US2353468A - Frequency modulation receiver circuit - Google Patents

Description

` July 11, 1944. P. lF. G. HoLsT ET AL l2,353,468

FREQUENCY MODULATION RECEIVER CIRCUIT Filed May 22, 1942 2 sheets-snee; 1 EM DETECTOR ff INPUT J0 -v Y /Vsrn/o/r `/M/rf/ K v 7l Z C Ai, l f f@ 13 I Pavo/a ff l @E -f- 4MM/H5@ j f l c5 .9 ,T f/va/CA raf? 7 TUBE? Tlc'la. Tllb- T lcic.Tlzrldlcfl'. I

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\ ATTQRNEY FREQUENCY July 11, 1944. P. F. GjHoLsT'ET AL 2,353,468

FREQUENCY MODULATION RECEIVER CIRCUIT Filed May 22, 1942 Tlzf.

` ff fo 2 Sheets-Sheet 2 l T T Y Tfr-- INVENTORS ,0401. F. a, #@sr oef/v e. we K14/005;

ATTORNEY Patented July 1l, 1944 FREQUENCY MODULATION RECEIVER CIRCUIT Paul F. G. Holst and Loren R. "Kirkwood, ak1yn,

N. J., assignors to Radio 'Corporation-of America, a corporation of Delaware Application May 22, i942, seriaiNo. 444,046

(ci. 25o- 27) 8 Claims.

`Our presentinvention relates generally to detectors of `angular velocity-modulated carrier waves, and more particularly to a detector of frequency modulated carrier waves.

, One of the main objects of our invention is to provide ina simplified type of detector ,of angular velocity-modulated carrier waves networks whose constants are so related as to provide optimum and eflicient performance.

Another important object of our invention is to provide Va Vfrequency modulation detector circuit of simplified andefiicient construction with which is associated .a visual indicator circuit which isA capable of indicating correctvtuning of the receiver; incorrect tuning; and no-signal state.

Another important object of this invention is to providey a simplified form of frequency modulation detector circuit of -the type employingiop1 posed rectifiers having a resonant circuitemploying only one inductive element'as the input circuit thereof, the latter being fed with signal en* ergy from the resonant output circuit of a prior signal transmission tube; and avisual indicator tube being energized from a point intermediate the aforesaid resonant circuits.

Still other `objects of our invention are to pro- .vids la highly eicient and simplified type of frequency modulation detector circuit, i and indicator therefor, in an economically manufacturable form. l

The novel features which we believe to be characteristic of our invention 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 we have indicated diagrammatically a circuit organization whereby our invention may be carried into effect.

In the drawings:

Fig. l shows a circuit embodying the invention,

Figs; lato le vinclusive each show the appearance of the indicator tube target for different conditions of resonance of the receiver,

Figs. 2, 3 and 4 respectively show different circuit considerations in analyzing the construction of the resonant input circuit of the detector, Fig. 5 shows the characteristic of the indicator control voltage,

Fig.,6 illustrates the pass-band characteristic of the detector input circuit,

Fig. 7 illustrates a circuit used for analysis,

Fig. 8 graphically shows various characteristics relating to the detector input circuit.

Referring now to the accompanying drawings, wherein like reference characters in the difierent iigures designate similar circuit elements,

there is shown in Fig. 1 that portion of a frequency modulation receiver which is located between the limiter input circuit andthe input electrodes of the audio frequency amplifier. Those skilled inthe art are fully acquainted with the general construction of receivers designed for the reception of frequency modulated carrier waves (FM) as employed in the assigned FM broadcast band of Li12-50 `megacycles (mc). Each station channel in this band has a maximum deviation range of l0() kilocycles (kc.) on each side of the center frequency, although at each FM transmitter the `carrier actually is deviated to a maximum of kc. on either side of the center, or mid-band, frequency. At the transmitter the carrier is deviated in accordance with ,the amplitude of the modulation signals; the rate of deviation is a function of the modulation frequencies per se. rlhe term angular velocitymodulated carrier waves is generic and is employed in thisl specification to designate either frequency modulated, or phase modulated, -carrier waves. y 1 l The FM receiver is usually 4of ythe superheterodyne type, and comprises the usual signal collector which feeds one or more ultra-high frequency amplifiers each tuned to the center frequency of the desired FM station. The FM signal energy is fed to a first detector, which is supplied with local oscillations from a tunable oscillator so that the FM signal energy can have its center frequency reduced to 'a desired intermediate f-re- Aquency (IF). While the operating IF value may be chosen from a wide range of Values, an IF value of 4.3 mc. hasbeen chosen. It will be understood, of course, that even though `the center vfrequency of the FM energy has .been reduced to `the IF Value, `the range of frequency deviation is ,the same as inthe original signal wave. Hence, ,the Various IF amplier stages each must be designed to have a pass-band in excess of 150 kc., so as to pass the maximum permissible frequency swings of the signal energy.

In order to prevent amplitude variationsin .the vcarrier from laffecting the performance of thedemodulator, `there eis usually employed a limiter network which functions to prevent vany .amplL tude effects in the FM signals from getting `through to the detector. `ter the carrier is swung in frequency in accordance with the modulation signals, it is desired to Since at the transmithave the detector responsive solely to frequency udeviation at theldetector input circuit. The limiter network may be `of any desired and Well known type. In Fig. 1 it -is shown as an lelectron discharge tube l, upon Who-se input electrode 2 is applied the IF energy. The liiniterhas `its grid bias, plate voltage and'input `circuit constants so chosen that the output increases with increasing input up to a predetermined input amplitude with the reactive elements of the resonant circuit. l

The purpose of the resistor 3 is to provide suicient damping so that the required pass-band characteristic is secured. The law potential side of resonant circuit L1--C1 is connected to the plate potential source through voltage reducing resistor 4 whose upper end is by-passed to ground by condenser 5.

The high potential side of the resonant circuit Li--Ci is connected by condenser Cz to the anode 6 of the double diode rectifier l.4 While separate diodes may be used, it is preferred to employ a double diode tube of the GHS type. The anode S and the condenser C2 are connected in common to the high potential side of the parallel resonant secondary'circuit Lz-Cs. The coils L1 and L2 may have adjustable iron cores, as shown. The anode 8 of the second diode of tube 'I is connected to the low potential side of resonant circuit Lz-Ca, and the low potential side of that circuit is lconnected to ground through condenser C4. The cathode 9 associated with anode V8 is connected to the grounded end of the detector load resistor, while the cathode I associated with diode anode 6 is connected to the upper end of the load resistor. The midpoint of the load resistor is connected by lead I I to the anode 6, and the load resistor is thus divided into two resistive sections R1 and R2. If desired, the lead II can be made from.l the midpoint of the resistor 'load to the anode v8. The audio voltage developed across the load resistor of the detector is transmitted to one or more stages of audio amplification. YThe lter F prevents any I. F. components from Abeing transmitted to the audio utilization'network.

It will be explained at a later point how the network between'tube I and tube 'I is Vdesigned to provide efcient and optimum performance. lIt is suicient for the present purposes'to point out that the detector functions to-develop across load resistor Ri-Rz in series the modulation signal voltage corresponding to the-same modulating signal voltage utilized at the transmitter. To-inform the operator of the receiver that he has accurately tuned to the center, ormid-hand, frequency of the desired FM station, there is provided a tuning indicator tube of the fluorescent target type. The numeral I2 denotes such an4 indicator tube. .For example, a tubeV of the 6E5 type may be employed for the tube I2. As those skilled in theart fully know, such a.tube comprises a direct current voltage amplifier section upon whose input grid I3 maybe applied control voltage. l

The numeral I4 schematically denotes the iluorescent target employed in such a tube. The numeral I denotes the electron control electrode which controls the emission of electrons from cathode I6 to the target I4. The deflecting electrode I5 is variable in potential in the same manner ,as the voltage of the plate circuitin the voltage amplifier section. Attention isy directed to U. S. Patent No. 2,051,189 granted August 18, 1936, to H. M. Wagner for a disclosure of an indicator tube which can be employed as the tube l2. It is sucient for the purpose of this application to point out that control voltage f or grid I3 may be derived from across a load resistor 20 which is arranged between the anode and the cathode of diode rectifier 2l. The diode anode 22 is conacross resonant circuit L1-C1.

nected by a direct current blocking capacitor and lead 23 to the left hand terminall of ,condenser C2, while the diode cathode is grounded.

It will, therefore, be seen that the diode 2l functions to rectify the signal energy developed The magnitude of the rectified voltage will be directly proportional to the I. F. signal voltage appearing across the resonant circuit L1-C1. For a usable signal input to the antenna connection of the receiver, the signal across the resonant circuit L1-C1 will be a function of the receiver tuning, and will vary as shown in Fig. 5. In the latter .fo represents correct tuning, While f1 and f2 represent tuning condition of the receiver to respectively lower and higher values than the correct tuning frequency. The means by which this characteristic is obtained will be explained fully later in this specification.

If the receiver selector circuits are adjusted to either side of center frequency fo the voltage, across resistor 2U will have a magnitude which diers from its value when the selector circuits are accurately adjusted to the center frequency of the FM station. The control grid I3 is connected through filter resistor 30 to the anode end of resistor 2E), the condenser 3| being connected from the grid end of the resistor 3l) to ground. The'network 30-3I functions as a lter for alternating currents and voltages so that grid I3 will vary solely in accordance with direct current voltage variation.

Figs. la-le inclusive show respectively Ydifferent appearances of the fluorescent target of the indicator tube as the operator tunes from one side of exact resonance to the other side through the center frequency. It will be understood that the non-shaded area 4I) is the fluorescent area upon which electrons from cathode I6 impinge upon target I4. The numeral 4I indicates the shaded area which represents that portion of the target which does not receive any electrons because the deflection voltage on electrode I5 is positive and therefore will attract electrons. In other words, area 4I is the electron shadow of deflection electrode I5 when the latter has a positive potential.

The width of the shadow will depend on the value of the positive voltage on the deflection element I5 in such a manner that the greater the positive voltageV the less is thev shadow width; and conversely the smaller the positive voltage of element I5 the greater is theV width of the shadow. As is well known, a greater negative voltage onV the control grid I3 will produce a less positive voltage on the anode, and therefore a less positive voltage on the deflection electrode I5. Conversely a smaller negative voltage on the control grid I3 will produce a more positive potential on the anode and a more positive voltage on deflection element I5. From Fig. 5 itis, therefore, evident that the shadow width will vary with tuning as indicated on Figs. la to le inclue sive, corresponding to the response at the tuning frequencies fa-fr-fc-fa-fi as indicated in Fig. 5. In particular it should be noted that the shadow will have its maximum width at the point of correct tuning, and consequently the indicator circuit described here will enable an operator to determine when the receiver is tuned to the center frequency of an angular velocity modulated signal.

There will now be considered the manner in which the detector input network should be clesigned to provide optimum and ecient operation. It is desired in the design of FM receivers 4b andground. q the following three frequencies:

'that the detector circuit satisfy the following requirements: i i

(l) It should be vof the balanced type to obtain a reduction in noise, and insensitivity to amplitude modulation. l

(2) The response characteristic must be rea` sonably linear v'within the Working range of the system to obtain a low distortion.

A(3) The linearity should be an inherent characteristic. That is, it should not be the result of two complementary -nonlinear characteristics, sincecomplementary"circuits are subject to excess'ive change from -normal slight circuit constant changes.` i Y i (4) It should possess a slope sensitivity vhigh enough to overload an .audio amplierof average gain with frequency deviations equivalent to `less than-l% modulation. j

(5) Stability `should be Vmaintained with fairly largeitemperature changes. i

(6) Thefrequency detector shouldbe of economical design;A and easy to adjust in production. i

With these requirements in mind the performance of the detector circuit shown in Fig. 1 will bean-alyzed. Assume that there exists an I. F. voltage between the point a and ground. This voltage will be rectified by dioder 8 9 in a manner which will cause a `negative voltage to exist across the resistor Rz. This same negative voltage will be impressed on both the cathode and anode of diode B-Hl. This latter diode will detect the I. voltage between b and ground so that the voltage across R1 will be positive. The resulting voltage from. c to ground will `be the sum of the two 4individual voltages, audit may lbe positive, negative or Zero depending on the value of the rectifiedV voltages.

Next consider the circuit between b and ground. The performance of this circuit may be analyzed as follows .(see Fig. 2). Two resonances will exist, namely (1) the parallel resonance between L2 and C3 which will cause the circuit to provide a high impedance Z between b and ground, and (2) the series resonance of Lz-Cs in conjunction with C4, which will cause the circuit to provide a low impedance Z between We are principally interested in (a) The frequency at which 'the series resonance 'occurs (b) The frequency at which the parallel resonance occurs fp (c) The frequency for which ezzei fo fFZN-LJCF (l) femm 2) `We can, therefore, state with good accuracy "that thepoint (frequency) for which the discrimina'tor has zero output will fall mid-way -`between'the two resonances-or that fo="1/2` fs+fp Consider next -that the circuit `shown in Fig. 2

is 'coupled to a prior network by means of condenser Cz. This is shown in Fig. 3. As far as the load on the other circuit L1-C1 is concerned the circuit may be redrawn as shown in Fig. 4. Here C5 represents the series connected capacitors C2 and C4. Let us next consider that the preceding circuit is another tuned circuit as shown in Fig, l. Then, as far as the primary perfomance is concerned the performance is simply that of a capacity-coupled I. F. transformer.

If We desire to line up a transformer of this type, then it is required that the following circuits shall resonate at the center frequency (fo) In order to obtain symmetrical performance of the entire circuit around the center frequency (fo) (which may also be termed the cross-over frequency), then it is required that the aforesaid resonances (fl) and (5) `should occur at the frequency fo. There has been placed no restrictions on the constants in (4), but (5) must be considered together with Equation 3, which will require that:

which will be the case if C2=C4. For later use it is noted that if the above requirement is complied with, then the frequency characteristic across the primary circuit L1-C1 will be symmetrical with respect to fo.

If the above condition is fulfilled, the impedance characteristic of the circuit inserted in the anode supply of the limiter tube will .be symmetrical with respect to fo, and it may through proper choice of constants be made to appear as shown in Fig. 6. However, if the same characteristic is measured from the antenna circuit it will be 'modified as indicated in Fig. 5, due to the effects of thermal and other noise on the circuit. It should be noted that the voltage at the center frequency may be made lower than the voltage existing for a completely detuned circuit, which explains the performance of the indicator described previously in this specification.

We will now return to our original problem which was to produce a dscrirninator characteristic which is linear over the operating range, and symmetrical with respect to the cross-over frequency fo. Let us take the latter requirement. First referring to Fig. 3, ywe have determined the conditions under which the frequency characteristic of the voltage is symmetrical with respect to fo. For our present purpose we may assume that the voltage e is constant; this being one kind of symmetrical response.

For the special case where L2 has no losses the symmetry of the circuit shown in Fig. 3 is easily shown for the case where C2=C4 for the resonance frequencies fp and fs. The resonance frequency fp gives e1=e, and e2:=0, because the circuit Lz-Ca has infinite impedance. The resonance frequency fs gives e1=0, and e2=e, because the circuit across which e1 would exist has zero impedance and the voltage across C2 and C4 will, therefore, be equal. Elaborate calculations indicate that the symmetry will hold at all other frequencies.

6, and that: Y

For. the case wherev loss exists in the coil and the :diode adds further losses, it has been found that the circuit will be substantially symmetrical as long as the losses are not excessive. The iconclusion drawn from the above is, therefore, that the symmetry of the circuit will be satisfactory. Let us next consider the proper choice of the L/ C ratio of the circuits L1-C1i and Lz-Ca. First, assume that both circuits have the same L/C ratio. Calculations in this case indicate that the impedance characteristic of the primary with the secondary connected will be as shown in Fig.

Itwill be noted that these frequencies are the same as those indicated for the secondary in the previous Equations l, 2 and 3. This condition is not very desirable because it is not possible to adjust the curvatures in a manner which will produce a linear detector characteristic, inas much as the curvatures are in general the same and of the character shown in Fig. 6. Itis, 110W- ever, possible to locate the primary and secondary resonances at different frequencies. Assume that the primary and secnodary circuits have different L/C ratios, and that the resonances are still adjusted so that:

f 21a/Law. 17h 21a/LIT( d+ d) These values refer to the analysis `circuit of'Fig. 7. We, then, find that the primary resonance peaks will be located at frequencies which are respectively higher and low'ervthan the resonances within the secondary circuit. That is, the primary peaks will have a greater separation if the L/C ratio of the primary is higher than the L/C ratio of the secondary. The effect of these relations may be visualized by reference to Fig. Let curve I represent the discriminator curve as it would be if a fiat voltage characteristic is presented to ,condenser C2V in Fig. 3. Curve II represents a characteristic of the primary Where the peaks are farther apart. The combined dotted curve III will then be-seen to be flatter over a greater frequency range and to possess greater sensitivity. The primary load'- ing resistor 3 may be used as a convenient means to adjust the flatness of the discriminator curve, because it controls the primary peaks.

The following purely illustrative constants have been found satisfactory: Y

Example l Example 2 C1=l50 microfarads (mi.) 100 mf. Ca=270 microfarads (mf.) 170 mi. Cz=33 micro-microfarads (mmf.) 18 mmf. C4=33 mmf. Y i8 mmf. C5=O.01 mf. 0.01 mf.

' R3=5,600 ohms 10,000 ohms Ri Ri= 106,000 ohms 150, 000 ohms While we have' indicated and described several that many modifications may be made without departing from the scope of our invention, as set forth in the appended claims.

What we claim is:

l. In a detector of angular velocity-modulated carrier waves, a pair of rectiers, a source of such waves, a first resonant circuit, a second resonant circuit, means connecting the opposite sides of the second circuit to a respective one anode of said rectiers, means capacitatively coupling the high ipotential sides of said two circuits, means capacitatively connecting the low potential side of the second resonant circuit to a point of relatively fixed alternating potential, an output load connecting the cathodes of said rectiers, means establishing a point on said load at a relatively fixed alternating potential, means for deriving modulation Voltage from another point on said load, the L/C ratio of the two resonant circuits being different, and the constants of the network between said source and said rectifiers being so chosen that the resonance peaks of the response curve of one resonant circuit are of greater separation than the resonance peaks or' the other resonant circuit.

2. In a frequency modulation detector of the type comprising a pair of diode rectiers, a resonant primary circuit, a resonant secondary circuit, means coupling the opposite sides of the secondary circuit to respective anodes of the diodes, means capacitatively coupling the high potential sides of the primary and secondary circuits, means capacitatively coupling the low potential side'of the secondary circuit to ground, an output resistor connecting the diode cathodes; the improvement which comprises said primary circuit having a higher L/C ratio than the L/C ratio of said secondary circuitwhereby the primary resonance peaks have a greater separation than the secondary resonance peaks.

3. In a detector of the type defined in claim 2, a visual current indicator `tube of the uorescent target type, a rectifier other than said diode rectifiers coupled to said primary cir-cuit, and means responsive to the rectified voltage output of the latter rectifier for controlling thendications of said indicator tube.

4. In a detector of the type defined in lclaim l, a rectifier other than said pair of rectiers coupied to said first resonant circuit, a tuning indicator tube, and means responsive to the rectified output of the last named rectifier for controlling the indicatortube. v

5. In a detector of angular velocity modulated waves, a source of said waves, a discriminator circuit comprising av primary circuit and a secondary circuit coupled thereto, said primary circuit having its constants so chosen that its frequency response characteristic has Va. pair of widely spaced peaks located on either side of the mean frequency of said waves and a minimum response at said mean frequency, a rectifier coupled to the primary circuit, said rectifier rectifying minimum response voltage at said mean frequency to Vindicate correct tuning, and an indicator indicating the rectied voltage.

6. In a demodulator of frequency modulated carrier waves, a pair of diode detectors, a source of such waves, a first resonant circuit, a second resonant circuit, means connecting the opposite sides of the second circuitto a respective one anode of said diodes, a condenser coupling the high potential sides of said two circuits, a. condenser connecting the low potential side of the second resonant circuit to a point of relatively fixed alternating potential, an output load resistor connecting the cathodes of said diodes, means establishing a point on said load at a relatively fixed alternating potential, means for derving modulation voltage from another point on said load, the L/C ratio of the two resonant circuits being different, and the constants of the network between said source and said diodes being chosen to render the resonance peaks of the response curve of one resonant circuit of greater separation than the resonance peaks of the other resonant circuit.

7. In a frequency modulation detector of the type ycomprising a pair of diode rectifiers, a resonant primary circuit, a resonant secondary circuit, means coupling the opposite sides of the secondary circuit to respective anodes of the diodes, means capacitatively coupling the high potential sides of the primary and secondary circuits, means capacitatively coupling the low potential side of the secondary circuit to ground, an output resistor connecting the diode cathodes; the improvement which comprises said primary circuit having a higher L/C ratio than the L/ C ratio of said secondary circuit, an indicator tube of the fluorescent target type, a third rectifier coupled to solely said primary cncuit, and means responsive to the rectified voltage output of solely the third rectifier, for controlling the indications of said indicator tube.

8. In a frequency modulation detector of the type comprising a .pair of diode rectifiers, a. resonant primary circuit, a resonant secondary circuit, means icoupling the opposite sides of the secondary circuit to respective anodes of the diodes, means capacitatively coupling the high potential sides of the primary and secondary circuits, means capacitatively coupling the low potential side of the secondary circuit to ground, an output load connecting the diode cathodes; the improvement which comprises said primary circuit having a higher L/C ratio than the L/C ratio of said secondary circuit whereby the primary resonance peaks have a greater separation than the secondary resonance peaks, a diode coupled to said primary resonant circuit, an indicator tube, and means responsive to the rectied output of the last named diode for controlling the indicator tube.

PAUL F. G. HOLST. LOREN R. KIRKWOOD.

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