US2376126A - Frequency modulated wave discriminator - Google Patents

Description

May 15, 1945. CROSBY 2,376,126

I FREQUENCY MODULATED WAVE DISCRIMINATOR 2 Sheets-Sheet 1 Filed Jan. 29, 1943 147M Our/W7- INVENTOR Mummy 6. (2055K ATTORNEY Patented May 15, 1945 FREQUENCY MODULATED WAVE DISCRIMINATOR Murray G. Crosbnniverheatl, N. Y.,-assignor to Radio Corporation of America, a corporation E of Delaware d Application January 2 Mypresent invention relates to signal modulated carrier wave detectors, and more particularlyto a frequency modulated carrier wave detector having improved discriminator input 'circuitsof the oppositely mistuned type.

In his U 'S. Patent No. 2,280,530, granted April 21,1942, G.- Mountjoy has disclosed and claimed a back to back detector whose signal input circuits are oppositely and equally mistuned from the mean, or center,'frequency of applied frequency modulated carrier wave' energy. 'Sym- 'metry is provided in the detection characteristic by employing mutual inductance betweenthe input circuits to cancel the .efiects of any inadvertent capacitance coupling therebetween. Spe-' cifically, he provides rectifiers whose outputs are in phase opposition, while the mistuned input circuits are in series relation across the prior network. Characteristics of such a detector are good linearity, low impedance and relative'ease of alignment of the mistuned circuits.v

It is found that a relatively high magnitude of inherent capacitance coupling between sections of the discriminator network renders it difficultto employ mutual inductance between the coils' of the discriminator circuits as a cancellation means. The detection characteristic will remain unsymmetrical wherethe inherent capacitance coupling is too high, and solely the mutual inductance between the detector input circuits is relied upon for compensation. Only by taking Steps resulting in a reduction of gain can the mutual inductance be employed to cancel out such high undesired capacitance coupling.

wave energy, perfect symmetry being provided in the detection characteristic by preventing any coupling between the input circuits, sufficient neutralizing capacity being provided between the latter to cancel out the effects of any undesired capacitance coupling therebetween. I 7

Still other objectsof this invention are to improve the FM detector. circuit of the aforesaid 9, 943, Serial No. 473,9 1 6Claims. 101.250-27) Mountjoy patent, and to provide the improveganizations whereby my invention may be c'arried into eiiect.

In the drawings: I Fig. 1 shows a circuit employing the invention,

Fig 2 shows a modification,

' Figs. 3 and 4 show equivalent circuits for the circuit of Fig. 1, d v

Fig. 5 shows theequivalent circuit in the case of Fig. 2. v

Referring now to the accompanying drawings, wherein like reference characters in the different figures denote similar elements, there are shown' by way of illustration in Fig. 1 the demodulator and tire-demodulator stages of an FM receiver[ The receiver may be of the superheterodyne type. ,ceivers are so Well known at the present time that a general'description thereof should suifice. It is to be understood that while the present description is given in connection with the reception of wave energy, the invention c'ouldbe employed for the reception of phase modulated (PM) energy as well. As will be shown at a later point, in addition, the present circuit is also capable of employment for the reception of amplitude modulated (AM) Wave energy. The generic expression angle modulated carrier wave energy is used in this specification to denote either FM signal energy, PM signal energy or hybrids thereof. In such case the generic expression specifically excludes the reception of AM wave energy.

7 Considering the circuit from the viewpoint of the specific reception of FM wave energy, it is wllknown that the FM broadcast band at the present time occupies the 42 to 50 megacycles (mc.) range. The signal collector device,usually a dipole, feeds collected signal energy to one or more tunable ultra-high frequency amplifiers. The amplified signalswould then be applied to a converter stage to reduce the center, or mean, frequency of the 'FM signals to an intermediate frequency value (IF); It is the general practice at the present time to utilize an IF value of 4.3

Such superheterodyne remc. Of course, the invention is not limited to this specific IF value, since any value may be entployed in this connection. It is also pointed out that the present circuit is applicable in any of the megacycle ranges.

Assuming, for the purposes of illustration, that the FM carrier is deviated up to '75 kc. (kilocycles) on either side of mean frequency by the audio modulation signals, then the IF amplifiers will feed to the rare-demodulator stage FM signals whose carrier is at the IF value of 4.3 mc. while the over-all frequency swing may cover a band of 150 kc. Of course, the modulation, whether of audio frequency or higher, is represented in the IF energy as carrier or mean frequency variation. The amplitude of the audio modulation corresponds to the extent of frequency deviation of the carrier, while the modulation frequency corresponds to the rate of carrier frequency deviation. The IF signal energy is applied to the resonant primary circuit I of the IF transformer 2, and the resonant secondary circuit of the transformer acts as the input circuit of the amplifier tube 4. It will be understood that each of circuits I and 3 is tuned to the operating IF value, while the network l--3 has a band width of the order of 150 kc. so as to pass with efilciency the entire swing of the received carrier energy.

The amplifier 4 may be of any well known type, and is specifically illustrated as of the pentode type. This amplifier may be of the well known IF amplification type, or it may function as an amplitude limiter, if desired. In the case of FM signal reception, it is desirable to have the stage feeding into the discriminator circuit function as an amplitude limiter. Let it be assumed, for purposes of illustration, that since the amplifled FM signals of IF value must be applied to the FM detector with uniform carrier amplitude that the tube 4 and its associated circuits functions as an amplitude limiter. Those skilled in the art are fully aware of the constants that such a limiter stage has. For example, the limiter tube may be a 6J7 pentode whose input electrodes are coupled to the resonant input circuit 3. The plate 5 of tube 4 is connected to a source of positive voltage through the series coils 6 and of the discriminator circuits. For a reason to be pointed out later, the lead from the lower end of coil 6 is connected to a predetermined tap I on coil 1. The lead from the lower end of coil I is connected to the +3 terminal of the voltage supply source, and the voltage lead is bypassed to ground by condenser 8. The coil 6 is shunted by condenser 9, while coil 1 is shunted by condenser ID. A dotted square, designated by numeral ll, surrounds the tuned circuit 9-6, while a dotted square I2 surrounds the tuned circuit Ill'l. These dotted squares signify shielding devices which are employed to isolate the tuned discriminatorcircuits from each other.

The limiter'tube 4 is designed, and has its constants chosen, to produce an amplifier which is very easily overloaded. That is, plate current is caused to flow, and plate current saturation is produced on one-half of the signal cycle. Plate current cut-off is provided on the other half of the signal cycle. The primary purpose of the limiter stage is to Wipe out all amplitude variations present in the output of the IF network 2 prior to the limiter tube, and to transmit to the discriminator circuits 9-5 and Ill-T a constant amplitude-variable frequency signal. It may be desirable, for reasons of simplicity, to dispense with the advantage of amplitude limitation. In

such case reliance may be placed on the balanced demodulator to remove any AM effects during FM reception.

The reference character C denotes the inherent coupling capacitance which exists between the discriminator circuits. This inherent capacitance is designated by dotted lines. It will be understood that the capacitance coupling is due to the plate-to-ground capacity of tube 4 plus th attending plate wiring and circuit capacitance to ground. This inherent capacitance tends to render the detection characteristic unsymmetrical. According to my invention, the effect of this coupling capacitance is substantially eliminated by providing no coupling between the discriminator circuits. Additionally, there is connected between the plate end of coil 6 and the high potential end of coil 1 a condenser designated as Cn. The function of this capacity is to provide neutralization of the inherent capacitance C. The magnitudes of C and Cu should be-equal. There are many cases where the value of C is so high (as between 10 and 15 mmf.) that there cannot be employed the mutual inductance between coils 6 and T, as was disclosed by the aforesaid Mountjoy patent. The present invention provides a simple and-effective method of eliminating such undesirable inherent capacitance coupling.

The limited FM signal energy is applied to the opposed diode rectifiers l3 and I4. While shown as separate diodes, these diodes may have their electrodes in a common tube envelope as in a 6H6 type of tube. The diode 53 comprises an anode l5 and cathode l6, while the diode l4 comprises a cathode l1 and an anode 18. The cathodes l6 and H are connected together by series load resistors l9 and 20, each of these resistors being properly by-passed for carrier frequencies. The anode I5 is connected to the junc tion of resistors IS and 20 by a resistor 2i. The anode I8 is connected to the junction of resistors l9 and 2!] by resistor 22. The anodeend of resistor 22 is established at ground potential. The audio output corresponding to frequency deviations of received FM signals is taken off from the anode end of resistor 2!, while audio variations corresponding to amplitude variations of received AM carrier energy is taken off from the cathode end of resistor E9.

The anode end of resistor 21 is connected by condenser 30 to the plate end'of coil 6. The junction of resistors 2| and 22 is connected by condenser 3| to the junction of coil 6 and point 1" of coil '1. Condenser 32 connects the grounded end of resistor 22 to the +B lead. The leads for taking off the audio modulation from the detected FM wave energy are represented by numeral 40, and the high potential lead of these output connections may include a resistance filter 4| for preventing transmission of carrier frequency currents. The leads 40 designate the output connections for audio modulation of the detected AM wave energy, and resistance filter 4| serves the same function as 4|. It will now be appreciated that resistors 2l22 function as the load impedances of diodes I3 and I4 respectively, insofar asdetection of FM wave energy is concerned. Resistors l9 and 2!) function as load impedances of the rectifiers with respect to detection of AM wave energy.

Considering the discriminator circuits circuit 9- 6 is tuned to one side of the mean, or center, frequency of 4.3 me. For example, the frequency value of this circuit could be 4.4 me. That is,

opposition.

the circuit would be resonated to a frequency 100 vkc. above the mean frequency of the applied FM wave energy. The tuned circuit of the other diode rectifier llwould be resonated to 4.2 mc., or 100 kc. below the center frequency of the applied signal energy. The condensers 30,

3| and 32 function as directcurrent blocking condensers. Proper choice of the magnitude of condenser C11 will result. in cancellation of the effect of the undesired capacitance C, and a symso' as to give thehighest impedance and the greatest gain for the driving tube.

Considering the manner in which the detection network functions, it will beunderstood that the maximum rectified voltage appears across resistor 2| when the instantaneous frequency of the FM signals is substantially that of circuit 9'6. Similarly, maximum rectified voltage appears across resistor 22 when the instantaneous frequency of the FM signals is equal to the resonant frequency of circuit lll- -l. Between these two instantaneous frequencies of the FM signals the rectified voltage.- across each of resistors 2| and 22 will depend upon the frequency displacement of the instantaneous frequency with respect to the resonant frequencies of each of the discriminator circuits. Hence, at the center or mean frequency of applied FM waves the over-all output voltage taken off at the upper end of resistor 2| will be equal to zero, since the diode output resistors are arranged in phase This follows from the fact that at the 'mean frequency of the applied FM signals rectified voltages across resistors 2|-22 are equal. For frequency deviation from the FM carrier frequency value effective output voltage at the upper end of resistor 2| will'depend upon the diiference between the rectified voltages produced across resistors 2| and 22. Since the audio frequency voltage components are represented by frequency swings of the FM carrier, it is obvious that the voltage taken oif at the upper end of resistor 2| is the audio modulation voltage which was applied at the transmitter to the carrier.

It will be understood that modulation voltage output corresponding to modulation of detected AM wave energy may be, had concurrently with, or alternatively with, reception of FM wave energy. In such cases where detection of amplitude modulated carrier wave energy is to be had,

it is necessary to eliminate any amplitude limitations of. signal energy prior to the input circuits of the rectifiers I3 and 14, since such amplitude limitation would seriously affect the amplitude modulation of the desired signal energy. AM reception would be used for bands where'AM and FM signalling are carried on. The AM reception is readily had, because the rectifiers give additive outputs. When applying AM carrier waves to the circuits 9-6 and |D-'I, each respective rectifier I3 and I4 has applied thereto amplitude variable currents. The AM carrier frequency will, of course, fall between the resonant frequencies of the input circuits. The leads 40' provide the sum of the rectified voltage across the resistor I9 and resistor 22. This will be the additive 'voltage across both of said resistors;

Considering, now, the specific aspects of the improved discriminator, Fig. 3 shows the equivalent circuit of the original Montjoy discriminator, but redrawn in a manner to show how the undesired capacity C from the plate 5 of the tube 4 to ground, acts as undesired capacity coupling between the two tuned circuits 6-9 and The coefficient of coupling for such a capacitatively coupled circuit is: Ic=C/ (Co-l-C), where C5 is thecapacity of 9 and 10. For proper adjustment of the discriminator, coils 6 and-1 are in- .ductively .coupled so that the coefiicient of coupling is equal to the capacitative coefficient of coupling is. If Co is'made small to give'the greatest gain from the driver tube 4, then It becomes large, and the required degree of inductive coupling may not be obtained even though the coils are placed tightly adjacent. Hence, to make the degree of inductive coupling obtainable, the usual.

procedure is to'increase the magnitude of Co and thereby sacrifice some gain. That is to say, the capacitative magnitudes of each of condensers 9 and I0 would have 'to be increased greatly to use mutual inductance between coils 6 and I to eliminate the effect of C. According to my invention, as shown by the equivalent diagram of Fig. 4, loss of gain is prevented by isolating the two tuned circuits from each other, and'using Cn to provide neutralization of the effect of C. Sufficient turns'would be added to coil 1', below point 1', to provide the requisite neutralizing voltage. Fig. 2, and the equivalent diagramof Fig. 5, shows how Cn may be connected from the ground end of coil"! to the lower end of coil 6. This isa forward'form of neutralization. Coil 6 would, in this case, have the added turns in place of coil I.

While I have indicated and described several systems for carrying my invention into effect, it

will be apparent to one skilled in the art that my lar organizations shown and described, but that many modifications'may be made without (lo-'- parting from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a network supplying angle modulated carrier waves, a detection network comprising a pair of rectifier circuits, each rectifier circuit having a resonant input circuit and an audio output circuit, said rectifier input circuits being oppositely mistuned with respect to the center frequency of said modulated waves, said output circuits being arranged in phase opposition, and said input circuits being arranged in series relation with each other and with the space current paths of the rectifiers, said input circuits having inherent undesirable capacity coupling of relatively high magnitude, means operatively associated with said input circuits to render them free of any magnetic coupling with each other, and neutralization means to cancel out the effect of said undesirable capacity coupling thereby to provide a detection characteristic which is linear and symmetrical.

2. In combination with a network supplying frequency modulated carrier waves, a detection network comprising a pair of rectifier circuits,

each rectifier circuit having a resonant input circuit and an audio output circuit, said rectifier input circuits being oppositely and equally mistuned with respect to the center frequency of said modulated. waves, said output circuits being arranged in phase opposition, and said input circuits being arranged in series relation with each other and with the space current paths of the rectifiers, said input circuits being free of coupling other than stray undesired relatively high magnitude capacity coupling, and a neutralizing condenser equal to said stray capacity connected between the input circuits whereby the detection characteristic is symmetrical and linear.

3. In combination with a network supplying frequency modulated carrier waves, a detection network comprising a pair of rectifier circuits, each rectifier circuit having a resonant input circuit and an audio output circuit, said rectifier input circuits being oppositely and equallymistuned with respect to the center frequency of said modulated waves, said output circuits being arranged in phase opposition, and said input circuits being arranged in series relation with each other and with the space current paths of the rectifiers, said input circuits being coupled to each other by a coupling which has an undesirable capacity component of high magnitude, a neutralizing capacity between the input circuits to cancel out said capacity component and means to prevent magnetic coupling'between the input circuits.

4. In combination, a diode rectifier circuit provided with a resonant input circuit and a resistive output load, a second diode rectifier circuit providedwith asecond resonant input circuit and a second resistive output load, means connecting said resistive loads in phase opposition, a source of frequency modulated carrier waves, said input circuits being arranged in series relation with each other and in series with thespace discharge paths of said rectifier diodes, coupling means between the source and input circuits havingundesirable capacity of relatively high magnitude to ground, means to maintain said inputcircuits free of magnetic coupling with each other, and a neutralization condenser equal in value to said undesirable capacity connected between said input circuits for cancelling out the effect of said frequency modulated carrier waves, a detection network comprising a pair of rectifier circuits, each rectifier circuit having a resonant input cirsuit and an audio output circuit, said rectifier input-circuits being oppositely and equally mistuned with respect to the center frequency of said modulated waves, said output circuits being arranged in phase opposition, and said input circuits being arranged in series relation with each other and with the space current. paths of the rectifiers, undesirable inherent capacity coupling of excessively high magnitude between said input circuits, means to prevent said input circuits from having any magnetic coupling with each other, means at one input circuit to develop out-ofphase voltage, capacitative means between said one circuit and the other input circuit to employ said out-of-phase voltage to cancel out the effect of said capacity coupling.

6. In combination, a first diode rectifier circuit provided with a resonant input circuit and a resistive output load, a second diode rectifier circuit provided with a second resonant input circuit and a second resistive output load, means connecting said resistive loads in phase opposition, a source of frequency modulated carrier waves, said input circuits being oppositely mistuned relative to the center frequency of said wavesand arranged in series relation with each other and in series withthe space discharge paths of said rectifier diodes, said resistive loads being connected in series between the anodes of said diodes, said loads being connected directly in shunt with said series-arranged input circuits, a second pair of resistive loads connected-in series between the cathodes of said diodes, undesired capacity coupling of excessively high magnitude between the input circuits, means to prevent any magnetic coupling between said input circuits, and neutralization means between the latter to cancel out the effect of the undesired capacity.

MURRAY G. CROSBY.

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