US2218524A

US2218524A - Frequency modulation system

Info

Publication number: US2218524A
Application number: US319877A
Authority: US
Inventors: Joseph G Chaffee
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1940-02-20
Filing date: 1940-02-20
Publication date: 1940-10-22
Anticipated expiration: 1957-10-22
Also published as: FR878762A; FR869048A; GB546677A; BE441132A; NL67475C; DE882424C; US2250104A

Description

Oct. 22, 1940. .J. G. CHAFFEE FREQUENCY MODULATION SYSTEM Filed Feb. 20, 1940 HA RMON/ C GENERA TOR FREQUENCY MOM/LA TCD OSCILLA TOR SIGNAL FIG/B FIG/A v IAIIVENTOR J. G. CHAFFEE A T TORNE' V Patented Oct. 22, 1940 2 t smas -15 M115 PATENT or rice Q U v i ranooiinoyriiefiirlonsYsTniyl Q I M h Q BellTelephone Laboratories, Incorporated, New

York, N. Y a corporation of New York Application February 20, 1940, Serial No. 319,877 i n e p 21Clai msr ((1250-47) 8 i;

The present invention relates to electricalwav e nected in the cathode-ground lead and by-passed control for signaling and kindredpurposes. More by a radio frequency condenser. For a fuller particularly it relates to thesuppression of un disclosure of typical embodiments the detailed p desired variations in the strength of the Waves, description to follow shouldfbe consulted. such as arise from undesired amplitude'modula- Referring. to the drawing, v tion, distortion or noise in the system. Fig. 1 is a schematic circuit diagram of the In signaling "by means of frequency modulated invention as applied to the harmonic generator or phase modulated waves, undesired amplitude portion of a frequency modulated system; and

10 changesoccur from various causes either in the Figs. 1A,.1 B, and. 10 show modifications that wave generating" and modulating part of the may be made in the circuit ofFig. 1.

system or in parts of the system in which the Fig. 1 shows the invention applied to a harmonic transmission is not constant as to frequency generator in a frequency modulated wave trans-v throughout the entire band of interest. Power mitting system. Harmonic generators have been variations produce noise,,besides which there suggested for use in frequency modulation sys- 0 may be noisefrom extraneous sources, tube noise, terns for the double purpose of increasing the etc, all of which may give rise to amplitude varig carrier frequency and increasing the modulation ations. l I v index soas to preserve a constant percentage of It is a general object of this invention to supfrequency shift. This permits generation of the press undesired amplitude variations with infrequency modulated wave at arelatively low fre- 9 creased effectiveness and in a simplified manner. quency and with small frequency shift. The reln It has been proposed heretofore to provide a ativelyhigh frequency wave actually transmitted special pick-up circuit and detector for detectand the required freq'uencyps'hiftof such high it ing amplitude variations in a frequency modufrequency wave. may be secured by frequency 11 lated wave and to impress the detected variamultiplication in the harmonic generator. In tions on the grid of an amplifier in the main the figure a sourceofsignal waves is. indicated signaling path to vary the amplifying properties at 26 connected to frequency modulated oscilof the amplifier in such a way as to compensate lator 2| of any suitable type for producing a relfor the amplitude variations in the waves passatively low frequency, frequency modulatedwave ing through the amplifier. Onedifficulty enwhich is impressed, through a tuned input cir- Q countered with this system arises from the fact (suit, on the harmonic generator '22 comprising 30 that there is a certain amount of phase shift tubes23 and 24 whose inputs are in push-pull rearound such feedback path, and when the numlation and whose plates are. parallel connected ber of meshes included in the feedback loop is to one terminal of tank circuit 25, which is couat all large, this is likelyto result in singing" at pled to transmitting antenna 26. The desired some frequency. l p harmonic frequency is selected by proper tun- According to the present invention a simpliing of the tank circuit 25. A by-pass condenser fied arrangement with improvedjeffectiveness'is 29 is shown aroundthe plate source H which attained by deriving the detectedamplitude variin the case of a system using'considerable power ations in some circuitdevice in the system which would preferably consist of a power rectifier and 40 has detecting properties and using the detected filtering system as common in the art. 40 variations to produce compensating modulation An audio frequency impedance 9, to be dev in the same device. For example, harmonic gen-- scribed more fully presently, is connected in the erators or frequency changers are commonly cathode lead from the negative pole of source present in a frequency modulation transmitting H which is preferably grounded. The grids are system and are used for shifting the frequencies connected to the ground terminal of impedance 45 to a different position in the frequency spectrum. 9. A grid bias battery I0 is indicated and may Such devices are efficient detectors and modube desirable in some cases. if lators. Thepresent invention comprises the use In the operation of the circuit o f-Fig. 1, it is of a negative feedback coupling from the outassumed that the frequency modulated Waves put to the input of such a device, effective to contain some undesired amplitude modulation 50 d feed back low frequency waves havingthe frewhich is to -be eliminated. The harmonic genquency of the undesired amplitude variations but erator 22 has a curved input-output characterineffective to feed back the high frequencies. 'istic and has the property of producting a certain Thus, in its preferred form the inventionv may amount of amplification together with an efiicomprise an audio frequency impedance concient degree of distortion for performing the 55 functionof harmonic generation. As a result of the non-linear characteristic of this device the amplitude modulation assumed to be present in the frequency modulated waves is detected and appears as a low frequency detected component flowing in the anode circuit and therefore in series through the low frequency impedance 9. This current develops a voltage across the terminals of the impedance 9 which is applied to the grid circuit in such phase as to oppose the variations of plate current of envelope frequency, resulting in a reduction of the amplitude modulation of the high frequency waves. The action may be regarded as a negative feedback of the envelope frequencies present in the applied wave, resulting in a reduction of the amplitude modulation appearing in the output. The action may also be looked upon as comprising a recovery of the audio frequency modulating component in the plate circuit and remodulation of the radio frequency wave in reverse sense by application of the recovered modulating component to the grid circuit of the frequency changer.

The form which the impedance 9 of Fig. 1 may take may vary widely in practice and illustrative examples are given in Figs. 1A, 1B, and 1C. In Fig. 1A the impedance 9 is shown as comprising a resistance l3 and radio frequency by-pass capacity [4. In this case the resistance I3 serves both for the feedback of the detected modulating component and also for biasing the grid. In case the grid receives an excessive negative bias an opposing battery may be used with its positive pole connected to the grid. The manner in which the substitute network may be connected into the circuit of Fig. 1 is shown by the dotted rectangle and the three terminals passing through it marked respectively G, C and B.

In the form of network shown in Fig. 1B the cathode resistor I3 is replaced by inductance Hi. This circuit would permit use of a large degree of feedback over some desired range of frequencies where the impedance of the combination l4, I is high, with zero or small feedback for both higher and lower frequencies including direct current where the impedance respectively of condenser I 4 or inductance I5 is much lower than that of the combination.

In the form of the network shown in Fig. 1C a transformer consisting of windings l6 and I1 is used for the feedback of the detected modulating component, while avoiding the feedback of direct current voltage. This circuit permits the use of a suitable voltage ratio in the transformer itself. The secondary winding l! is shown terminated in resistance [8 provided with a slider to which the grid terminal is connected. Any high frequencies appearing on the grid side of this transformer which are not to be fed back may be shunted out by condenser l9. Obviously each of the circuits shown in Figs. 1A, 1B and 10 contains sufficient elements to enable a certain amount of shaping or control of the amplitude frequency relations of the fed back waves. Also, obviously, other types of cathode networks will appear to those skilled in the art, and the forms that have been disclosed are to be construed as illustrative rather than limiting.

Instead of the one stage of harmonic generation disclosed there may be a plurality of stages in tandem for raising the frequency to the desired frequency level. The cathode impedance for reducing amplitude modulation may be used in one or more of these tandem connected harmonic generators.

The amplitude modulations to be reduced may arise at any point in the system including the tubes in the frequency shifting circuits by which the compensating operation is carried out. For example, such undesired moduation may arise from tube noise, power fluctuations or other local sources, or elsewhere.

Circuits embodying the invention may be modified and varied widely within the scope of the claims, which follow.

What is claimed is:

1. In combination, a source of frequency modulated waves, a harmonic generator comprising a space discharge device having a grid, a cathode and an anode, means to impress waves from said source on said grid and cathode, an outgoing circuit connected to Said anode and cathode containing filter means for selectively passing waves of desired harmonic frequencies, and an impedance connected on one side to said cathode and on its other side to said grid and anode, said impedance providing an audio frequency negative feedback path from the output to the input of said device to oppose transmission into the outgoing circuit of amplitude variations of audio envelope frequency in said waves.

2. In combination, a source of frequency modulated high frequency waves, a utilization circuit for waves whose frequencies are harmonics of the frequencies of the waves from said source, and a spacedischarge amplifying device having an input circuit and an output circuit, and having a non-linear characteristic, connected between said source and said utilization circuit, for stepping up the frequencies to the level of said harmonics, and means for opposing audio frequency amplitude variations in the waves of stepped-up frequency comprising an audio frequency negative feedback impedance common to the input and output circuits of said device.

JOSEPH G. CI-IAFFEE.