US2416795A

US2416795A - Phase or amplitude modulation receiver

Info

Publication number: US2416795A
Application number: US489924A
Authority: US
Inventors: Murray G Crosby
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1943-06-07
Filing date: 1943-06-07
Publication date: 1947-03-04
Anticipated expiration: 1964-03-04

Description

March 4, 1947. M. Rosav 2,416,795

PHASE QR AMPLITUDE MODULATION RECEIVER Filed June 7, 194s v INVENTOR. MIJ/PRA Y 6. P055 Y ATTORNEY QMSSN,

Patented Mar. 4,

PHASE R AMPLITUDE MODULATION.

RECEIVER Murray G. Crosby, Riverhead, N. vYurassignor Yto Radio Corporation of America, a corporation of Delaware Application J une 7, 1943, Serial No. 489,924

6 Claims.

My present invention relates to receivers of phase, or amplitude, modulated carrier waves, and more particularly to a system for receiving phase modulated, or amplitude modulated, waves with carrier exaltation.

In my U. S. Patent No. 2,063,588, granted December 8, 1936, I have shown a receiving system for receiving either phase modulated (PM hereinafter for brevity) carrier wave energy or amplitude modulated (AM hereinafter for brevity) carrier waves which is characterized by its employment of a multi-grid detector tube capable of detecting either the PM or AM wave energies, with substantially unmodulated carrier energy being applied to one control grid of the detector tube and the modulated carrier wave energy being impressed upon a second control grid in predetermined phase relation. In that system a separate network is employed to provide automatic frequency control (AFC hereinafter for brevity) in order to maintain the receiver properly tuned to the desired carrier frequency.

l In my copending application Serial No. 476,052, led February 16, 1943, I have disclosed a novel detection network for PM, or AM, wave energy. The detection network utilizes a piezo-electric crystal element to provide substantially unmodulated carrier energy. AFC voltage is, also, provided by the network in response to a relatively slow shift in mean frequency of applied signal waves from a predetermined reference frequencyA It is one of the main objects of my present invention to supply the multi-grid detector circuit of .my aforesaid patent with PM or AM waves, and to employ the crystal iilternetwork of my said copending application for the supply of substantially unmodulated carrier energy to said detector whereby the latter functions as a PM de-` tector or as a carrier-exaltation AM detector.

Another important object of my invention is to provide a compact circuit which is capable of receiving either PM or AM signal waves with a minimum of tubes.

Another object of this invention is to provide a receiving` circuit employing a converter to reduce the mean 'frequency of either applied PM or AMV carrier waves, a multi-grid detector tube having the waves applied to a grid thereof, a crystal filter network providing filtered carrier energy fora second grid of the detector tube,

and there being derived AFC voltage from the crystal lter network for controlling the effect of a reactane tube on the ,oscillator section of the converter.

1 Still other objects of my invention-are'to ,im- 55 prove the efficiency and reliability of PM-AM receivers, and more especially to provide a highly compact adapter for PM reception or for exaltedcarrier AM reception.

Still other features of myinvention 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. Y

Referring now to the accompanying drawing, there is shown a pentagrid converter tube I which may be of the 6SA7 type. Its circuitsv are very well known. It acts to reduce the mean or center frequency of applied PM, or AM, carrier wave energy in conventional conversion manner. The coupling condenser 2 will be understoodas functioning to couple the signal grid 3 to any desired signal source. The specific nature of the signal receiver circuits prior to condenser 2 is immaterial to this invention. The collected PM, or AM, carrier waves may be in the megacycle (mc.) or in the kilocycle (kc.) ranges. Prior to condenser 2 the collected waves will be heterodyned, either one or more times, until the mean frequency of signal energy at signal input grid 3 is equal, for example, to 450 kc. ventional intermediate frequency (I. F.) value of the usual superheterodyne receiver.

Whether the wave energy at grid 3 is PM or AM the treatment thereof, according to my in vention, is the same. Generally speaking, it is desired to reduce the mean frequency of the signal energy to a lower Value. Thereafter, it is desired to derive from the reducedfrequency energy substantially unmodulated carrier energy, modulation voltage and AFC voltage. These functions are accomplished in the following manner: l

The local oscillator tank circuit 4 is resonated to 550 kc. Grid 5, the oscillation grid, isfcoupled to the high potential side of the tank circuit 4. 'I'he cathode 6 is connected to a tap on the tank circuit coil whose lower end is grounded. The plate 'l isA 4connected to a positive potential point +B through the winding yrofran iron core kc.A The screen grids I0 are connected to theAV positive potential +B through a properly bypassed, voltage reducing resistor Il. There will bek developed across tuned primary circuit 9-8;

signalenergy whose mean frequency is that Of This is the con--Y the resonant frequency of the circuit. The pass band of transformer T should be sufciently wide to pass all the side band components in the case of AM carrier energy and the maximum phase deviations in the case of PM carrier waves.

The signal wave energy appearing across secondary winding 8' is passed through` a crystal filter to provide the substantially unmodulated carrier energy. The filter consists of piezo-electric crystal P whose inter-electrode capacitancel is neutralized by shunt condenser Cn. The crystal P is tuned to 100 kc. application Serial No. 476,052 has described the functioning of the crystal filter network and its associated opposed rectiers in detail, only a general description is given herein. The filtered carrier output of crystal P is transmitted through condenser I2, of a phase shifter I2-I3-I3, to the grid I4 of multi-grid detector tube I5. The latter is shown as a pentagrid tube of the 6SA7 type. The resistor I3 `of phase shifter I2-I3-I3 is connected to ground from grid I4. The grid IB of tube I5 has applied to it the PM or AM signal energy as the case may be. The grid I6 is connected to the high potential end of primary winding 8 through a series path consisting of attenuating resistor I1 and direct current blocking condenser I8. The resistor I9 acts as a direct current return path for grid I6 so that it can be biased by the voltage drop across a suitably bypassed cathode bias resistor 20.

The plate 2| of the detector tube I5 is connected to a point of suitable positive direct current potential through the output load resistor 22, the latter being bypassed toground for high frequency currents by condenser 23. The screen grids of tube I5 are similarly connected to the source of positive voltage through a voltage reducing resistor 24 Which is properly bypassed for high frequency currents. It is noted that in connection withtube I5 it may be of the metal envelope type. In that case the suppressor grid thereof may be connected to the metal shell of the tube. This may, also, be true in the case of converter tube I.

As explained in my aforesaid U. S. Patent No. 2,063,588, the multi-grid detector tube of the type shown in this case functions to demodulate either.

PM signal waves or AM signal waves.- Since my patent describes the electrical phase relations and functioning of the detector tube in detail it is not believed necessary to repeat such explanation in this description. It is believed sufficient to refer to the detailed and mathematical analysis of my said patent, and to make it clear that such mathematical analysis is to be considered as part of the disclosure of this application. The phase shifter I2-I3--I3 may be adjusted to properly phase the substantially unmodulated carrier energy applied to grid I4, so that the modulation voltage across resistor 22 Will correspond to the modulation of the originally-modulated signal energy for either AM or PM detection. The phase shifter switch S has two contacts or positions indicated as AM and PM. The AM contact is provided at vthe upper end of resistor I3. vGrid I4 of detector tube I5 is connectedl to the junction of resistors I3 and I3. Hence, when switch S is adjusted to contact AM the output electrode of crystal P is connected to grid I4 through a path Vconsisting of S, contact AM and resistor I3'.

The ltered carrier energyundergoes no phase shift inthis case due to the apericdic nature of resistor I3'. The switch S is adjusted tovcontact PM in thedrawing, and the PMT contact is provided at the input side of condenser I2. The grid I4 is connected to the output electrode of condenser I2, or at the junction of condenser I2 and I3. In the position of switch S the filtered output of crystal P is connected to grid I4 through a path consisting of S, contact PM and condenser I2. In other words, if there is transmitted to the grid I6 unfiltered AM signal energy, then there will be simultaneously applied to grid I4 energy of the mean frequency of the AM signal enn ergy which has its phase properly adjusted by switching S to contact AlVL However, the energy at grid I4 will be unmodulated due to the ltering action of crystal P, and its phase will be such relative to the phase of the AM signalenergy of gridlii that the tube Will function to provide across output resistor 22 the ldesired modulation components. The proper phase relation between the modulated carrier energy and the filtered carrier energy is zero or 180 degrees for AM detection. In other words when switch S'is adjusted to contact AM the filtered output of crystal P will be applied to grid I4 through re. sistor I3'. Hence, the unmodulated carrier energy transmitted to grid I4 will have its phase substantially the same as the phase of the carrier of the unfiltered AM signal energy transmitted through condenser I8 and resistor Il to grid I6. With this, in-phase relation between the energies applied to grids I4 and IS of tube I5, the latter will function to provide across output resistor 22 the desired modulation signal components.

Assume, now, that there is transmitted to the grid I6 PM signal energy. In this case, the subu stantially unmodulated carrier energy applied to grid I4 will be adjustedto be 90 degreesout of phase with the PM carrier energy applied to grid I0 by switching S to the contact PMJ In this PM position of switchl S the condenser .I2 feeds the unmodulated carrier energy output of crystal P to grid I4. Condenser I2, in orderv to impart the aforesaid phase shift to the unmodulated carrier energy, is chosen sufciently small' for this function. The filtered carrier voltage at grid I4 normally, therefore, has a phase quadrature relation to the unfiltered PMsignal energy at grid.

I6. This normal phase quadrature relation iside-r parted from the instantaneous phase deviations ofthe PM signal wave. Detector tube I5 translates the departures from. the normal phase quadrature relation. of the voltages of grids I4.

and .I6 into modulation output voltage inthe manner described in'rny aforesaid Patent No. 2,063,588. The action of the detector. tube 'will therefore be to demodulate. the PM signal energy,

and to provide demodulaticn.components thereof' In the case of: both sideband components relative to thestrength ofthecarrier at the respective grids I4 and I6. The filtered carrier at grid I4 has been exaltedr or augmented relative to the attenuated sideband componentsat grid` I6.. This action'is, also,true for PM signal waves, the path Iii- I1 attenuating the sideband components of the unfiltered PM signal wave relative to the ltered'PM- car. rier energy appliedthrough',condenser I2 to grid Hence, selective `fading effects: are reduced' Y I4. Such carrier exaltationdetectionl lsffofesp'e-A by the action of the opposed rectifiers 26 and 21.y

While these rectiers are symbolically represented as separate diodes, it is preferred that the diodes be housed within a tube of the doublediode type, suchas a GHG type tube. The load resistors.

of the

rectifiers

26 and 21 are represented by

numerals

26 and 21 respectively. The cathodeof rectifier 21 is grounded, and each ofthe load resistors is bypassed for high frequencycurrents..

The junction of the load resistors is connected to the junction of a pair of direct

current return resistors

28 and 29. The junction point of?r these latter return resistors is connectedto the .midpoint of secondary winding 8',

As explained in my aforesaid application Serial No. 476,052 the filtered carrier energy is ap-` plied in like polarity or parallel to thev anodesof the respective rectifiers 261 and 21.

Condensers

30 and 3| function as isolation condensers and apply the filtered carrier energy from the output electrode of crystal P to the respective anodes of the diode rectifiers. The modulated, or unltered, signal energy existing at the opposite ends of center-tapped winding 8 is appliedto the

rectifiers

26 and 21 in opposite polarity push-pull by virtue of the similar shunt coupling condensers 32 and 33 respectively. As explained fully in my aforesaid pending application, each

rectifier

26 and 21 has applied to it filtered and unfiltered signal energy in phase quadrature relation. This phase quadrature relation of the two voltages at each

rectifier

26 and 21 results from the fact that the unfiltered signal energy is applied tothe recti fiers by condensers 32 and 33 which arev suflicient` ingly permit all signal components to pass to both rectilers 26 and 21.r Crystal P, however, effects no substantial phase shift of the filtered carrier wave at modulation frequencies, but by its inertia effect substantially removes the modulation of the carrier thereby restoring -the latter substan-l tially to the phase and wave' form which it had before modulation at the transmitter. rllhe crystal P due to its sharp selectivity is of course,A

selective against frequencies off resonance, but will nevertheless pass a band of frequencies varying over a narrow range. The rectiers function to rectify the respective resultant Vector voltages of each'pair of quadrature-related energies. When 4the applied signal energy at transformer T has a mean frequency equal to the predetermined frequency ofl the primary tuned circuit -9-3, then the respective resultant vector voltages lapplied tc rectifiers Z6 and 21 are equal in amplitude. This follows from the fact that each of the respective resultant vector voltages is the vector sum of the filtered carrier voltage from crystal P Without phase change and of an unfiltered carrier voltage substantially 90 different in. phase from the filtered carrier. Accordingly, the'difer-v ential direct current voltage output 'of the recti-l fiers is substantially zero, since the `voltages across resistors 26 and 21' areequal and of opposite i 6 polarity. However, upon a relatively slow dei parture ofthe said mean frequency fromthe pre'.. determined resonant reference frequency vthere will be a direct current .voltage'or AFC bias devel.-

L oped at the cathode end of resistor 26. This follows from the fact that crystal P can respond to frequency changes over a limited range, and, in particular, to slow frequency changes such as may, for example, result from oscillator drift.

As to such frequencies within the narrow band of frequencies passed by it, the crystal P acts as series-connected inductance and capacity. When" the received signal energy frequency is exactly that of the crystal frequency, the crystal effects no phase change in the filtered carrier energy `and the-normal phase quadrature relation exists between the latter and the unfiltered energy. passing through respective condensers 32 and 33.` When, however, the frequency of the signal en'- ergy applied at transformer T tends Aslowlyto change toward one side of the narrow passband of crystal P, the filtered output energy of the crystal changes in phase in a direction toward the phase of the current in one of condensers 32 and 33, and

away from the phase ofthe current in the other of such condensers. In this way there is pro-4 duced increased energy on one of the

rectifiers

26 and 21 and decreased energy on the other. Should the signal energy frequency change in the. opposite direction, the phase of the filtered output of the crystal changes in the opposite direc-V tion thereby reversing the relative magnitude of energies applied to

rectifiers

26 and 21. Hence, there appear direct current voltages across each of

resistors

26 and 21 whose relative magnitudes vary as the applied signal frequency slowly shifts relative to the reference frequency of crystal P. These voltages are differently combined toprovide a resultant direct current voltage (AFC bias). The magnitude and polarity of the result-1 ant direct current voltage will be functions respectively of the extent and direction of the' aforesaid slow frequency shift. Reference is-v made to my aforesaid pending application for a further description of the electrical reactions existing inthe crystal discriminator-rectifier net-'- work.

The direct current voltage output of the opposed rectiers is utilized as AFC bias, and is applied over lead 34 tothe reactancetube 25. 'The coil 36 whose lower end is bypassed for high fre'-'- quency currents by condenser 36. The screen: grid 6| of tube 25 may, also, be connected to the? source of positive potential +B through a prop-- erly bypassed, voltage-reducing resistor 25', The' plate 31 of the reactancetube 25 is connected back-.to ground through a pathwhich comprises.

a phase shifter. The phase shifter is shown asl a dash line rectangle, and Consists of condenser 40 and resistor 4l. connected back to ground for high frequency' cur'- rents through the condenser 42. The AFC lead 34 is connected through filter resistor 43 to the lowerend of phase shifter resistor 4l. In this way, the

control grid 44 is variably biased'so as to con-n The plate 31 isiconl'f* nected by the Vdirect current blocking' condenser' trol the gain of tube 25.

The lower end of resistor 4I is andres;

- 7 S'to the high potential side ofthe oscillator tank circuit '4. A tuning meter 62 is shown arranged in series with choke 36. The meter 62 has'a fixed deflection for the in-tune condition of the oscillator.V For off-tune conditions the meter B2 will be deflected from its mean correct tuning indication. The reactance tubeA plate current ows through the meter- 62, Hence the latter is capable of indicating correct tuning of the oscillator circuit. In explaining the functioning of the reactance tube, it is rst pointed out that its plate tocathode impedance is effectively connected in shunt across the tank circuit 4. The alternating voltage existing at plate 31 is applied across the phase shifter 40-4I. If the capacitatve. reactance Vof condenser 4U is chosen sufficiently small compared to the resistance of resistor 4i, the. alternating voltage developed across resistor 4l' and applied to grid 44 will be substantially in phase quadrature with the alternating plate voltage. Under these conditions the plate to Cathode impedance. of tube 25 will function as if it was capacitative, and it will appear effectively across tank circuit 4. The magnitude of this simulated capacitative reactance will then be aV function of the gain.

pend upon the variation in AFC bias transmitted over lead 34, This, of course, is the well known AFC actionwhich is familiar to those skilled in the art. In this case the AFC circuit functions to maintain the mean frequency of the signal energy applied to circuit 9-8 substantially equal to the predetermined reference frequency which is the resonant frequency of the crystal P. Hence, the filtered carrier energy fed to grid i4 of detector tube I will be synchronized in frequency for proper detection.

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

What I claim is:

1. In a system of the type including a converter' network provided with a signal input circuit and a local oscillator circuit, means for applying to said signal input circuit either phase modulated, or amplitude modulated, carrier wave energy, an output circuit 'connected to said converter for developing thereacross signal energy whose mean frequency is the difference between the signal carrier frequency and kthe frequency of. said oscillator circuit, and means coupled to said output circuit for derivingy therefrom substantially unmodulated `carrier energy, the improvement which consists of an electron discharge tube provided with at least a pair ofspaced input electrodes, means connecting one of the said input electrodes to said output circuit thereby to apply to the input electrode modulated signal energy, a phase shifter connecting the second input electrode to said deriving means thereby to apply to the second input electrode said substantially unmodulated carrier energy, said phase shifter comprising a resistor and a condenser having individual input terminals, a common out-put connection from the output terminals of the resistor and condenser to said second input electrode,

means for selectively connecting said deriving for connecting one of the saidv input electrodes to cation to the respective resistor andcondenser of the unmodulated carrier energy derived from.

the amplitude modulated and phase modulated waves respectively, and an output circuit connected to said tube for developing modulation voltage components.

2. In a system of the type including a converter i the frequency of said oscillator circuit, the improvement comprising piezo-electric crystal lter means coupled to said output circuit for deriving therefrom substantially unmodulated carrier energy.. an electron discharge tube provided with at least a pair of spaced input electrodes, means including a condenser in series with a resistor said output circuit thereby to apply to the input electrode said developed phase modulated energy, a phase shifter consisting of a resistor and a condenser provided with a common output terminal, means connecting the latter to said second input electrode, said resistor and condenser having respective input terminals, a switch for selectively connecting either of said inputl terminals to the f output of said iilter, and an output circuit connected to said tube for developing modulation voltage components.

Il. In a system of the type including a converter and a local oscillator circuit, means for applying to said converter phase modulated carrier wave ence between the applied carrier frequency and the frequency of said oscillator circuit, the improvement which consists of a piezo-electric 4crystal lter coupled to said output circuit for deriving therefrom substantially unmodulated carrier energy, an electron discharge tube provided with at least a pair of spaced input electrodes, means for connecting one of the said input electrodes to said output circuit thereby to apply to the input electrode said developed phase modulated energy, a phase shifter comprising a resistor and a condenser having a common output connection at the junction thereof to said second input electrode, means for selectively connecting the output vof said filter to either input terminal of the resistor and condenser, and an output circuit connected to said tube for developing modulation voltage components, and means coupled to the output of the` crystal filter respon-- sive to a departure of the mean frequency of the modulated energy in said output circuit from a predetermined reference frequency for automatically controlling the frequency of said local oscillator circuit.

4. In combination with a converter network provided with a signal input circuit, means for applying to said signal input circuit either phase modulated or amplitude modulated carrier wave energy, and an output 4circuit connected to said converter for developing thereacross signal energy whose means frequency is lower than signal carrier frequency, piezo-electric crystal filter-means coupled to said output circuit for deriving therefrom. substantially unmodulated carrier energy, a detector tube provided. with at least a pair of spaced input grids, means for' connecting one of the. said input grids to said output circuit thereby to apply to the input grid modulated signal energy, a phase shifter consisting of a resistive element and a capacitative element, said elements having a common output terminal and independent input terminals, said second input grid being connected to the common output terminal, means selectively connecting either of the input terminals tothe output of said filter means whereby the phase of substantially unmodulated carrier energy may vbe adjusted relative to the phase of the modulated signal energy applied to the first input grid whereby the detector is selectively conditioned for detecting said phase modulated or amplitude modulated carrier wave energy, and an output circuit connected to said tube for developing modulation voltage components.

5. In combination with a signal input circuit, means for applying to said signal input circuit either phase modulated or amplitude modulated carrier wave energy, and an output circuit, piezoelectric crystal filter means coupled to said output circuit for deriving therefrom substantially unmodulated carrier energy, an electron discharge tube provided with at least a pair of spaced input electrodes, sideband attenuation means for connecting one of the said input electrodes to said output circuit thereby to apply to the input electrode modulated carrier energy with substantially attenuated sidebands, a selective phase shifter connecting the second input electrode to said deriving means thereby to apply to the second input electrode said substantially unmodulated carrier energy, said phase shifter comprising a resistive impedance and a reactive impedance, said impedyances having a common output terminal connected to said second input electrode, said impedances having separate input terminals, means for selectively connecting either of said input terminals to said deriving means, and a modulation routput circuit connected to said tube for developing modulation voltage components. l

6. A system of the type including a converter network provided with a signal input circuit and a local oscillator circuit, means for applying to said signal input circuit either phase modulated or amplitude modulated carrier wave energy, and an output circuit connected to said converter for developing thereacross signal energy whose mean trodes to said output circuit thereby to apply to the input electrode modulated signal energy with attenuated sidebands, a phase shifter connecting the second input electrode to said deriving means thereby to apply to the second input electrode said substantially unmodulated carrier energy, said phase shifter comprising a condenser for imparting a quadrature phase shift to unmodulated carrier energy for receiving phase modulated carrier waves, a resistor having a common output connection to the output terminal of said condenser, said resistor and condenser having separate input terminals, means for selectively disconnecting the said deriving means from the condenser input terminal and connecting it to the resistor input terminal thereby eliminating the condenser and replacing it by said resistor for the reception of amplitude modulated carrier wave energy, and an output circuit connectedto said last tube for I developing modulation voltage components.

MURRAY G. CROSBY.

REFERENCES CITED The following references are of record in the i'lle of this patent:

UNITED STATES PATENTS Number Name Date 2,063,588 Crosby Dec. 8, 1936 2,233,778 Foster Mar. 4, 1941 2,173,907 Kirkwood Sept. 26, 1939 2,195,290 Shofstall Mar. 26, 1940 2,263,615 Crosby Nov. 25, 1941