US2363652A - Frequency modulation discriminator circuit - Google Patents

Description

Nov.` 28, 1944. M, cs. cJRoslaYV 2,363,652

` FREQUNCY MODULATION DISCRIMINATOR CIRCUIT Filed April 2s, -1943 2 sheets-sheet 1 .O FF- TUNE "ATTORNEY Nov.4 28, 1944.' I M. G. cRosBY 1 2,363,652

FREQUENCY MODULATION'DISCRIMINATCR CIRCUIT l 'S C Fc -FfEgz/s/vcr I IH 7b A j L 3) M.. Mmmm-.- '576ML r1 j l 007.007 5 9' al; ///`=E I INVENTOR ATToRNEY Patented Nov. 28, 1944 FREQUENCY MoDULA'rToN DISCRIMINA- 'ron CIRCUIT Murray G. Crosby, Riverhead. N. Y., assignor to .Radio Corporation oi' America, a corporation of Delaware .I

I Application April 29, 1943, Serial No. 484,973

j 9 claims. (ci. 25o-27) MyV present invention relates generally to frequency discriminator circuits, and more particularly to novel forms of discriminator circuits for frequency modulated (FM) carrier wave energy.

An object of my invention is to provide an FM detector circuit wherein a pair of opDOSed rectiers is fed by a discriminator which consists of a resonant circuit path for feeding both rectiflers in parallel with retarded FM 'signal energy, and additional paths for feeding the rectiiiers with unretarded FM signal energy, but in opposite polarity.

Another object of my invention is to provide l a discriminator for FM signal energy wherein there is provided a parallel resonant circuit path functioning to feed retarded FM signal energy to a detector for frequencies oif the predetermined frequency of the parallel resonant circuit, there Vbeing provided a second path to the detector circuit for feeding to the latter unretarded FM signal energy.

The novel features which I believe to be characteristiocf myfinvention are set forth with particularity in the appended claims; the invention itself, however. as to both its organization and method of operation will best be understood by reference to the following description, taken in connection with the drawings, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings: Fig. 1 shows ,one form of a discriminator-recti fier circuit embodying the invention,

Fig. 1a shows a modification of the circuit arf rangement of Fig. 1,Y

source of the signal energy is not disclosed, because it may be ofany well known form. For example, where the invention is used in a superheterodyne receiver of FM signal energy it will.

be clear that the transformer 3 maybe an intermediate frequency (I. FJ transformer which is fthe series resonant circuit S. This series reso- .or low deviation ranget fed fromV the'plate 'circuit of a prior stage of I. F. amplification, or from the plate circuit of an amplitude limiter tube. In any event the primary circuit l of transformer 3 is xedly tuned to theoperatlng I. F. value of the receiver. The FM signal energy itself may either have a high A ain, the FM signal energy received at the signa collector device of the receiver may be in the megacycle (Mc.) range,

and the I. F. value itself may be in the megacycle or kilocycle bands.

'I'he secondary winding "I of transformer 3 has its upper end connected to the anodes of rectiiiers I and 2 through s. path which includes nant circuit S is tuned to the mean, or center, frequency of the applied FM signal energy. This frequency will, of course, be the Vfrequency to which the primary and 'secondary-circuits of 4 transformer 3 are tuned. The series resonant cir- Figs. 2a and 2b show graphically the vector?,

relations between the retarded and unretarded signal energies of Figs. 1 and 1a for the "in-tune condition as well as for the off-tune"v condition, A

Fig. 3 shows a modified form of the invention, Fig. 3ashows the frequency-amplitude char-f acteristic lof the parallel resonant circuit of thev `discriminator in Fig. 3,

Fig. 3b vectoriallyY illustrates the olf-tune condition of the discriminator in Fig. 3, y

Fig. 4 shows a circuit embodying a'. modiiica-A tion' of the discriminator of Fig.13.

Referring now to the accompanying drawings.- wherein like reference characters in the different figures designate similar-elements, in Fig. 1 the opposed rectifiers I and .2, specifically disclosed as of the diode type, function as a means for rectifying 'signal energy applied thereto. The

cuit specifically consists of condenser 8 and inductance 9. The output terminal of the series resonant circuit S is connected to the respective anodes of diodes I and 2 through resistors I0 and II respectively. 'I'he opposite ends of winding I are respectively connected to the anodesA of rectiersJ and 2 through non-selective paths s consisting of respective condensers 5 and 6. The

" condenser 5 shunts the series resonant circuit'S and resistor l0. The condenser 6 is connected directly from the lower end of winding 1 to the anode of rectifier 2. The` midpoint of winding 1 is effectively 'grounded for radio frequencies by virtue of its connection to lground through a path which includes the load resistor I2.

This load resistor I2 is arranged in series with load resistor I3, and the cathode end of resistor I2 is grounded. Each of resistors I3 and I2 is shunted by a respective carrierfrequency bypass condenser. Resistor Il is a direc current return resistor connected between the node of rectier I andthe junction of resistors I3 and I2.4

Resistor I5 functions as a direct current return resistor, and is connected between the anode ofrectier 2 and the aforementioned junction point. It will be understood that from the cathf odeend of resistor I3 there is derived the difage of each of rectiiiers I and 2,. This diiferenferential resultant of the rectiiied output volttial resultant voltage may be the modulation signal voltage, such as audio frequency voltage, which is utilized in subsequent audio frequency amplifiers. There may, also, be taken off from the cathode end of resistor I3 automatic frequency control (AFC) voltage for correcting the frequency of the local oscillator of the super-` heterodyne receiver so as to compensate forany relatively slow shifts in the mean frequency of the FM signal energy applied to the input transformer 3.

Considering the functioning of the network of Fig. 1 it is pointed out that for the in-tune condition, depicted in Fig. 2a, Vthe Voltage which is -fed to each of the rectiiers through the series resonant path S may be y represented yby the Vector Es. In the in-tune condition of the applied signal energy the mean frequency of such applied energy is, of course, equal to the resonant frequency of seriesresonant circuit S. y The circuit S, as is well known, retards or shifts the phase of Ithe signal voltage applied thereto to an increasing degree for frequencies departing from the mean or carrier frequency value. Hence,

l there will be no phase shift of the signal voltage applied to S at the instant when its mean frequency is equal to that of circuit S. Since the resonant circuitI S feeds the rectiilers through resistive elements I and II, it follows that at the instant when the applied signal energy is at the mean frequency there will be no phase shift provided in the signal energy developed Es in Fig. 2a. The signal voltage which passes through condensers 5 and 6 respectively has not applied to the respective rectifiers I and 2 are now substantially as represented in Fig. 2b. One of these resultant vectors exceeds the other in magnitude, and the differential voltage derived from the cathode end of resistor I3 has some value.

The value of the differential voltage is dependent at the output terminal of S which is fed in like j polarity to the rectiflers. This would' alsobe true at other instantaneous frequencies of the signal energy.

However, the signal Voltage fed through condensers E and 6 is derived from the opposite ends of winding 1. Since the midpoint of winding 1 is effectively grounded for radio frequency energy,

it follows 4thatthe voltages fed through condensers 5 and 6 are respectively of l different polarity. Furthermore, these voltages have a phase shift'of 90 degrees with respect to the retarded voltage fed through the series resonant circuit S. The condensers 5 and 6 do not have the selective phase shift characteristic of circuit S. 'I'hey affect the signal energy in a non-selective manner at all instantaneous frequencies.

Fig. 2a shows the vector representations of the voltages Ee and E5 which are fed through con'- vdensers 6 and 5 respectively. It will, therefore, be

plied signal energy is different from the resonant frequency of series resonant circuit S, there is produced aphase shift in the voltage fed therethrough. As previously explained this phase shift is increasingly greater in proportion to the departure of the instantaneous signal frequency from the vmean frequency thereof. The vector E; in Fig. 2b represents the voltage now'fed through the series resonant circuit, and it will be seen that some phase shift has been provided in this vector relative to the angular' 110511.71013 Qf upon the magnitude of frequency difference between the instantaneous frequency of the applied signal energy and the predetermined frequency of series resonant circuit S. In the case where there occurs a relatively slow departure ofthe signal energy mean frequency from the aforesaid predetermined frequency of circuitlS, the differential voltage is used for AFC voltage. The

relatively fast frequency deviations of the signaly energy from said predetermined frequency show up as modulation variations of said differential voltage.

In Fig. 1a. there is shown a modification wherein the series resonant circuit 8-9 is fed from a single resonant circuit 3 located in the plate circuit of the prior amplifier tube 4. The coil of tuned 4circuit 3' is center-tapped by the +B voltage line which feeds the plate of tube 4. The +B line is bypassed to ground for signal frequencies, and therefore the center tapis at ground potential for signal frequencies. In this way there is provided the opposite polarity voltages for the paths 5 and 6. The circuit is otherwise similar in action to that of Fig. 1. The advantage of this circuit is that the two circuits 4 and I of Fig. 1 have been replaced by a single input circuit.

It is not necessary to employ a series resonant circuit in the retarding or common selective phase shift path to the opposed rectifiers. In Fig. 3 I have shown a parallel resonant circuit T which specifically consists of an inductance 9' and a shunt condenser 8. The parallel resonant circuit Tis tuned, as was the series resonant circuit S, to the desired or predetermined mean fre- 'quency of applied signal energy. That is, the

circuit T is tuned to the same frequency as the input transformer 3. In the case of a superheterodyne receiver the transformer circuits and T will be tuned to the operating I. F. value. There is a difference in operation in this modification by virtue of the nature of the frequency- V amplitude characteristic of circuit T.

resonant circuit T is'zero. For frequencies less than, or greater than, the resonant frequency ol circuit T, -the amplitude of the transmitted current increases in magnitude in a positive or negative sense respectively. The positive increase in current is inductive in nature, whereas the negative increase in current is capacitive in nature.

Those skilled in the art are fully aware of this` exist for theoff-tune condition of the circuit of Fig. 3. The vectors Es and E5 are similar to those Shown in Fig. 2b. That is, the FM signal energy aaeacta' transmitted throughcondensers V and "6 are shifted 90 degrees in phase, and are appliedto` the rectifiers 4I and 2 in opposite polarity. At the in-tune condition, that is when the mean-fre quency of the applied signal energy is exactly equal to the frequency Fc of tuned circuit T, the vector Et representing-the voltage transmitted through circuit T is zero. Therefore, there will be applied to the opposed rectiiiers I and 2 voltages E5 and Es respectively, and the differential outputvoltage will be zero. However, for the `olf-tune condition the vector Et will have a length which depends on the frequency difference between theinstantaneous frequency of the applied signal energy and the frequency Fc. The vector. Erwill be in aiding phase with eitherof vectors Es or Es depending upon the direction in which the instantaneous frequency of applied signal energy differs from Fc. This follows from the amplitude-frequency characteristic of Fig.'

3a. The signal current through circuit T will be in phase with the current through condenser 5 or condenser 6 depending upon the 'direction of frequency deviation of the signal energy relative to Fc. Accordingly, the vector Et will be added on either. of vectors Es or Ea, and in this way-one of the rectifiers will have signal voltage` of larger magnitude .applied thereto than the other rectifier. It will, therefore, be noted that for thearrangement shown in Fig. i the resultant vectors are produced by virtue of an angular dierence between the retarded'and unretarded voltages, whereas-for the arrangement shown in Fig. 3 the' resultant vectors arise by virtue of a Vsimple additive, or aiding, phase relation between one of the unretarded voltages Ee or E5 and the retarded voltage Et. V

In Fig. 4 I have shown a further modification which is essentially basedr on thel arrangement shown in Fig. 3. In the case of Fig. 4 the 90 degree phase shift effected by the tuned transformer 3 is utilized to properly phase' the combination of voltages fed to each diode. transformer is tuned for band pas's action, and may be properly damped by primary and secondary resistors 20 and 2|, as shown, or the damping may consist of the losses in the primary The with voltages induced 'in 'in the manner of Fig. 3b.

While I have indicated and described several systems for carrying my invention into eect, it will be apparent to one skilled in the art that my invention is by-no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set of opposed rectiiiers having a differential'output circuit, a discriminator circuit feeding the op posed rectiers, a source of frequency-variable signal energy coupled to said discriminator, the

' improvement inthe discriminator comprising a by to apply retarded signal voltage to said rectilfiers in opposite polarity.

2. In combination with a source of frequency variable signal energy and a pair of opposed rectiers having a differential output circuit, a discriminator circuit comprising a retarding path consisting of a parallel resonant circuit tuned to the mean frequency ofapplied frequency-variable signal energy. means 'for applying the output of said parallel `resonant circuit in parallel to said rectiiiers, a second path feeding the signal energy to one of the rectiiiers with a relative phase shift of 90 degrees, and a third path feedingthe signal energy to the second rectifier in opposite polarity to the signal energy fed through the second pathA but'with a similar quadratureV phase shift.

and secondary coils, 'I'he midpoint of winding 1 is connected to the junction of the load resistors I3 and I2'through a parallel resonant circuit T'.

The midpoint of winding 'l is additionally con` nected to the high potential side of resonant circuit I through a path consisting ofrblocking condenser 5 in `series with the parallel resonant circuit T. The opposite side of resonantl circuit 4 is connected to the midpoint of -winding 1 through a. path which consistsof blocking condenser 6 in series with an adjustable resistor R.. 'I'he midpoint of the primary winding of transformer 3 is connected to the +B line of the direct current voltage supply source for the plate of the immediately preceding amplifier tube. Here. again, the +B line is bypassed to ground for signal frequencies.

In Fig. 4 thev voltages Es and Es are. fed to diodes l and 2 through mid-tapped winding 1.

En is fed through T which has its resistance balirnpedance across which voltage of T and R appear"The highside of 'IT' is connected'to the mid-tap of 1 so that the voltage across T is fed inphase to the opposed diodes, and combines 3. In a system of the type 'comprising .a pair of opposed rectiflers having a differential output circuit, a discriminator circuit feeding the opposed rectiers, lafsource of frequency-variable 'Y signal energy coupled to said discriminator, the improvement in the discriminator comprising a signal energy retarding path consisting of a series i resonant circuit" tuned to the mean frequency of applied signal energy, aperiodic means connecting said series resonant vcircuit in parallel to said rectiflers, and a pair of non-retarding paths connecting said source to respective rectiflers thereby to apply retarded signal voltage to saidrectifiers in opposite polarity, and said aperiodic means functioning to isolate said retarding -path from the non-retarding paths.

4. In combination with `a source of frequencyvariable signal energy and a pair of opposed rectiers having a differential output circuit, a discriminator' circuit comprising a retarding path consisting of a parallel resonant circuittuned to the mean frequency of applied frequency-variable signal energy, meansV for applying the output of said parallel resonant circuit in parallel to said rectiers, a second path feeding the signal energy to one of the rectiflers with a relative phase shift of degrees, and a third path feeding the signal energy to the second rectifier in opposite polarity to the vsignal energy fed through the-second path but with a similar quadrature phase shift, and

isolation` resistors'being inserted between said parallel resonant circuit and third'paths.- y

5. In a frequencyidlscriminator network inand each of said second -cluding a pair of opposed rectiilers, a signal retarding path' correlatingl of a tuned circuit composed of separatereactances of opposite sign,

means for applying retarded signal voltage to the rectifiers in like polarity. separate non-selective paths for feeding unretarded signal `voltages to the rectiilers in opposite polarity, and said applying means being constructed and arranged to isolate the signal retarding path from the separate paths.

6. In a frequency modulation detector, a pair of opposed rectifiers, a discriminator circuit for the rectiers comprising a resonant circuit signal retard path, isolation resistors connecting said resonant circuit in parallel to the rectiilers, and lnon-selective means for feeding the latter with unretarded signal voltages of opposite polarity.

'7. In a system of the type comprising a. pjur of opposed rectifiers having a diierential output circuit, a discriminator crcuit feeding.the opposed rectiilers, a source of frequency-variable signal energy coupled to said discriminator, the

improvement in the discriminator comprising a signal energy retarding path consisting of a series resonant circuit tuned to the mean frequency'of applied signal energy, aperiodic means connecting said series resonant circuit in parallel to said rectier's, and a pair of non-retarding paths connecting said source to'respective rectiiers therebyto apply retarded signal voltage to said recti- 8. In combination witha pair oi opposed diode rectiers having a differential output circuit, a

'diode rectiiiers having a dinerential output circuit, a discriminator circuit comprising a retardy ing path consisting of a parallel resonant circuit,

tuned to the mean frequency of applied signal energy, isolation resistor means `for applying the output of said parallel resonant circuit in parallel to said rectiiiers, a capacitative path feeding the signal energy to one of the rectiers with a relative phase shift of 90 degrees, a second capacitative path feeding the signal energy to the second rectifier in opposite polarity to thesignal energy fed through the first capacitative path but with a similar quadrature phase sluit. 1

- MURRAY G. CROSBY.

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