US2374746A - Frequency modulation receiver - Google Patents

Description

Patented May 1, 1945 2,374,746 FREQUENCY MODULATION RECEIVER Clarence W. Hansell, Port Jefferson, N. Y.,'as-

signor to RadioCorporation of America, a corporation of Delaware Original application June 1'7,

1941, Serial No.

398,391. Dividedand this application September 21, 1942, Serial No. 459,111

5 Claims. 31. 250- 21 This application is a division of my application Serial No. 398,391, filed June 17, 1941, U. S. Patent No. 2,323,596, granted July 6, 1943. This invention relates to new and improved method of, and means for, reception of frequency modulated carrier current. In its broader aspects it involves new and improved methods of, and means for, carrying out the fundamental principles of frequency modulation (FM) 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 change, per cycle of an intermediate frequency (I. F.) carrier current in a superheterodyne frequency modulation receiver. That means was followed by an additional mean for integrating and utilizing the successive energies, or currents, to provide an output current, or increment of output current, proportional tothe 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 The novel features which I believe tobe characteristic of my invention are set-forth with particularity in the appended claims; the invention itself, however, a to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I hav ind-icate'd diagrammatically several circuit organizgftioihs whereby my invention may be carried into e ec In describing my invention reference will b made to the attached drawing'wherein:

Figs. 1, 2 and 3 each show respectively different embodiments of a frequency modulated wave energy demodulator arranged in accordance with my invention.

In the arrangement of Fig. 1 received radio signals (FM energy) are'supplied .from antenna A to circuits in unit 2 comprising radio frequency amplifiersra source oflocal 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 respective grids .Ill and I2 of a pair of vacuum tubes l4 and IS.

The I. F. currents from unit 2 are supplied by coupling condensers 60 and 62 to the respective resistances 6 and 8. grids HI and I2 is supplied through a tap on an inductance 64. The anodes 24 and are tied together, and coupled by condenser 66 to a, rectiiier 40. The applied I.F. currents cause anode currents to flow in one tube, or the other, at all Biasing potential for the times, except while the input intermediate frequency potential I is passing through zero. At

these timesfor'very brief periods," which occur twice per cycle ofinput current, the anode currents of both tubes cease.

parallel-connected anodes,-tends to keep an approximately cofistant flow of current through itself. Consequently, when both tubes cutoff their anode currents a current pulse is delivered through the condenser 66 to the rectifier 40. Rectified current derived from the pulses is fi1-' te'red by network 52 and associated circuit elements to remove current of the pulse frequency thereby leaving direct current and modulation current components. The direct current may be bypassed by transformer windings, and the remaining modulation frequency current is ampli fied in amplifier 54 and applied 'to the loudspeaker 55.

' In this circuit arrangement the current pulses tend to vary in lengthas 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 anode may be adjusted to a lower value so that, at lower pulse frequencies, the cur rent inthe inductance willchange during a pulse period. Then the variation in the reactance with variation, in frequency assists in obtaining uniform frequency response in the demodulation process. If desired, one may employ frequency response equalizers in association with amplifier '54 in addition to, or in .place of, lowering the valueof inductance 68. I

In Fig. 2 I have shown :asecond form of frequency modulation receiving system in which frequency modulated intermediate frequency currents are applied differentiallyto the grids l0 and 12 of a multi-grid vacuum .tube 14. In the arrangement shown I have indicateda pentode type of vacuum tube 14 having threegrids l0, l2 and 16 arranged in succession in the electron stream between the cathode 18 and anode 80.

Thefirst and-second grids l0. and 12 are main,- tained at zero, or at somewhat positive average,

, potentials, andthe thirdgrid is maintained at asomewhat more positive average potential. Directcurrent potentials for so biasing thecgrids are supplied by taps on resistance 15.

Due to the intermediate frequency inputpotentials applied to grids"!!! and 112 one gridor the other'will always be at a negative potential, and will'block 011" the anode current, exceptfforf 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 Hismade, conducting between anode and cathode and, dis- The inductance 68, between the direct current power supply and the or liquids, such as submarine signalling.

charges the condenser 88 connected between anode and 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 1 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 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 amplifier 54 and applied to a loudspeaker 56.

The arrangement of Fig. 3 is'a modification having features of the arrangement of Fig. 1. In Fig. 3 asingle pulsing tube l4 replaces the pulsing tubes I4 and I6 of Fig. 1. A transformer 05 replaces the coupling including condensers 60, 62, inductance 64 and resistances 6 and 8 of Fig. 1. The pulses in Fig. 3 are fed to a condenser dischargetube H8. The tube H8 controls the'rate of discharge of condenser l2l which is charged at a rate dependent principally on the value of resistance I23. The conductivity of tube H8 is controlled by the pulses from tube M; 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 theamplifier in network 3. A separate heterodyne oscillator I is schematically represented as feeding the heterodyne detector in network 3. The time constant of the network including condenser |2l and resistance I23 is made less than the time between pulses (output from M) at the highest pulse rate which corresponds to the highest modulation frequency. The audio output transformer I03 feeds the audio energy to network 54".

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 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 Decl 24, 1935. l

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

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 organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a system for demodulating frequency modulated wave energy, an electron discharge tube pulse generator, said tube having acathode,

. of said tube in push-pull, and means responsive substantially only to the rate of repetition of said pulses coupled to the plate of'said tube.

2. In means for demodulatingangular'veloc-' ity-modulated wave energy, an electron discharge device having a pair of input grids and an output circuit, said device being capable of generating dependent on the frequency of applied input energy, means for applyingsaid modulated wave energy to the input grids of said device in pushpull relation, and a pulse integrating systemcoupled to said generating device output circuit 7, 3. In means for demodulating frequency modulated wave energy, a pair of electron discharge tubes, each tube including'an anode, a cathode and a plurality of control electrodes, means for impressing negative potentials on said'control electrodes relative to said cathodes, means connecting said anodes in parallel to a sourceof positive potential, means for impressing modulated wave energy in phase opposition onthec'onpositively to bias the anode, means for impressing frequency modulated wave energy in phase opposition on the control electrodes of said tube whereby pulses are produced on the-anode of said tube, and a condenser, the charge on'which is controlled by the rate of repetition of said pulses,

coupled to the anode of said tube.

5. In means for demodulating frequency modulated wave energy, a. pair of electron discharge devices each including an anode, a cathode and a control electrode, means for impressing frequency 'modulated Wave energy on the control electrodes in phase opposition whereby impulses are produced on the anodes, a space discharge device provided with a cathode, anode and control grid, means coupling said control grid to said anodes of said pair of devices, and an integration condenser connected between the anode and cathode of said space-discharge device.

' CLARENCE W. HANSELL.

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