US2323596A - Frequency modulation receiver - Google Patents

Description

July 6, 1943. c. w. HANSELL 2,323,596

FREQUENCY MODULATION RECEIVER i Filed Jxine 17, 1941 3 Sheets-Sheet 1 INVENTOR July 6, 1943. c, w, HANSELL 2,323,596

EREQUENGY MODULATION RECEIVER Filed June 17, 1941 3 Sheets-Sheet 2 low/ ads July 6, 1943. c. w. HANSELL FREQUENCY MODULATION RECEIVER 3 Sheets-Sheet 3 Filed June 17, 1941 @MMLeJ'J I w u a. W n 6 W. my w. w m 1/ Y M B Czar-made ods of and means for carrying Patented July 6, 1943 iJNl'i'ED STATES FREQUENCY MODULATION RECEIVER Clarence W. Hansell, Port Jefferson, N. 31., assignor to Radio Corporation of America, a corporation of Delaware Application June 17, 1941, Serial No. 398,391

6 Claims.

This invention relates to new and improved methods of and means for the reception of wave length modulated carrier current. In its broader aspects it involves new and improved methout the fundamental principles of frequency modulation reception which were described in my United States Patent No. 1,813,922. In the said patent I described a means for providing constant energy, or constant electrical charge, per cycle of an intermediate frequency carrier current in a superheterodyne frequency modulation receiver, followed by means for integrating and utilizing the successive energies or currents to provide an output current, or increment of output current, proportional to the frequency of the intermediate frequency current. That is, I provided means for producing equal energy or current pulses at a rate determined by the frequency of an intermediate frequency carrier current and utilized these pulses to produce a flow of power, or of current, which varied in proportion to frequency variations of the carrier current.

In describing my invention reference will be made to the attached drawings wherein: Figures 1 to 6 inclusive each show a modification of a wave length modulated wave demodulator arranged in accordance with my invention and Figs. '7, 8, 9 show further modifications. In the arrangement of Figure 1 received radio signals are supplied from antenna A to circuits in unit 2 comprising radio frequency amplifiers, asource of oscillations, a first detector and I. F. amplifiers, The radio signals are heterodyned down in unit 2 to an intermediate frequency, amplified and applied by way of resistances 6 and 8 to the grids Ill and 12 of a pair of vacuum tubes I4 and 56 which are provided with regeneration sufficient to produce a condition of unstable equilibrium. The regeneration may be obtained in various ways. In a preferred embodiment the grid I8 of tube I4 is coupled by resistance 20 and condenser 22 to the anode 24 of tube it, while the grid 25 of tube !6 is coupled by resistance 25 and condenser 28 t the anode 30 of tube l4. These couplings provide regeneration to produce an unstable condition so that over the operating frequency range, one tube or the other carries a relatively large anode current while the opposite tube carries zero or very low anode current.

coupled in push-pull relation by the primary winding ill of a transformer 42, the secondary winding 44 of which is coupled to the anodes 46 and '53 of a double diode 50. The tubes l4 and I6 are coupled by the transformer 42 to the rectifier 5B which, in turn, supplies current through a low pass filter 52 to an audio amplifier 54 and loudspeaker 56,

In operation the intermediate frequency current controls the frequency rate at which the two tubes, with their circuits, throw the current balance from one tube to the other but the regenerative feedback determines the quickness of the transition. Therefore, the wave form of current pulses, and the magnitude of electrical charge per pulse delivered to the rectifier 5B are substantially independent of the pulse frequency rate. Consequently, the mean direct current passed through the rectifier 50 and low pass filter 52 t0 the load 54 and 56 is substantially proportional to the pulse rate, provided the load impedance is not too high in comparison with the effective impedance of the source of current for the load.

With this combination, when a frequency modulated radio carrier is being received, there is a corresponding modulation of the intermediate frequency and of the pulse rate so that the rectified current, after going through the low pass filter, varies in magnitude substantially in proportion to the frequency and has variations corresponding to the modulation of received cur-- rents.

In the arrangement of Fig. 1, I have shown tubes having three grids placed successively in the electron stream between cathode and anode. In each tube one grid receives the intermediate frequency control potential, another acts as a screen between tube input and output circuits and the third is utilized for feedback potentials to provide the desired instability of current balance between the two tubes- The two tubes and their circuits are closely related to the tripping circuit invented by my associate, J. L. Finch, as, for example, disclosed in United States Patent No. 1,844,950, dated February 16, 1932, which has found wide application in communications systems.

Since, as pointed out above, the magnitude of the electrical charge per pulse delivered to the rectifier 50 is substantially independent of the pulse frequency rate the arrangement of Figure 1 automatically performs the functions of an amplitude limiter and frequency modulation detector, as used in prior art frequency modulation receivers, but performs the functions with simpier" and lower cost equipment which produces less distortion when correctly proportioned.

In Figure 2, I have shown another form of frequency modulation detector in which intermediate frequency currents from unit 2 are supplied by coupling condensers 6G and 62 and resistances B and 8 tothe control grids l6 and I2 of two tubes l4 and i5. Biasing potential for the grids I and i2 is supplied through a tap on an inductance 84. The anodes Ed and 38 are tied together and coupled by condenser 65 to a rectifier The applied I. F. currents cause anode currents to flow in one tube or the other at all times eff-- cept while the input intermediate frequency potential is passing through zero. At these for very brief periods, which occur twice per cycle of input current, the anode currents of both tubes cease. The inductance 68 between the anode direct-current power supply and the anodes tends to keep an approximately constant flow of current through itself, Consequently, when both tubes cut off their anode currents a current pulse is delivered through the condenser 66 to the rectifier ii). Rectified current derived from the pulses is filtered by 52 and associated circuit elements to remove currents of the pulse frequency leaving direct current and modulation current components. The direct current may be by-passed by transformer windings, and the remaining modulation frequency current amplified in 54 and applied to the loudspeaker 55.

In this arrangement the current pulses tend to vary in length as the input intermediate frequency varies and this tends to decrease the useful modulation output. To overcome this effect the inductance 68 between the power source and the tube anodes may be adjusted to a lower value so that, at lower pulse frequencies, the current in the inductance will change during a pulse period.

In Fig. 3 I have shown a third form of frequency modulation receiving system in which frequency modulated currents are applied differentially to the grids Ill and 12 of a multi-grid vacuum tube M. In the arrangement shown I have indicated a pentode type of vacuum tube 74 having three grids l0, l2 and 76 in series along the electron stream between the cathode l8 and anode 8! and second grids 1E] and 72 are maintained at zero or at somewhat positive average potentials and the third grid is maintained at a somewhat more positive average potential. potentials for so biasing the grids are supplied by appropriate tabs on potentiometer resistance 15.

Due to the intermediate frequency input potentials applied to grids l0 and 12 one grid or the other will always be at a negative potential, and will block off the anode current, except for a brief interval of time when the intermediate frequency input potentials are passing through zero. At these brief time intervals, repeated twice per cycle of input potential, the tube 14 is made conducting between anode and cathode and discharges the condenser 86 connected between anode cathode.

The rate or frequency at which the condenser is discharged is determined by the frequency of the intermediate frequency current and varies in accordance with the modulation of the frequency. Consequently, the average or direct-current potential across the condenser 86 tends to vary in inverse proportion to the frequency while the average current through the tube tends to vary in proportion to the frequency. Then, so long as the condenser discharge through the tube has a time constant which is shorter than the time duration of the tube conducting periods, and the circuits supplying charge to the condenser have time constants which are long compared with the longest time period between conducting pulses, I obtain a power input current tending to intermediate frequency Direct current The first 1 vary in proportion to the input frequency. This current variation, and a corresponding potential variation may be utilized to supply modulation frequency output power, which, in the arrangements illustrated, is amplified in 54 and applied to a loudspeaker 56. Audio transformer couples amplifier 54 to the plate circuit of tube 14.

In Fig. 4 I have shown a frequency modulation receiver in which I employ What is commonly known as a multivibrator circuit. This circuit is capable of self-oscillation with a. very distorted wave form of vacuum tube currents. As shown, the multivibrator comprises tubes and 92 with the grid 94 coupled by condenser C to the anode 95 of tube 92. The grid 95 of tube 92 is coupled by condenser C2 to the anode 91 of tube 90. The tubes so cross connected operate to generate waves of a frequency controlled in part at least by the Values of condensers C and C2 and of the resistances RI, R2, R3 and R4 of the circuits. The multivibrator consists essentially of a twostage resistance coupled amplifier, with output from the anode 95 of the second stage fed back to the control electrode 94 of the first stage. When the circuits are oscillating the second tube 92 draws large pulses of current followed by longer periods of no current during which the condenser 98 connected from anode 95 to the cathode 99 of the second tube is being charged. The time between pulses, and therefore the average current through the second tube is controlled by the intermediate frequency input power supplied across Rl, which controls the frequency of tripping of the circuit. This average current varies in accordance with the frequency modulation. The modulation frequency ciurents are supplied from the resistance R4 to the transformer I03 and thence through a low pass filter included with amplifier 54. In this manner the modulation frequency currents are derived, amplified and utilized.

In Figure 5 I have shown a frequency modulation receiver in which a blocking oscillator, as it is called in the art, has its frequency of action determined by input intermediate frequency currents. This blocking oscillator comprises a tube I00 having its grid I02 regeneratively coupled to its anode IE4 by the windings of a transformer IE6. A grid leak I08 and condenser H0 is included in the grid connection and this resistance and condenser combination taken with the circuit elements determines in part the frequency of operation of the oscillator. The circuit is such that when an oscillation is started it builds up until a blocking voltage is developed therein on the grid I02. The building up and blocking action is rapid and current flows in short high peaks. The intermediate frequency input current to the blocking oscillator supplied across resistance H2 determines the time of starting of the pulses but, once the pulses are started, the blocking oscillator feedback through transformer I06 takes over control of the tube and forces the tube to operate and pass a short high peak of current. The tube I00 then blocks due to high negative potential, developed by the action of the grid leak and condenser, and is ready for the neXt cycle of wave length modulated intermediate frequency input from stage 2. The building up and blocking action is made faster than the time of a half cycle of the highest intermediate frequency, but the time period within which the anode current of tube I00 would remain cut 011 between pulses is made equal to or greater than the time of one cycle of the lowest intermediate frequency produced by the useful modulation.

Across grid to cathode of the oscillator tube I there appear short high peaks of potential. These peaks are supplied by lead H4 to the grid H6 of the tube II 8 of the following stage. The tube H8 is a condenser discharge tube. This tube has a condenser I connected between its anode H1 and cathode H9. In consequence, at each cycle of the intermediate frequency input current the grid H6 becomes less negative and the condenser discharge tube H8 becomes conductive and discharges the condenser I29 connected between its grid and cathode. The condenser then is charged again partially or wholly from the positive anode current supply. The

mean or average potential appearing across the condenser 129 is dependent upon the frequency at which tube H8 is biased conductive and at which the condenser discharges. This average potential therefore varies up and down in accordance with the frequency modulation present in the intermediate frequency input. Consequently, the potential across the condenser I20, after elimination of frequency components at frequencies above the useful modulation frequencies, including the intermediate frequency and its harmonics, represents the useful modulation which it is desired to receive. Consequently, output taken from the condenser through a low pass filter and audio amplifier 5 3 may be used to drive a loudspeaker or any other suitable output device.

In the arrangement of Fig. 5, as well as in the arrangements shown in the other figures, a variation in amplitude of the intermediate frequency currents and potentials has very little, if any, efifect upon the operation of the detecting system, provided the input is above some minimum required level. Consequently, the systems shown act as amplitude limiters as well as frequency modulation detectors.

In Fig. 6 is shown a modification of Fig. 5 in which pulses from a blocking oscillator are passed by coupling condenser I into a rectifier I26 to provide a rectified current having an average value which varies substantially in proportion to the frequency of the intermediate frequency current. This arrangement also provides amplitude limiting and frequency modulation detection.

The system of Figure 1 may be modified as illustrated in Figure '7. In Figure '7 the antenna I and heterodyning and intermediate frequency amplifying means reduces the frequency of the received wave length modulated wave while increasing the percentage modulation thereof and feeds the same to transformer 5 and thence by way of resistances 6 and 8 to the control grids I8 and 25 of tubes I4 and I6 which in this modification are triodes. These grids I8 .and 25 are cross connected by resistances 23 and 2!, respectively, to the anodes 24 and 30, respectively, to form a tripping circuit similar to that of Figure 1. The tubes I4 and I6 as explained in detail in connection with Figure 1 alternately pass current. The duration of the impulses depends on the constants of the tube circuits and is substantially independent of the amplitude of the input. The frequency of reversal, twice per cycle of applied intermediate frequency current, depends on the frequency of the input. The alternating current output is fed to transformer 42 and rectified as in Figure 1.

The arrangement of Figure 8 is a modification I 20 passed having features of the arrangements of Figures 2 and 5. In Figure 8 a single pulsing tube I4 replaces the pulsing tubes I4 and I6 of Figue 2. A transformer replaces the coupling including condensers Si}, 62, inductance 64 and resistances 6 and 8 of Figure 2. The pulses in Figure 8 are fed to a condenser discharge tube H8 in many respects similar to tube H8 of Figure 5. The tube H8 controls the rate of discharge of condenser I2I which is charged at a rate dependent principally on the value of resistance l23. The conductivity of tube H8 is controlled by the pulses from tube I4 and the rate at which these pulses, which are of substantially constant amplitude and time duration, are fed through condenser 66 is a. function of the frequency of the intermediate frequency output of the amplifier in 3. The time constant of the assembly including condenser I25 and resistance I23 is made less than the time between pulses (output from I4) at the highest pulse rate which corresponds to the highest modulation frequency.

In Figure 9, which is in some respects similar to the modification of Figure 5, currents picked up by the antenna are first heterodyned to a lower frequency and amplified to a higher power level, after which they are delivered through a coupling transformer to a pulsing oscillator comprising one-half of the twin element triode vacuum tube Illil. This oscillator serves the same purpose as the oscillator associated with tube I00 of Figure 5. The oscillator is so designed and adjusted that, by itself, it would cause short high peaks of anode current at a frequency somewhat less than the lowest intermediate frequency delivered to it.

When the frequency modulated intermediate frequency current is delivered to the oscillator it increases the pulsing oscillation rate to make it equal in frequency to the frequency of the intermediate frequency current. Then the 0scillator frequency is modulated as the intermediate frueqency current is modulated by the useful signals.

By means of a connection from the control electrode I92 of the pulsing oscillator to control electrode H6 of a discharge circuit a condenser IBI connected with the anode circuit is discharged once each cycle of the intermediate frequency current. The condenser I0! is connected with a positive potential current source through an impedance, which may be the resistance I01 shown.

Preferably the adjustments are made such that the condenser is discharged to a rather low fixed value of potential at each pulse from the pulsing oscillator. The potential across the condenser then rises due to charging current from the source until the next pulse discharges the condenser again. As a consequence of this action the mean effective potential of the condenser is modulated by a modulation in the frequency of the pulsing oscillator under the control of the received current. This modulation in mean effective potential causes modulation frequency currents to flow through the primary winding of the output transformer, which may be filtered and utilized as explained in connection with the other figures.

In the operation of the arrangement of Figures 4, 5, 6 and 9 it is not essential that the frequency of operation of the pulsing oscillators be equal to the frequency of the intermediate frequency current. Instead, the oscillators may operate at /2, /3, 2;, etc. of the frequency 'of the intermediate frequency current provided the percentage modulation of frequency of the intermediate frequency current is not too great.

I have illustrated my novel frequency modulation detector circuits as applied to radio receivers but it should be apparent that they are also applicable to any kind of carrier wave frequency modulation communication or telemetering system, including electrical communication over wire circuits, through wave guides, and signalling by vibrational waves through gases, solids or liquids, such as submarine signalling. They may be used advantageously in receiving sub-carrier frequency modulated signals in multiplex systems and in facsimile communications systems such, for example, as disclosed in C. W. Hansell Patent No. 1,819,508, dated August 18, 1931; C. W. Hansell Patent No. 2,103,847, dated Dec. 28, 1937; R. H. Ranger Patent No. 1,830,242, dated Nov. 3, 1931; J. N. Whitaker appln. Ser. No. 311,495, filed Dec. 29, 1939; H. 0. Peterson appln. Ser. No. 384,628, filed Mar. 22, 1941; H. Tunick, appln. Ser. No. 369,800, filed Dec. 12, 1940; H. 0. Peterson appln. Ser. No. 341,285, filed June 19, 1940; and H. H. Beverage Patent No. 2,025,190, dated Dec. 24, 1935.

The invention, and the detail arrangements shown, provide for substantial suppression of amplitude modulations, or amplitude limiting, as well as for demodulation of frequency modulated waves with simpler and less expensive equipment than heretofore used.

I claim:

1. In a receiver of angular velocity-modulated carrier waves, a demodulator comprising a first electronic device having at least a cathode, a wave input electrode and an output electrode, means regeneratively coupling the output and input electrodes to provide an oscillator, means operatively associated with the oscillator normally to cause high peaks of output electrode current of short duration at a frequency close to the lowest wave frequency supplied to the receiver, a second electron discharge device having electrodes coupled to said oscillator device to derive therefrom a voltage representative of the modulation of applied waves, and a modulation voltage utilization circuit coupled to the second device.

2. In a receiver of angular velocity-modulated carrier waves, a demodulator comprising a first electronic device having at least a cathode, a Wave input electrode and an output electrode, means regeneratively coupling the output and input electrodes to provide an oscillator operating at the frequency of applied waves, a second electron discharge device having electrodes coupled to said oscillator device to derive therefrom a voltage representative of the modulation of applied waves, means connected to the said oscillator input electrode adapted to cause the oscillator to function as a blocking oscillator whose blocking action is faster than the time of a half cycle of the highest wave frequency supplied to the receiver, and a modulation v0ltage utilization circuit coupled to the second device.

'3. In a receiver of angular velocity-modulated carrier waves, a demodulator comprising a first electronic device having at least a cathode, a wave input electrode and an output electrode, means regeneratively coupling the output and input electrodes to provide an oscillator, means operatively associated with the oscillator normally to cause high peaks of output electrode current of short duration at a frequency close to the lowest wave frequency supplied to the receiver, a second electron discharge device having electrodes coupled to said oscillator device to derive therefrom a voltage representative of the modulation of applied waves, said second device having an input electrode coupled to the oscillator device input electrode, and a modulation voltage utilization circuit coupled to the second device.

4. In a receiver of angular velocity-modulated carrier waves, a demodulator comprising a first electronic device having at least a cathode, a wave input electrode and an output electrode, means regeneratively coupling the output and input electrodes to provide an oscillator operating at the frequency of applied waves, a second electron discharge device having electrodes coupled to said oscillator device to derive therefrom a voltage representative of the modulation of applied waves, means connected to the said oscillator input electrode adapted to cause the oscillator to function as a blocking oscillator whose blocking action is faster than the time of a half cycle of the highest wave frequency supplied to the receiver, said second device having an input electrode coupled to the oscillator device input electrode, and a modulation voltage utilization circuit coupled to the second device.

5. In a receiver of angular velocity-modulated carrier waves, a demodulator comprising a first electronic device having at least a cathode, a wave input electrode and an output electrode, means regeneratively coupling the output and input electrodes to provide an oscillator, means operatively associated with the oscillator normally to cause high peaks of output electrode current of short duration at a frequency close to the lowest wave frequency supplied to the receiver, a second electron discharge device having electrodes coupled to said oscillator device to derive therefrom a voltage representative of the modulation of applied waves, means connected to the said oscillator input electrode adapted to cause the oscillator to function as a blocking oscillator, said second device having an input electrode coupled to the oscillator device input electrode, and a modulation voltage utilization circuit coupled to the second device, said devices being separate electron discharge tubes, said second tube having an output electrode, and a charging condenser connected in circuit with the latter output electrode.

6. In a receiver of angular velocity-modulated carrier waves, a demodulator comprising a first electronic device having at least a cathode, a wave input electrode and an output electrode, means regeneratively coupling the output and input electrodes to provide an oscillator, means operatively associated with the oscillator normally to cause high peaks of output electrode current of short duration at a frequency close to the lowest wave frequency supplied to the receiver, a second electron discharge device having electrodes coupled to said oscillator device to derive therefrom a voltage representative of the modulation of applied waves, and a modulation voltage utilization circuit coupled to the second device, said second device being a diode rectifier, and the electrodes thereof being coupled to the output electrode and cathode of the oscillator device.

CLARENCE W. HANSELL.

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