US2425923A

US2425923A - Frequency divider and discriminator circuit

Info

Publication number: US2425923A
Application number: US581448A
Authority: US
Inventors: Murray G Crosby
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1945-03-07
Filing date: 1945-03-07
Publication date: 1947-08-19
Anticipated expiration: 1964-08-19

Description

Aug. 19, 1947. CROSBY 2,425,923

FREQUENCY DIVIDER AND DISCRIMINATOR CIRCUIT Filed March '7, 1945 I fPzgaz/vcr 0/1405? 1 /6 .7

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Patented Aug. 19, 1947 FREQUENCY DIVIDER AND DISCRIMINATOR CIRCUIT Murray G. Crosby,

Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application March 7, 1945, Serial No. 581,448

8 Claims.

My present invention relates generally to reception of angle modulated carrier waves, and more particularly to a combined frequency divider and discriminator circuit for frequency modulated (FM) carrier waves.

One of the important objects of my present invention is to provide an FM discriminatorrectifier network, wherein the discriminator circuits are normally tuned to frequencies so widely spaced apart that the resonance curves of the discriminator .circuits do not overlap at any points thereof, there being employed balanced rectifiers in conjunction with the discriminator circuits to provide a Inodulatio-n-representative signal.

Another important object of my invention is to provide a frequency divider of the oscillating converter and amplifier type, 50 adjusted that two tuned circuits thereof are off-tuned on opposite sides of respective carrier frequencies, whereby impression of an FM signal on the tuned circuits causes the amplitude of the oscillations in the two tuned circuits to be modulated in opposite directions, in association with a differential detector network for detecting the amplitude modulated oscillations.

Another object of my present invention is to provide a novel FM detection system which eliminates the need for an amplitude limiter, and which may be adjusted automatically tosquelch noise in the absence of received FM signals.

Still another object of my invention is to add further utility to a frequency divider network of the type disclosed and claimed in my U. S. Patent No. 2,344,678, granted March 21, 1944.

Still other objects of this invention are to improve generally the efiiciency and reliability of frequency divider networks in FM receivers, and more especially to provide frequency division networks functioning to discriminate and to provide an output of substantially uniform amplitude without the use of an amplitude limiter.

Still other features of my invention will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect. I

In the drawing:

Fig. 1 shows diagrammatically an embodiment of the invention;

Fig. 2 shows typical resonance curves of the discriminator circuits; and

Fig. 3 shows an alternative relation between the resonance curves of the discriminator circuits.

and particularly No. 2,064,106, granted December 15, 1936, as well as in my U. S. Patent No. 2,230,231, granted February 4, 1941. In the circuits of these patents, prior to demodulation of the frequency modulated carrier waves the latter are subjected to frequency division in such a manner that the frequency swing of the modulated carrier wave is reduced in [the same proportion as the center frequency of the wave.

It will be understood that the frequency divider shown in Fig. 1 may be considered, for a specific utilization, as being located following the intermediate frequency (I. F.) amplifier network of an FM receiver of the superheterodyne type. It is, however, to be clearly understood that the present invention is not limited to a superheterodyne receiver, nor to the specific frequency values given, nor to the specific frequency modulation. The generic expression angle modulated is intended to cover both phase modulation, frequency modulation and hybrid modulations which possess characteristics common to :both phase and frequency modulation.

For the purpose of the present application, let it be assumed that the superheterodyne receiver is one employed in the present FM broadcast band of 40-50 megacycles (mc.), and that the first detector or converter reduces the center or carrier frequency of the modulated carrier waves to an I. F. value chosen from a range of 2 to 20 me. In such case advantages are secured if a frequency division network is utilized to reduce the center frequency, say 4 mc., by a desired factor which is an integer. As explained in my aforementioned patents, the frequency swing of the I. F. energy is simultaneously decreased by the same factor. If, for example, the center frequency is reduced by a factor of 4, then the frequency swing would be reduced by the same factor of 4. Since, in accordance with present standards of FM broadcasting, the maximum permissible frequency deviation in use in the 40-50 mc. band is kilocycles (kc.) to either side of the center frequency, it will be seen that reducing the center frequency by a factor of 4 transmitted through will result in a reduction of the maximum overall frequency swing from 150 kc. to 37.5 kc.

Specifically considering the circuit of Fig, 1, let it be assumed that numeral I designates the usual intermediat frequency transformer coupled to the plate circuit of the last intermediate frequency, amplifier tube. In that-case .each .of the primary and secondary resonant circuits 2 and 3 would be tuned to the center frequency F. As stated before, and merely by way of specific illustration, F is assumed to have a value of 4 mc. As is well known, the center or carrier frequency is deviated at the transmitter in dependence upon the amplitude of the modulation signal, and'the rate of overall frequency deviation isdependent.

upon the modulating frequencies themselves. The band width of transformer. I will be in excess of the maximum frequency swing and will in the case assumed preferably be approximately 200 kc.

The-numeral 4 designates the converter tube which may be of the-6SA'7 type, althoughthe inyentionisnot restricted to this type-of tube nor to a; tube of the pentagridtype. Generally, the

tube may comprise a-cathode 5 and an output .electrode, or plate, 6. Between the cathode 5 and plate. 6 are arrangedin sequential relation a signal input grid. Lapositive screen grid 8, a secnd, signal, or oscillation, input electrode 9, a

- positive screen grid I0 and the usual suppressor grid II.

1 The cathode is connected to ground I through the customary biasing resistor I2 shuntedban intermediate frequency b -pass condenser. -'Ihe- 1ow potential side of secondary circuitf3 .issestablished at ground potential, while theopposite s de is connected to the signal grid .7. .Theelectrode 9.is connected through resistor i3 to ,the grounded-end of biasing resistor I2. The functions of grids 'I .and .9 may be interchangedif desired.

,The plate-,6 is connected to the positive terminal +13 of a direct current source: through a resonant =output circuit comprising a coil I4 shunted by'condenser I5. The circuit I4, I5 is presentlyassumed tobe tuned to a frequency which is an integer. Thesignalmodulated car rier YQ .tage, develop,ed across circuit I4, I5 is the condenser. I6. which has a low impedance to the carrier voltage. As heretofore explained, it is,desired tojdivide the center frequency'of themodulated carrier waves and se-- curesimultaneousproportional reduction of the frequency swing, whichv is. taken as twice the fr quency deviation. ,Hence, the effective frequencyswing-of the energy in circuit I4, I5 would be, reducedun times. 'If. the effective band passwidth ,at 1 transformer .I .was 200 kc., .and .n equalled'4 then. thepass band widthat .tuned circuit I4,, I5 ,would be.approximately,.50 kc.

,The frequency multiplier tube is designated by numeral 2B,,and it may .bea tube of the pentode' type suchas ,is.,now known commercially as the GSKI. The, cathode 2| ofthis tube includes a self-biasingresistor- 22 connected to ground-and shunted by anappropriate .bypass condenser.

"The input grid}23 .isconnected-by the coupling condenser 24 to the, highpotential side .of'circuit I4, I5, the grid being connected toground through the: leak resistor,- 25.

is-connectedby lead.,3I and coupling condenser :32 to the grid. 9. ,The'plate 30 is, also, connected.

/ presently assumed to be equal to F (Hi1);

In the specific illustration given, the resonant frequency of circuit 40, 4I would be either 3 mo.

or 5 mc. The biasing resistor 22 is given a value such that, in the absence of input voltage to grid 23, directcurrent voltage developed across resistor 22 is. sufficient to bias grid 23 close to plate current cut-off. It will now be realized that the resonant frequency voltage developed across the circuit .40, 4| is impressed upon the electrode 9 simultaneously with the impression of the frequency-modulated carrier wave energy of center r frequency F upon grid I.

The frequency modulated carrier wave voltages impressed on respective grids I; and 9 are heterodyned by virtue of electron coupling, and produce the beatfrequency. voltage Whose center. frequency is plication ofnil times, and this multiplied output is. then. fed to the electrode 9 .of the converter tube 4 so that the output of the converter consists of the heterodyne beat between the center frequency R and the frequenc of the voltage developedacross circuits, M. It will be seen that under these circumstances an oscillationwill exist in circuit I4, I5 when the converter gain and the multiplier gain exceedunity. The converter gain depends upon the strength of the signal applied from the input transformer I. Hence, in

the absence of signal energy-at the input transformer I there-will be no output voltage at circuit I4, I5- since there=will be no oscillations produced through the multiplier tube. Tubes 4 and -2G cooperate with the associated tuned circuits to provide a re-entrant circuit i.-e.-, a regenerative modulation-circuit, The constants of the tubes 4 and 20 arechosen-so'thatwhen the signal inputlevel exceeds a predetermined amplitude'then the re-entrant action becomes sufficient to sustain oscillation.

. Additionally, the network-may have its constants adjusted so when the signal input voltage is raised. above the level required to start oscillations the-output voltage-does not appreciably .increase. Accordingly,-when used todivide the frequency,- -or phase, deviation in a frequency, or

phase, -modulat-ion receiver, the division'network .also. permits the usual limiter stage employed rior. to the detector tobe dispensed with.

A further advantage of this type'of frequency divider, when employed in a frequency, or, phase,

modulation receiver, is-in the selective ,action thereof. Due to the resonant frequenciesbf the circuits employed in 1 the re eneration circuit,

there is a limited range-19f frequency-F whichwill cause the system to'oscillate and produce an out- ;put' voltage. "Thus; the receiver-may be tuned to the proper center frequency F so as to produce oscillations in the divider, and interfering signals which do not have the frequency F, but which come through the usual selective circuits, will be rejected.

A still further advantage which this division circuit has for use in frequency, or phase, modulation receivers is its threshold action. The latter can be utilized to eliminate noise from the receiver in the absence of signals. The usual frequency modulation receiver produces a roar of noise when there is no signal present. This is due to the fact that the limiter increases the gain of the receiver so as to amplify the noise to full volume. However, with the present type of frequency divider network, the gain of the receiver may be adjusted so that the threshold of the divider is just above the noise level. Hence, there will be no output voltage unless a signal is present which is stronger than, and overrides, the noise level. It will therefore, be appreciated that the present frequency division network improves the signal to noise ratio at the demodulator input circuit by virtue of the conjoint action of the division of the frequency swing of the frequency modulated carrier waves and the threshold action of the network itself. Furthermore, by virtue of the improved selectivity and the substantiall constant output, distortion is greatl minimized.

In accordance with my present invention I utilize both tuned circuits Hi, l5 and 40, 4| as discriminator input circuits. Instead of tuning these circuits to the respective frequencies n and I tune them respectively to frequencies below and above said frequencies or vice versa. Assuming the specific illustrative examples given above, circuit M, l5 may be tuned to a frequency less than 1 me. by a predetermined amount, and circuit 10, 4| is in such case tuned to a frequency greater than 5 me. (or 3 Inc.) by a predetermined amount. As a result of such relatively opposite detuning of circuits l4, l5 and 43, 4| there is produced amplitude modulated carrier wave energy across each of the circuits.

Rectifiers 42 and H are shown as being of the diode type; it is to be understood that any other well known form of detector device may be utilized in place of the diode rectifiers. Anode 43 of diode 42 is coupled to the plate side of circuit l4, l5 through condenser l6. Cathode 44 of diode 42 is connected to ground through load resistors 46 and 50 arranged in series. I. F, bypass condensers t7 and 5| shunt respective resistors 46 and 5D. Resistor 45 provides a direct current return path from anode 43 to the negative end of resistor 46. Resistor 52 is also a direct current return path and connects cathode 49 of diode 4| to ground. An I. F. choke of high impedance at the frequency of circuit 40, 4| may replace resistor 52. Condenser it? couples the cathode end of resistor 52 to the plate side of circuit fill, 6|. It will be obvious that condensers l5 and 46 function as direct current blocking condensers.

The modulation-representative voltage is taken off between the cathode end of resistor 46 and ground by leads 53. In other words, the rectified signal voltages across resistors 45 and 59 are combined in differential or polarity opposing relation so as to provide the usual advantages of a balanced FM detector. The leads 53 may feed the modulation signal voltage to any desired and suitable amplifier which may be followed by an ultimate reproducer.

The functioning of the detector circuits will now be described, it being pointed out that Figs. 2 and 3 show curves illustrative of the invention. In Fig. 2 I have shown the relation between the amplitude vs. frequency curves of circuits |4, I5 and 40, 4| for the condition where the normal frequency of the latter would be while in Fig. 3 the curves are shown for the case where circuit 4|), M has a normal frequency of In Fig. 2 the input signal frequency is shown located by the vertical dotted line F- at 4 me. The output circuit I4, I5 is tuned, in accordance with my invention, to a frequency sufficiently less than 1 me. to permit the frequency of 1 me. to fall on a substantially linear portion of the upper flank of resonance curve a. In other words, if curve 0. represents the resonance curve of circuit i i, I5 for the off-tune condition, the value may be indicated to fall at point I) of resonance curve a. The dotted line curve 0 shows what the resonance curve of circuit I l, l5 would be if it were tuned to the frequency In the same way curve d illustrates the resonance curve of circuit Gil, 4|, and shows circuit 40, 4| to be tuned to a frequency somewhat higher than the frequency Dotted line curve e shows what the resonance curve of circuit 48, 4| would be if it were tuned to 5 me. The frequency or 5 mc., falls at point g 0f the lower flank of resonance curve 01. Here, again, point g should fall at a substantially linear portion of the lower flank of resonance curve :2.

It is important to locate points b and g on respective curves at and if so that the maximum frequency swings at circuits I 3, I 5 and Mi, 4| can be accommodated. The maximum frequency swing at input circuit i has been assumed to be kc. Under these conditions the maximum frequency swing at circuit |4, H5 is 150 kc. divided by 4, which is 37.5 kc. Hence, point b should be located so that the maximum frequency swing of the I. F. energy at circuit l4, l5 will be 18.75 kc., above and 18.75 kc., below point 1) without going off the linear part of the resonance curve. At circuit 46, 4| the maximum frequency swing is 37.5 kc. multiplied by 5, or 187.5 kc. Therefore, point 9 is located on the resonance curve at, and the resonance curve at is chosen accordingly, so that the maximum frequency swing of the energyatizcircuit 40,f4|: will be.93.7 5.

The frequency F of. the inputsignals falls above the peak frequency of circuit 40, 4| in that case. However, the point g' on curve at will be located as explained in connection with curve (1. An examination of points b and g (or g) reveals that operation along these points is productive of variable-amplitude carrier energy at respective circuits |4, l and 40-, 4|.

Itcan be seen from Figs. 2 and 3 that when the signal input energy instantaneously deviates higher in frequency, that is shifts above the value F, the energ developed at circuit |4, I5 is modulated'towards a lower amplitude while the energy developed at circuit 40; ll-is concurrently modulated towards a higher amplitude. The reverse is; of course, true as the instantaneous signal input frequency deviates to a frequency less than F. In other words, Figs. 2 and 3 clearly show that as the instantaneous frequency of the signal input energy deviates below and above frequency F the amplitudes of signal energy at circuits l4, I5 and 40, 4| will correspondingly vary, but in opposite senses.

This is the familiar condition required for operation of a balanced FM detector. By rectifying the variable amplitudesignal energy at each of circuits |4, I5 and 40, 4| by means of respective rectifiers 42 and 4|, there is provided rectified signal voltage across each of load resistors 46 and 5B. These output resistors 46 and 50 are connected so as to provide a differential modulation signal voltage which is representative of the amplitude modulations superimposed on the FM waves in the separate tuned circuits |4, |5- and 40, ll. Although in the foregoing description, the circuit |4, |5 has been described as detuned to a frequency below and circuit 40, 6| has been described as detuned to a frequency above as the case may be, this action may be reversed in that circuit M, l5 may be detuned to a frequency above and circuit 40', 4 I- may be detuned to a frequency below ZOtil) as will be apparent to those skilled in the art.

By a proper adjustment of the grid leak resistors and electrode voltages of tubes 4 and 26, the amplitude of the oscillations in the re-entrant circuit may be mad independent of the amplitude of the FM signal. over a range of signal inputs. This characteristic tends'to give oscillator output of uniform amplitude despite amplitude modulation on the incoming FM signal, and, therefore, the circuit does not require an amplitudelimiter. The system, of course, possesses the various other characteristics described above and claimed in my aforesaid U. S. Patent No. 2,344,678; It will beparticularly noted that although circuits l4, l5 and 40; 4| function as discriminator circuits, yet the resonance curves thereof do not overlap at any point and are in fact relatively widely spaced in the frequency spectrum.

WhileI have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization. shown and described, but that many modifications may be made without departing from the scope of m invention.

What I claim is:

1. In combination with a pair of rectifiers having, respective load resistors and a differential output voltage circuit common to said resistors, a first tuned circuit connected to one rectifier, a second tuned circuit connected to the second rectifier, a source of angle modulated carrier waves having a predetermined center frequency, said first tuned circuit being resonant to a first frequency less than a predetermined subharmonic frequency of said center frequency, said subharmonic'frequency falling onthe upper flank of the resonance curve of the first tuned circuit, said second tuned circuit being resonant to a second frequency in excess of a third frequency which differs from the source center frequency by said subharmonic frequency, said third frequency fallingon the lower flank of the resonance curve of the second tuned circuit, frequency multiplying means coupling said first tuned circuit to said second tuned circuit, and means responsive to said source waves and frequency multiplied wave energy for developing in said first tuned circuit angle modulated waves whose center frequency is reduced to said subharmonic frequency value.

2. In a frequency modulation discriminator of the type comprising a pair of differentially connected rectifiers; the improvement comprising respective resonant input circuits for said rectifiers, a source of frequency modulated waves tuned to a predetermined frequency, one of the input circuits being tuned to a predetermined frequency difference from a desired subharmonic frequency of the source frequency, said subharmonic frequency being located on a flank of the resonance curve of said one input circuit, the second input circuit'being tuned to a predetermined frequency difference in an opposite sense from a frequency multiple of the subharmonic frequency, said frequency multiple being located on the opposite flank of the resonance curve of the second tuned input circuit, and connections including said source and two input circuits in a closed reentrant system.

3. In combination with a pair of diode rectifiers having respective load resistors and a differential output circuit common to said resistors, a first tuned circuit connected to one rectifier, a second tuned circuit connected to the second rectifier, a source of frequency modulated carrier waves having a predetermined carrier frequency, said first tuned circuit being resonant to a frequency less than a predetermined subharmonic frequency of said carrier frequency,- said subharmonic frequency being located on the upper flank of the resonance curve of the first tuned circuit, said second tuned circuit being resonant to a second frequency lIleXCBSS of a third frequency which differs from said carrier frequency by said subharmonic frequency, said third frequency being located on the lower flank of the resonance curve of the second tuned circuit, a frequency multiplier coupling the first tuned circuit to said second tuned circuit, and means for developing in said first tuned circuit frequency modulated waves whose carrier frequency is reduced to said subharmonic frequency value,

4. In combination with a pair of differentially connected rectifiers, respective resonant input circuits for said rectifiers, a source of angle modulated carrier waves tuned to a predetermined carrier frequency, one of the input circuits being tuned a predetermined frequency difference from a desired subharmonic frequency of said carrier frequency, said subharmonic frequency being located on one flank of theresonance curve of said one input circuit, the second input circuit being tuned a predetermined frequency difference in an opposite sense from a frequency multiple of the subharmonic frequency, said frequency multiple falling on the opposite flank of the resonance curve of the second input circuit, and connections including said source and two input circuits in a closed system.

5. In a detector of frequency modulated carrier waves, a pair of oppositely connected diode rectifiers, respective resonant input circuits for the rectifiers, said input circuits being tuned to resonant frequencies widely separated to prevent overlapping of the resonance curves thereof, and means, responsive to said waves, for applying to said input circuits modulated waves whose center frequencies are sufficiently different from the said separated resonant frequencies to cause ampli tude-variable wave energy to appear in each rectifier input circuit and the center frequency of the waves applied to each of said input circuits being located on respectively opposite flanks of said resonance curves.

6. In combination with a, pair of detectors having respective load resistors and differential output voltage circuit common to said resistors, a first tuned circuit connected to one rectifier, a second tuned circuit connected to the second rectifier, a source of angle modulated carrier waves having a predetermined center frequency, a frequency divider having said first tuned circuit as its output circuit, said first tuned circuit being resonant to a frequency less than a predetermined subharmonic frequency of said center frequency, said subharmonic frequency fallingon the upper flank of the resonance curve of said first tuned circuit, a frequency multiplier including said second tuned 10 lated waves whose center frequency is reduced to said subharmonic frequency value.

'7. In a frequency modulation discriminator of the type comprising a pair of differentially connected rectifiers; the improvement comprising re- 15 spective resonant input circuits for said rectifiers,

a source of frequency modulated waves tuned to a predetermined frequency, a frequency divider including one of the input circuits tuned a predetermined frequency difference from a desired subharmonic frequency of the source frequency, said subharmonic frequency being located on one flank of the resonance curve of said one input circuit, a frequency multiplier including the second input circuit tuned a predetermined frequency difference in an opposite sense from the frequency multiple of the subharmonic frequency, said frequency multiple being located on the opposite flank of the resonance curve of the second input circuit, and connections including said source and two input circuits in a closed re-entrant system.

8. In combination, a pair of difierentially connected detectors, respective input circuits for the detectors, said input circuits being tuned to spaced resonant frequencies to prevent overlapping of the resonance curves thereof, and means, responsive to angle modulated waves, for applying to said input circuits angle modulated waves whose cen ter frequencies are substantially different from said spaced resonant frequencies and the center frequencies of the waves applied to the respective input circuits being located on respectively 013- posite flanks of said resonance curves.

MURRAY G. CROSBY.

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

UNITED STATES PATENTS