US2273771A - Frequency modulated carrier detector - Google Patents

Description

Feb. 17, 1942. s, -r

FREQUENCY MODULATED CARRIER DETECTOR Filed April 10, 1941 2 Sheets-Sheet l INVENTOR .9

BY %i"2" ATTORNEY Feb. 17, 1942.

S. HUNT FREQUENCY MODULATED CARRIER DETECTOR Filed April 10, 1941 2 Sheets-Sheet 2 elgl VENTOR gymaar BY flaunt ATTbRNEY circuit of Fig Patented F b. 17, 1942 UNITED STATES PATENT OFFICE;

FREQUENCY MODULATED CARRIER DETECTOR H Seymour Hunt, Flushing, N. Y., assignor to Radio Corporation of Amcrica, a corporation of Dclaware Application April- 10, 1941, Serial No. 387,829

9 Claims.

My present invention relates to detectors for frequency modulated carrier waves, and more especially to detectors of the multi-grid' type capable of deriving modulation components from angular velocity-modulated carrier waves.

' It has been shown. in the past that if angular velocity-modulated carrier waves. are applied in phase quadrature relation to. spacedelectrodes in a. common electron stream to an output electrode, there will be produced. in the output ch- 3 cuit modulation voltage corresponding to the modulation-representative deviations of the cenvoltage at one grid-in phase quadrature with the [carrier-voltage at a second grid.

{Still another ob ect of my invention is .to improve frequency modulated carrier wave dejtectors of the wherein wave voltages are appliedin phase quadrature to spaced grids of a multi-grid tube, and theimprovementresiding ina tuned device employed in association with one ofthe grids for producing the phase quadrature relation.

connection with the drawings in which I have indicated diagrammatically several circuitor- 'ganizations whereby my invention may be carried into eife'ct.

Inthe drawings: Y, Fig. lshows a circuit embodying the invention, Fig. 2 illustrates a modification of the cathode Figs. 3 and 4 show' respectively ditional modifications,

Figs. 5 and 6 show two different embodiments,

Fig. '7 shows still another form of the invention.

Referringnow, to Fig. 1 of the accompanying I drawings, wherein like reference characters in "the difierentfig'ures designate similar circuit elements,the tube 1 may be a multi-grid tube ofthe6A8 type. The grid 2 is the signal voltage input grid, and'it'is connected to the cathode,

different ad .30 The novel features which I believe to be char- .acteristi'c of my invention are set forth in particularity in the appended claims; the invention itself,- however, as to both its organization and method of operation will best beunderstood by {reference to the following description taken in of tube I through a-path comprising, coil 3 and resistor 5. The coil 3. is shunted by condenser 4,, and the two are tuned to the operating intermediate frequency (I. F.) of the system, assuming the latter is a superheterodyne receiver. The I. F. may have a value of 4.2v mc. (megacycles), where the FM? (frequency modulated carrier wave) band covers 42 to 50 mo. As is well known, there are impressed upon circuit 1' frequency modulated carrier waves whose center frequency is-of the 4.2 mc. value. The circuits prior to tuned circuit 1 may include a signal collecting circuit, a converter, one or more I. F. amplifiers and a limiter stage. 7

As is well known to those skilled in the art, the function of the limiter is to. eliminate amplitude modulation effects'in thecarrier. The limiter tube may be a saturated amplifier, and the circuit 1 will be located in the plate circuit of'the limiter tube. vHence there will. be delivered to input circuit. 3- 4 frequency variable waves of constant amplitude. It is, of course, well known that in frequency modulation transmission the modulating signal amplitude ap: pears as a frequency deviationof the carrier, or center, frequency, while the modulating f refquencies themselves correspond to the rate;o f deviation of the center frequency. The function 'of tube and its associated circuits is to derive from the frequency-variable wave the modulation voltage whichcorresponds to the original modulating signals.

The grid 1 of tube l is located betweena pair oflpositively biased screen grids 8 and 9-, while the output electrode, or plate, H) is connected to the positive terminalof a source of direct current by a load resistor II. The plate end of resistor H is by-passedto'ground by condenser 12 which has a low impedance to I. F. currents. Modulation voltage developed across'resistor II is transmitted to a succeeding audio frequency utilization network by condenser l3' which has'a low impedance to modulation frequencyvoltage.

The cathode of tube I is connected to ground throu'gha path including jcoil M 'andby-passed bias resistor If. ,The dotted rectangle It enjcloses the leads n and is, the latter bann rminated by-the series termination M. The network within rectangle I6 is a one-quarter wave length transmissionline, and functions, as a phase shifting network. The grid 1 is connected by lead ZD'to' the junction of resistor 15 and coil i l. The .resistorv 5 develops'thereacrossa unidirectional voltage which maybe utilized' ffor automatic volume controliAvC), and the lead 2| is connected to the grid end of resistor through a filter resistor; the voltage transmitted over lead 2| is to be understood as being applied to the grid circuits of amplifier tubes preceding the limiter stage. Condenser 6 acts as an I. F. by-pass condenser across resistor 5.

It has been shown in the past that when frequency modulated carrier waves of the same center frequency are applied to grids 2 and 'l in substantially phase quadrature, then there will be developed modulation voltage across load resistor II. In other words, the circuit functions as a demodulator. Of course, where the bypassed resistor 5 is inserted in the grid circuit of tube I then there will be provided further limiting action in the grid circuit of the demodulator. In that case a special limiter stage prior to the input circuit of detector tube I may be dispensed with. Those skilled in the art know that the onequarter wave length transmission line in rectangle l6 produces a phase shift thereacross which is 90 degrees. Thus, the voltages on grid 2 and grid 1 differ 90 degrees in phase. The line impedance at the cathode end of lead l'l-l8 is very low and the radio frequency voltage drop at that point is very small. At the terminating endof the line the impedance is high andthe radio frequency voltage dropis high. In other words, the network in rectangle l6 may be conconnected to the cathode in Fig. 1 may be returned to ground. This is shown in Fig. 2. In this case grid 2 has degeneratively applied to it radio frequency voltage developed across l4. However, there is but small degeneration applied to grid 2 compared to a large voltage developed for grid 1 by the resonant rise due to the transmission line in the cathode circuit of the tube.

Another method of producing a 90 degrees phase shift between the frequency modulated voltages applied to grids 2 and 1 is shown in Fig.3. In this figure the cathode of tube l is connected to ground through a coil 22 which is shunted by condenser 23. Circuit 2223 is tuned tothe center frequency Fe of the applied FM waves. A small resistor 24 is placed in series with condenser 23 to provide quadrature voltage thereacross. Grid 1 is connected to the junction of condenser 23and resistor 24 and takes off the quadrature voltage across the latter. In this way FM voltage developed across the cathode network of tube l is applied to grid 1 in phase quadrature with the FM voltage applied to grid 2 by input circuit 3-4.

' In'Fig. 4 there is shown still another modification. This arrangement differs from that shown .in Fig. 3 in the arrangement of the cathode phase shifting network. Here coil 22' and condenser 23 are arranged in series between the cathode and ground. The series path 22-23' is series tuned to Fe, the junction of the series elements In shunt with condenser 23' is connected radio frequency choke coil 25in series with resistor 26. The radio frequency current through condenser 23' is 90 degrees out of phase with the voltage across tuned circuit 2223'. Thus, voltage applied to grid 1 is degrees out of phase with voltage at grid 2.

The circuit modification in Fig. 5 diifers from that shown previously in the other figures in that the grid 7 is connected to ground through a piezo-electric crystal element which is tuned to F0. Thus, the numeral 30 designates the crystal which is tuned or cut to Fe, those skilled in the art being fully aware of the manner of mounting the crystal between appropriate electrodes or crystal holder elements. One of the crystal electrodes is connected to grid 1, while the opposite one is connected to ground. The electrical equivalent circuit of the crystal would appear as an inductance, capacity and resistance, all constants of the crystal, arranged in series between grid 1 and ground, whereas the capacity of the crystal holder would be arranged in shunt with the aforesaid series constants. These equivalent components may be made to appear as a tuned circuit.

In Fig. 6 is shown another modification wherein the piezo-electric resonator 30 of Fig. 5 is replaced by a mechanical resonant line. The latter comprises an outer metallic tube 40 having one end thereof grounded, whereas there is disposed concentrically within the tube 40 a second metallic tube 4| connected to grid 1. Of course, the grid 1 will be tapped to a proper point on the inner tube 4|. The mechanical resonator 40-4I is adjusted to tune to Fe.

It often happens that it is desirable to arrange an FM detector so that it will deliver a pair of audio voltages degrees out of phase whereby a push-pull audio amplifier may be driven. In Fig. 7 there is shown such an arrangement. Here the tube I has its grid I connected to ground through the coil 50 of resonant circuit JO-5|. The latter circuit is tuned to Fe, and develops across it FM voltage which is in phase quadrature with. the voltage applied to grid 2. Screen grids 8 and 9 are connected to the positive terminal of a direct current source through a load resistor H, the latter being shunted by an I. F. by-pass condenser 12'. A modulation voltage developed across plate load resistor II is transmitted through condenser [3 to the grid of one of a pair of push-pull connected'amplifier tubes, while the voltage developed across resistor H is transmitted through condenser l3 to the opposite push-pull amplifier. As the FM voltage applied to grid 2 has its instantaneous frequency shifted above and below Fc more or less voltage is built up across circuit 5lI-5l by virtue of space charge coupling between grids 2 and I. The mutual conductance of tube I changes as the voltage of grid 1 changes, When more plate current flows, less screen grid current flows and vice versa. This is, of course, the proper polarity sense for the audio voltages developed across resistors H and ll. of course, the phase quadrature voltage for grid 1 may be developed by using any of the cathode load networks shown in Figs. 1, 2 and 3 and 4 instead of relying upon space charge coupling of the detector tube.

The following explanation of the operation of Fig. 7 is equally applicable to Figs. 5 and 6. Electrons are first attracted by the positive screen 8. Some go through the open mesh of the screen and are attracted by the positive plate. Due to grid 1 being less positive than the screen 8 many electrons are slowed down in speed. A cloud, or pile, ofelectrons gather in front of grid 1. This is the virtual cathode. The plate draws electrons from the virtual cathode. The

virtual cathode waxes and wanes in relation to the voltage applied to grid 2. This motion of electrons causes a current to flow in grid 1 and tuned circuit LDC5l. At resonance L5'o"-C'51 looks like a resistor, and it has been found that the voltage across Lao-C51 at resonance is 90 degrees out of phase with the voltage applied to grid 2. The actual plate current flowingis proportional to the voltage on grid 2 and the voltage on grid 1. As the carrier shifts L5o-C5i is not a resistor any longer, but either a capacity or inductance depending upon whether the applied frequency is above or below the tuned frequency of Law-C51 Thus, the voltage across L5o--C51 is either more or less than 90 degrees out from the applied grid 2 voltage. Thus, as the modula- I tioncauses the voltage of grid 2 to vary in frequency the plate current varies in step with the modulation, and provides the audio output.

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 invention is by no means limited to the particular or anizations snown and described, but that many mocuncations may be made without departing from the scope of my invention, as setforth in the appended claims.

What I claim isi 1. In an angular velocity modulated carrier wave detection network, a tube provided with at least a cathode, an output electrode and a pair of spaced control grids in the electron stream between the cathode and output electrode, a modu lated carrier input circuit connected between one control grid and the cathode, an output load element across whichis developed modulation voltage connected to the output electrode, a phase shifting network connected in the space current path of said tube between said cathode and a point of relatively fixed alternating potential, said shifting network developing modulated carrier voltage which is in phase quadrature with the voltage at said input circuit, and means for applying said quadrature voltage to the second control grid of said tube.

2. In an angular velocity modulated carrier wave detection network, a tube provided with at least a cathode, an output electrode and a pair of spaced control grids in the electron stream .between the cathode and output electrode, a

modulated carrier input circuit connected between one control grid and the cathode, an output load element'across which is developed modulation voltage connected to the output electrode,

a phase shifting network connected in the space current path of said tube between said cathode and a point of relatively fixed alternating potential, said shifting network developing modulated carrier voltage which is in phase quadrature with the voltage at said input circuit, and means for applying said quadrature voltage to the second control grid of said tube, said phase shifting network consisting of a quarter wave length transmission line.

3. In an angular velocity modulated carrier wave detection network, a tube provided with at least a cathode, an output electrode and a pair of spaced control grids in the electron stream between the cathode and outputelectrode, a modulated carrier input circuit connected between one control grid and the cathode, an output load element across which is developed modulation voltage connected to the output electrode, a phase shifting network connected in the space current wave detection network, a tubeprovided with at a least a cathode, anoutput electrode and a pair of spaced control grids in the electron stream between the cathode and output electrode, a modulated carrier input circuit connected between one control grid and the cathode, an output load element across which is developed modulation voltage connected to the output electrode, a phase shifting network connected in the space current path of said tube between said cathode and a point of relatively fixed alternating potential, said shifting network developing modulated carrier voltage which is in phase quadrature with the voltage at said input circuit, and means for applying said quadrature voltage to the second control grid of said tube, said phase shifting network consisting of a coil and condenser series resonant to the center frequency of the waves of said input circuit, and said second control grid being connected to the junction of said coil and condenser.

5. In an angular velocity modulated carrier wave detection network, a tube provided with at least a cathode, an output electrode and a pair of electrodes arranged to control the electron stream between the cathode and output electrode, a modulated carrier input circuit connected between one control electrode and the cathode, an output load element across which is developed modulation voltage connected to the output electrode, a reactive phaseshifting network connected between the cathode and a point of relatively fixed alternating potential and developing modulated carrier voltage which is in phase quadrature with the voltage at said input circuit, and means for connecting a point of said shifting network to the second control electrode of said tube.

6. In an angular velocity modulated carrier wave detection network, a tube provided with at least a cathode, a plate and at least two control grids in the electron stream between the cathode and plate, a modulated carrier input circuit connected between one control grid and the cathode, an output resistor element across which is developed modulation voltage connected to the plate, a phase shifting network connected in the space current path of said tube between said cathode and ground, said shifting network consisting of a transmission line of a length such.- as to develop modulated carrier voltage which is in substantial phase quadrature with the voltage at said input circuit, and means for applying saidquadrature voltage to the second controlgrid of said tube-..

'7. In an angular velocity modulated carrier Wave detection network, a tube provided with at least a cathode, an output electrode and at least a pair of spaced control grids in the electron stream between the cathode and output electrode, a modulated carrier input circuit connected betweenone control grid and the cathode, an output load element across which is developed modulation voltage connected to the output electrode, a phase shifting network connected in the space current path of said tube between said cathode and a point of relatively fixed alternating potential, said shifting network including an open ended transmission line of a wave length chosen to develop modulated carrier voltage which is in substantial phase quadrature with the voltage at said input circuit, and means for applying said quadrature voltage to the second control grid of said tube.

8. In an angular velocity modulated carrie wave detection network, a tube provided with at least a cathode, an output electrode and a pair of spaced control grids in the electron stream between the cathode and output electrode, a modulated carrier input circuit connected between one control grid and the cathode,

an output load element across which is developed modulation voltage connected to the output electrode, a phase shifting network connected in the space current path of said tube between said cathode and a point of relatively fixed alternating potential, said shifting network developing modulated carrier voltage which is in substantial phase quadrature with the voltage at said input circuit, and means for applying said quadrature voltage to the second control grid of said tube, said phase shifting network comprising a coil and condenser arranged as a parallel resonant circuit tuned to the center frequency of the carrier wave of said input circuit, a resistor in series with the condenser, said applying means including a connection between the second grid and the junction of the resistor and condenser.

9. In an angular velocity modulated carrier wave detection network, a tube provided with at least a cathode, an output electrode and a pair of spaced control grids in the electron stream between the cathode and output electrode, a modulated carrier input circuit connected between one control grid and the cathode, an output load element across which is developed modulation voltage connected to the output electrode, a phase shifting network connected in the space current path of said tube between said cathode and a point of relatively fixed alternating potential, said shifting network developing modulated carrier voltage which is in phase quadrature with the voltage at said input circuit, means in circuit with said one grid for limiting the response of the network to carrier amplitude variation, and means for applying said quadrature voltage to the second control grid of said tube.

SEYMOUR HUNT.

Images