shown in Fig. 1 comprises a generator of electrical oscillations 5 including an electric discharge device 1 which has an anode 9, a cathode II and a grid l3.

Anode potential is supplied to the discharge device from a source (not shown) through a resonant network |5 connected to the anode 9. The grid I3 is biased negative with respect to the cathode II by operation of a'biasing network including a resistor shunted by a capacitor I9. The biasing network is connected between the grid I3 and a frequency-determining network 2|. The latter comprises an inductor 23 and a variable capacitor 25, and is connected to ground at the terminal remote from the biasing network. The cathode I is connected to the inductor 23 at a point intermediate its terminals. The portion of the inductor between the cathode II and ground introduces a regenerative potential, causing the circuit including the discharge device I to oscillate.

Across the frequency-determining network 2|, a modulation network including a coupling capacitor 21 and a crystal assembly 29 is connected.

The crystal assembly 29 is preferably a 1N34 germanium detector such as is shown in Fig. 2 It comprises a cylindrical ceramic tube 30 closed at the ends by caps 32. From the center of one cap, a small metallic projection 34 extends. A particle 35 of crystalline germanium or other suitable material is secured to the projection. From the other cap 32, a fine wire 38 extends. The tip of the wire engages the germanium particle. The assembly is approximately 4" in diameter and long and its distributed capacity is small. The essential element of the assembly is the crystal 35. The other components are provided for effecting connection with the crystal.

The crystal assembly is supplied with current from a modulating source, suchas a microphone circuit 3|, for example. The microphone circuit includes an output transformer 33, the secondary 35 of which is connected across the modulation network 2| through a rectifier 31. This rectiher (31) may also be a 1N34 crystal assembly. Across the secondary 35 of the transformer, a condenser 39 is connected. This condenser and the rectifier 31 in series with the secondary operate to suppress the fiow of the oscillations produced in the generator through the microphone circuit 3|.

Signals impressed in the microphone circuit work. The currents vary the resistance of the crystal, thus varying the net reactive component of the frequency-determining network 2|. The magniudes of the components of the network 2|, the mean crystal resistance and capacity 21, are so set that the net resistive component of the frequency determining network is not varied. Accordingly, frequency modulations, but not amplitude modulations, are produced in the output of the oscillator.

The resonant network IS in the output circuit of the discharge device I is tuned to a multiple of the unmodulated frequency of the oscillator. This network (I5) is inductively coupled to another resonant network 4| in the control circuit of another electric discharge device 43. The latter resonant network (4 is tuned to the same frequency as the network I5 in the plate circuit of the first electric discharge device I. The circuit including the discharge device 43 thus operates as a frequency multiplier. Depending on the output frequency and amplitude desired, additional frequency multipliers and amplifiers 45 may be interposed between the multiplier (l5, 4|, 43) just described and an output device, such as a radiating antenna 11.

The signals radiated by the antenna are free of alternating-current hum because the crystal 29 at the sensitive point in the transmitter circuit does not require filament current. Distortions and amplitude modulations are suppressed because the interelectrode capacity of the crystal 23 and its capacity to ground is small. The crystal is small and its electrodes have no tendency to be set into vibration by transmitter station sounds, and, therefore, the output signal is free of microphonic noises.

We have conducted extensive tests with our modulation circuit and have found that the crystal circuit has a usable linear responsive over a range of approximately 3 to 4 per cent of the unmodulated frequency of the oscillation generator. This linear range is substantially greater than that available for the modulation circuit in which a vacuum tube is connected at the sensitive point. Accordingly, distortions in the output signal arising from deviations from linearity, are substantially less in a modulation system in accordance with our invention, than in the prior art modulation systems.

In the modifications of our invention shown in Fig. 3, the rectifier 31 in series with the secondary 35 of the microphone transformer is replaced by a reactor 49. This reactor and the capacitor 39 perform the function of suppressing oscilla tions from the generator 5. While our il'iVBl'1-' tion, in its broadest aspects, comprehends within its scope a frequency-modulation system incliirl ihg the reactor, the system including the rectifier 31 in the secondary circuit in lieu of the reactor 49 is superior and is to be preferred. The reactor has a tendency to introduce nonlinearity in the response of a modulation circuit.

In the modification of our invention shown in Fig. 4, a capacitor 5| is connected in parallel with the crystal 29. This capacitor cooperates with the coupling capacitor to properly divide the voltageacross the inductor 23 in the frequencydetermining network 2|.

"Fig. 5 shows a circuit designed for high r.-f. current. In this a plurality of crystals are connected in parallel.

Fig. 6 shows a circuit in accordance with our invention designed for situations where the amplitude of the radio frequency oscillations exceeds thera ting-of single crystals; In this case, a pair- (ora plurality) or crystals 53, 55-, 5,1 and 519 in series'may be: used.

Although we have shown and described certain specific embodiments of our invention, we are. aware that many modifications thereof are pos sible'. For example, the capacitor 21 may be replaced by a reactor of any other type such as an inductor. Other reactances may be correspondingly varied. For low carrier frequencies, the tube I: may be, of: the gas filled type; for very high frequencies, it may be of the klystron type. In; the latter case, the networkv 21- is a resonator. The crystal assembly 29 may in specific embodiments of my invention be of a difierent structure than the one disclosed. Where our invention is applied to relativelylow frequency systems, crystal assemblyhaving larger distributed capacity and capable of conductingcurrent or larger magnitudemay be used. Qur'invention, therefore, is, not toberestricted except insofar as is necessitated by the prior art and by thespiri t the appended claims. 7

We claim as our invention:

1'. In combination, a generator of electrical os cillations, a frequency-determiningnetwork for said generator, a network shunting said frequency-determining network including a crystal. the resistance of which varies with the current flowing therethrough, means. for varying the currentfiow through said crystal, and meansinterposed between said shunting network and said varying means. for suppressing, the flow of, oscillations from: said generator through said vary-v ing means, said suppressing meansincluding a crystal and condenser in series.

2. In combination, a generator of electrical oscillations, a frequency-determining network for said generator, a network shunting said frequency-determining network including a crystal, the resistance of which varies with the current flowing therethrough, means for varying the current fiow through said crystal, and means interposed between said shunting network and said varying means for suppressing the fiow of oscillations from said generator through said varying means, said suppressing means including a crystal rectifier.

3. In combination, a generator of electrical oscillations, a frequency-determining network for said generator, a network shunting said frequency-determining network including a crystal, the resistance of which varies with the current flowing therethrough, means for varying the current flow through said crystal, and means interposed between said shunting network and said varying means for suppressing the flow of oscillations from said generator through said varying means, said suppressing means including a crystal.

4. A source of frequency-modulated oscillations comprising a generator of oscillations including a frequency-determining network, a network shunting said frequency-determining network including a crystal rectifier, means for varying the current flow through said rectifier, and another crystal rectifier interposed between said varying means and said shunting network.

5. In combination, a generator of electrical oscillations, a frequency-determining network for said generator, a network shunting said frequency-determining network including a crystal, the resistance of which varies with the current flowing therethrough, and means for varying the current flow through said crystal, the magnitudes or the; components or said; networks: being.- so

that the; net: impedance produced by variationsin type only.

6.. In combination, a. resonant; network, a net.-

work shunting said resonant network, including a crystal; the resistance of which varies with the current; flowing. therethrough,, and means. for supplying varying current to said shunting network. to vary the resistance of. said shunting network thereby to vary the resonant: frequency of said resonant network.

7-. In combination, a generator. of electrical oscillations; a frequency-determining network for said generator, a. network shunting said .fretruancy-determining network including a crystal. the resistance of which varies with the current therethrough, and connections for varying the current through said crystal in accordance with a given signal to frequency modulate said generatori-n accordance with said-signal. I

8-. Incombination, 'a generatorv of electrical oscillations, afrequency-determining network for said generator, a network shunting said froquency-determining networkincluding a. crystal,

the resistance of which varies with the current flowing therethrough, and. a reactance connected in series, andconnecti-ons' for varying the cur rentthrough said crystal in accordance with a given signal to frequency modulate said: gen enator in accordance with said signal.

9. In combination, a generator of electrical oscillations, a. frequenoys-dete'rmining network for said generator, a network shunting said free quency-determi ning' network including a crystal of' the: type that is electro-mechani'cally independent of said generator and the resistance of which varies with the current flowing therethrough, and connections for varying the current flow through said crystal.

10. In combination, a generator of electrical oscillations including a frequency-determining network, a network shunting said frequency-dctermining network including an electromechani- 3 cally inert crystal, the resistance of which varies with the current flow therethrough, and connections for varying the current flow through said crystal.

11. In a modulation circuit, the combination of a generator of electrical oscillations, a frequency determining network for said generator, a modulation network including a crystal shunting said frequency determining network, a source of modu lating signals, a modulation transformer connected to said source of modulating signals and having a secondary connected to said modulation network, and a suppressing circuit connected across said secondary.

12. In a modulation circuit, the combination of a generator of electrical oscillations, a frequency determining network for said generator, a modulation network including a crystal shunting said frequency determining network, a source of modulating signals, a modulation transformer connected to said source of modulating signals and having a secondary connected to said modulation network, and a suppressing circuit connected across said secondary including a crystal and a reactance in series.

, 13. The combination of a generator of electrical oscillations, a frequency determining circuit for said generator, a modulation network including a low-capacitance crystal shunting said frequency determining network, a source of modulation signals connected to said modulation netquency multiplier connected between said generator and said output translating device and adapted to multiply the output oscillation frequency from said generator before feeding said translating device.

14. In apparatus for providing oscillations which are adapted to be frequency modulated by a modulating source,'the combination of a resonant network, a modulating network including a crystal element connected to said modulating source and shunting said resonant network, the resistance of said crystal element varying with the current flowing from the modulating source through the element, and a reactance connected between said resonant network and said modulating network. H

15. For use in apparatus for providing high frequency oscillations which are adapted to be frequency modulated by a modulation signal source, the combination of a frequency determining network for said oscillations, a modulat ing network connected between said frequency determining network and said modulation signal:

source, said modulating network including a crystal element having a resistance which varies with the current through the element, and a capacitor having a relatively low impedance to said high frequency oscillations and shunting said crystal element.

16. In apparatus for generating first oscillations which are adapted to be frequency modulated by second oscillations from a modulating source, the combination of a resonant network for determining the frequency of said first oscillations, and a modulating network shunting said 8. resonant network and connected to said modulating source, said modulating network including a crystal rectifier of the type that has an interelectrode capacity and a capacity to ground which are small compared to the capacity of said network.

- JOHN M. CARTER. JOHN R. BOYKIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 17,356 Cady July 2, 1929 1,447,793 Latour Mar. 6, 1923 1,471,418 Rodgers Oct. 23, 1923 1,998,119 Cox Apr. 16, 1935 2,033,231 Crosby Mar, 10, 1936 2,182,377 Guanella Dec. 5, 1939 2,222,048 Stevens Nov. 19, 1940 2,298,437 Usselman Oct. 13, 1942 2,306,555 Mueller Dec. 29, 1942 2,424,246 Mason July 22, 1947 2,461,307 Antalek Feb. 8, 1949 2,473,556 Wiley June 21, 1949- FOREIGN PATENTS Number Country Date 491,103 Great Britain Aug. 26, 1938 OTHER REFERENCES Characteristics and Applications of Selenium- ;Rectifier Cells, by Harty, Electrical Engineering,

October 1943, vol. 62, page 625.