US2375746A - Frequency modulation receiver - Google Patents

Description

Patented May 1, 1945 UNITED STATES PATENT 2,874,746 r nnounncv MODULATION ancnrvan.

Clarence W. Hansel], Port Jefferson, N. Y.,- assignor to Radio Corporation of America, a corporation of Delaware Original application June 17, 1941, SerialNo.

398,391. Divided and this application Septem- 7 her 21, 1942, Serial No. 459,111

Claims.

This application is a division of my application Serial No. 398,391, flied June 17, 1941, U. 8. Patent No. 2,323,596, granted July 6, 1943. This invention relates to new and improved method of,

l and means for, reception of frequency modulated carrier current. In its broader aspects it involves new and improved methods'oL-and means for, carrying out the fundamental principles o: 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 successiveenergies, 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 equalenergy, 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 to be 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 drawingin which I hav in dicated diagrammatically several circuit organizgtiotns whereby my invention may be carried into e co In describing my invention reference will 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 Figrl received radio signals (FM energy) aresupplied from antenna A to circuits in unit 2 comprising radio frequency amplifiers, a source of local 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 retimes, except while the input intermediate frequency potential is passing through zero. 7 At these times for very brief periods, which occur twice per cycle of input current, the anode currents of both tubes cease. The inductance 88, between the direct current power supply and the parallel-connected anodes, tends to keepan 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 40. Rectified current derived from the pulses is filtered 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 amplified in amplifier 54 and applied to the loudspeaker 56.

In this circuit 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 ef- 54 in addition to, or in place of, lowering the value of inductance 68.

In Fig. 2 I have shown a second form of frequency modulation receiving system in which frequency modulated intermediate frequency currents are applied differentially to the grids 10 and 12 of a multi-grid vacuum tube M. In the arrangementshown I have indicated a pentode type of vacuum tube It having three grids 10, I2 and 16 arranged in succession in the electron stream between the cathode l8 and anode 89. The first and second grids l0 and 12 ar maintained at'zero, or at somewhat positive average,

potentials, and the third grid is maintained at c a somewhat more positive average potential. Di-

sistancesfl and B to the respective grids I0 and 12 of a'pair'o'fvacuum tubes l4 and I6.

The I. currents from unit 2 are supplied by coupling-- condensers 60 and 62 to the respective rect current potentials for so biasing the grids are supplied by taps on resistance 15.

Due to the intermediate frequency input potentials applied to grids ill and 12 one grid or the other'fwill 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 thesebrief time intervals, repeated twice per cycle of input potential, the tube M is made conducting between anode and cathode and discharges the condenser 36 connected between anode and cathode. Y

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 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 trols the rate of discharge of condenser I'M 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 II. and the rate at which these pulses, which are of substantially constant amplitude and time duration, are fed through condenser 66 is afunction of the frequency of the intermediate fre quency output of the amplifier 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 l2l and resistance I23 is made less than the time between pulses (output from H) 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 or liquids, such as submarine signalling. They may be used advantageously in receiving subcarrier frequency modulated signals in multiplex systems and in facsimile communications sys terns such, for. example, as disclosed in: C. W. Hansell Patent No. 1,819,508, dated August 18, 1931; C. W. Hansell Patent No. 2, 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. 34l,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 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 efiect, 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 a cathode, a pair of input grids, a plate, the output pulse rate of said generator being dependent on the frequency of applied input power and substantially independent of variations in the amplitude of applied input power, means for applying frequency modulated wave power to the input grids 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 demodulating angular velocity-modulated wave energy, an electron discharge device having a pair of input grids and an output circuit, said device being capable of generating pulses wherein the rate of pulse generation is dependent on the frequency of applied input energy, means for applying said modulated wave energy to the input grids of said device in pushpull relation, and a pulse integrating system coupled to said generating device output circuit.

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 source of positive potential, means for impressing modulated wave energy in phase opposition on the control electrodes of said tubes whereby impulses are produced on the anodes, and a diode rectifier coupled to said parallel-connected anode electrodes of said tubes.

4. In means for demodulating frequency modulated wave energy, an electron discharge tube having an anode, a cathode and at least two control electrodes, means negatively to bias the control electrodes with respect to the cathode, means positively 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 wav 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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