US2484556A - Demodulator for frequency modulated signals - Google Patents

Description

Oct. 11, 1949. T. s. CUSTIN 2,484,556

DEMODULATOR FOR FREQUENCY MODULATED SIGNAL$ Filed Nov. 12, 1946 2 Sheets-Sheet l Fig.2.

C'L IPPE 19 Inventor": Thomas G. Custin,

by ,d'm

His Attorney.

Oct. 11, 1949. I T. G. CUSTIN 2,484,556

DEMODULATOR FOR FREQUENCY MODULATED SIGNALS Filed Nov. 12, 1946 2 Sheets-Sheet 2 Fig. 8. J\

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H is Attorney Patented Oct. 11, 1949 DEMODULA'TOR FOR FREQUENCY. MODULATED SIGNALS Thomas; G. Ciistimsyracuse, N..Y.', assignor to General. Electric Company, a. corporation of:

New York.

Application November 12, 1946, Serial 'No. 709,337.

1 Claim. 1.-

Myinvention relates to. an' improved counter type. demodulator for frequency or phase modulated signals.

An] objectrof myinventionis to provide. a demodulating device iorifrequency or phase modulated signals .which will providewa high output signal level;

A- further object is'sto provide. a demodulating deviceuincorporating means for preventing the passage: of; noisei impulses in. the absence of a received: carrier: of I at least: a predetermined intensity:.

Another object of my invention is to. provide a demodulator'for phase; or: frequency modulated signals: with 1 high. signaleto-noise level characteristics;.

Aniadditional-object is to:eliminate the necessitysf'or high-gainoutput amplifiers in a phase or frequency modulation" receiver and thereby reduce-the distortion andnoise level.

The novel featuresiwhichlbelieve toxbe characteristic of my inventionare set forth with particularity in the'appended claims. My invention itself-,.however,z.both as. to its organization and method of' operation; together with further objects. and advantages thereof may best be understood by referenceto' the following description taken in connection with the accompanying drawings, in which Fig. 1v is-a diagram of demodulator apparatus-embodying; the invention; Fig. 2 is a diagram of a'demodulator incorporating a squelch circuityand Fig. 3 is-amexplanatorydiagram illustrative of the operation of the apparatus.

Counter type'demodulators have been known in the. past. wherein a. frequency-modulated carrier wave is successively subjected to clipping, amplification and differentiation to produce a series of .short pulses, which'in turn arerectified and integrated to produce a demodulated audio output signal. Theoutput signal ofa demodulator of this. type. however, is of. relatively low intensity and-must beramplified inv high gain voltage amplifiers which may introduce undesired noise and distortion.

The.- demodulator,. according to my invention, provides a relatively high intensity output signalpermitting the use of amplifiers of low gain resulting in an output signal characterized by thepresence'ofa minimum of noise and distortion;

According. to my, invention, the frequencymodulated carrier wave is clipped, as in the prior art,. and is then applied to one winding of a three-winding. transformer in which differentiationofi the clipped wave occurs. The differentiation in this transformer provides very 'sharpa pulses, as hereinafter described. Associated with:

the transformer is an' electron discharge device sponding initiating plifier drives itself to complete saturatlon durmg: each pulse and provides substantially perfect? limitingof each pulse. The; electron discharge---. device also serves as a rectifier for the square.

pulses and feeds the rectifiedlpulsesrto atlowrpass filter, such as an audio passs filter; in which in'--- tegration' is. obtained to: produce; an output signal which is determined by the. rate at whichthes squarepulses are applied torthe filter; Thefilterw output therefore is a reproduction-.ofthe. original.

modulation of the incoming carrien.

In the circuit showniinFig. 1, antennazl fure nishes frequency, or phase, modulated; carrier signal energyto. a radio. frequency amplifier and clipper circuit 2', wherein the. incoming" signalr is: modified from a sine wave, suchasrthatlabelledi FM carrier in Fig. 3,.to aclippedsinewave as: shown at 3.111 Fig. 1 and labelled Clippedfinz. Fig. 3.. The clippedwave is applied towindingii. 4 of a. three winding. transformer 5, which is. preferably of the powdered iron. coretype. The? positive-going leading edgeof each clippedpulse;

as-it reaches winding 4 induces. apositive potential in the form of a sharp difierentiatedpulse:

at the-top of the secondary windingj' of trans? former 5 to drive the'control electrode of 81805 tron discharge device I sharply positive. There:-

sultant conduction of device It causes increasing; anode current flow: in" the. third. transformer-r windings, the direction'of the windings being? so chosen-that the increase in anodexcurrentjlnduces an even more positive potential at thecontrol electrode end of. windingli. This-feedback action quickly drives device-.1 til anode current? The device remains. saturated: fort: a predeterminedtimeduring which a maximum" saturation;

voltage drop occurs across load resistor 9iproviding, a shaped, amplified and rectified. pulse as shown in Fig. 3 for. each leading-e'dgei ofithez clipped signal. Thev duration of saturation. is established by the time.- constants as:- determined by the values of resistance;inductanceiand distributed capacity of the circuits including the transformer windings and,componentsconnectedx; therewith. The. control electrode of device" 1:" may be driven, d'uringthis: time; more; positive:

than the potential required for anode current saturation. When the control electrode potential decays, as the charge accumulated in the distributed capacity leaks off, the control electrode starts to fall below the saturation potential and the anode current starts to decrease. A negative-going potential is thus induced on the control electrode end of winding 6 which serves to hasten cutoif of device 1. The negative-going portions of the differentiated pulses, formed by the lagging edges of the clipped carrier waves, do not operate device I. The device operates, therefore, as a rectifier.

The circuit including the transformer and device 1 accordingly operates in response to the leading edge of each clipped sine wave to provide a positive-going difierentiated pulse, which is generated by induction in winding 6 by the current in winding l. These positive-going differentiated pulses initate negative-going voltage pulses across loadresistor 9 of large constant magnitude due to the self-saturation of device 1.

The negative-going pulses generated by device 1 are; integrated by filter network l9, which resonates at-a frequency below the frequency of pulse repetition, such as at less than 250 kilocycles, and theresultant output voltage, which may be an audio frequency voltage, represents a summation of the energycontained in the square wave rectifled pulses- The output voltage is, accordingly, an accurate reproduction of the frequency, or phase, modulation appearing on the received carrier. r

Operating potentials may be supplied for device 7 in a conventional manner, anode power being provided from a suitable direct current ptentialsource such as B battery H and control electrode negative bias potential from a voltage divider. comprising resistors I2 and I3 connected across C battery l4. heater for the cathode of device 1 is also provided though not shown in the drawing.

.The advantages'of demodulation as described above-over the prior art methods are immediately apparent from Fig. 3, wherein the curves under A indicate prior art operation, and those under B are exemplary of operation utilizing the'zdevic'e-of Fig. 1. In the prior art, as shown, the positive. diiferentiated pulses formed by the leading edges of clipped input signals waves are rectified'directly. These rectified pulses contain onlysm'allquantities of energy, being a little less than the energy in the positive differentiated pulses. Not only are the rectified pulses inadequately limited for many applications, since the rectified pulse energy is related to the energy in the corresponding differentiated pulse, but the low intensity of the rectified pulses requires high gain amplification to obtain-useable output signal strength as indicated by the low intensity output 'wave'shown in Fig. 3.

On the. other hand, according to the present invention,v the differentiated pulses initiate the generation of almost perfectly limited square pulses not dependent in energy content on the A conventionally excited 4 ing 4 of transformer 5. Windin 6 is arranged to provide positive differentiated pulses to the control electrode of discharge device I in response to the leading edges of the clipped sine waves, and feedback action from winding 8 to winding 6 causes device 1 to reach anode current saturation very quickly forjeach positive differentiated pulse appearing on winding 6 as explained in connection with Fig. 1. Filter network Ill integrates the substantially square wave voltage appearing across load resistor 9 to provide an output signal corresponding to the frequency modulation of the carrier received on antenna I.

In addition to the elements described above which are substantially the same as those of Fig. 1, the device shown in Fig. 2 incorporates a squelch circuit arranged to provide a strong negative cutoff bias to the control electrode of device I in the absence of a received carrier of greater than a predetermined minimum intensity. The squelch circuit comprises a diode discharge device .15 with acathode resistor it connected to receive they: amplified but unclipped carrier signal from the:- radio frequency amplifier section of amplifier and clipper circuit 2. When astrong carrier is received, discharge device I5.v conducts, drawing anode current through resistor IT to produce a negative potential on the-anode. 'A threshold potential for conduction of device I5 is established a by selecting the position of the slider of poten-y tiometer I8, which is arranged as a voltage divider across a negative bias potential source- [9 indicated as a C battery.

Electron discharge device .20 is arranged to be conductive when no strong: carrier: is being received. Power source [9 serves to provide operat- 3- ing potentials for device 20,- the negative terminal being connected to the cathode, the positive (grounded) terminal to'the anode, and a selected Y intermediate potential "from the 'slider" of po-' 1 tentiometer I 8 to the control electrode. 5

In the absence of a strong carrier, diode 'device I5 is non-conductive and there is substan- 3 tially no voltage drop in resistor I]. The con- 3 trol electrode of device 20 is thus maintained slider of potentiometer l8, and device 20 conducts to produce a strongly negative potential with respect to ground on that end of resistor- 2| which is connected to the anode of device at. This negative potential, which may be as'hi'gh as volts or more, biases the control electrode of device 1 far beyond cutoff preventing the op U eration of the device in response to any noise impulses which may be received on the antenna I orotherwise. I 7 When a carrieris received which is strong enough after rectification by diode device l5 to produce sufficient voltage drop in resistor 11 to overcome the positive bias on the control elec trodeof device 20, device 20 is cut off, conduction through resistor 2! ceases and the con'trol electrode of device I is made self-biasing through resistors 25 and 22. Condenser 23 funcof device 1.

While I have shown only certainpreferred" embodiments of my invention by way of illustration, many modifications will occur tothose skilled in the art and I therefore wish'to'have-it understood that .I. intend ,in the appended claim to cover all .such modificationsas-fall Within the true spirit and scope of my invention? What I claim as new and desire to secure by Letters Patent of the United States is:

A demodulator for frequency modulated clipped carrier wave signals comprising a transformer, said transformer having an output winding and a feedback winding, an electron diocharge device having a control electrode, an anode, a screen electrode and a cathode, a source of operating potential having a positive terminal and a negative terminal, the negative terminal being connected to said cathode, one end of said output winding being connected through an impedance to said positive terminal and the other end of said output Winding being connected to anode, said feedback winding being connected in regenerative polarity at one end to said control electrode and at the other end through a control electrode leak resistor to said cathode, means to impress said signals on said control electrode to drive said device to anode current saturation in response to impression 0n said control electrode of the positive-going portion of each cycle of said signal thereby to produce current pulses in said anode circuit, a low pass filter connected between said REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,113,214 Luck Apr. 5, 1938 2,261,643 Brown Nov. 4, 1941 2,358,297 Bedford Sept. 19, 1944 2,441,957 DeRosa May 25, 1948

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