US2312977A - Frequency modulation - Google Patents

Description

March 2, 1943'.

w. 8. PERCIVAL FREQUENCY MODULATION Original FiledFeb. 24, 1939 INVENTOR WILLIAM s95 05 PERC/VAL BY fig ATTORNEY 1,1 lLLLl Hg-Ju Patented Mar. 2, 1943 UNITED STATES PATENT OFFICE FREQUEN CY MODULATION William Spencer Percival, Ealing, London, England, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain L aJ I February 24, 1938 4 Claims.

This invention relates to thermionic valve circuits and has particular, but not exclusive, reference to circuits for transmitting frequency modulated signals of high frequency deviation, the invention being applicable also for other purposes for which it is desired to obtain oscillations of varying frequencies as, for example, for automatic frequency correction in radio receivers and transmitters. This application is a division of my United States application #258,141 filed February 24, 1939, now Patent No. 2,231,854, dated February 11, 1941.

In the field of radio communication by means of carrier waves there are three fundamental methods of modulation of a carrier wave, namely, amplitude, phase and frequency modulation At the present time the most commonly employed method of modulation is amplitude modulation, wherein the amplitude of the carrier wave is varied in sympathy with the modulation frequencies. Now it is known that by using frequency modulation instead of amplitude modulation a more efiicient transmission can be effected, thus resulting in economy, especially in the operation of large transmitting stations. In addition, it is also known that frequency modulation, whereby the frequency of the carrier wave is altered in response to modulation frequencies, provides an improvement over amplitude modulation in the signal-to-noise ratio of reception, and this is of great importance in the transmission of television and other signals which occupy wide frequency bands.

Known methods of frequency modulation have been found difficult to apply to the wide pass band required for television, and there are two main reasons for this. In the first place, it is not easy to deviate the carrier frequency over a sufiiciently wide band, and secondly, it is not a simple matter to maintain a wide pass band for the side bands, it can be shown that theoretically frequency modulation produces an infinite number of side bands. For example, let it be required to vary the frequency of a carrier between 42 megacycles per second and 48 megacycles per second at a low periodicity to represent a low frequency component of the modulation signal. At the same time let there be superimposed a high modulation frequency compo nent of say 3 megacycles per second, but of a very small amplitude. The minimum side bands required for the higher frequency component will be 42-3 megacycles/second and 48+3 megacycles/second depending on the instantaneous amplitude of the low frequency component. From this it will be seen that for television a very wide pass band is required in the radiofrequency circuits even if the higher sidebands are neglected.

It is the object of the present invention to provide improved means for frequency modulation of a carrier frequency which is especially suitable where the modulation frequencies comprise television or other wide band signals.

According to a feature of the invention a thermionic valve circuit is provided for generating varying frequency oscillations comprising a thermionic valve oscillator including a frequency determining circuit, a plurality of valves in cascade the output circuit of the cascade valves being effectively connected to the input circuit of the cascade valves for the generation of oscillations, and means for applying a controlling potential for determining the frequency of the generated oscillations.

In order that the said invention may be clearly understood and readily carried into effect the same will now be more fully described with reference to the accompanying drawing in which the single figure illustrates a thermionic valve circuit arranged in accordance with the invention.

In the drawing, the screen grid valve III has its grid I2 connected in a grid circuit with an inductance IIlI which, in conjunction with the stray capacities of the circuit, forms a frequency determining circuit. The anode I4 of valve I9 is coupled by condenser 48 and resistance 29 to the grid 62 of a valve 60. The anodes I4 and 64 of the two valves Ill and are arranged in parallel with anode impedances 65 and i5 and a suitable de-coupling resistance 14 in conjunction with a tie-coupling condenser I2. A broadly tuned circuit I02, which is suitable (heavily) damped by a resistance I05, is connected effectively in the anode circuit of valve I0 and is designed so that its resonant frequency is preferably situated in the middle of the frequency band over which modulation is to be carried out. The output of the oscillator is taken from terminals 4| and 42 which are connected across a part of the inductance I02. Valves I0 and 60 are provided with suitable cathode biasing circuits, namely, resistances I8 and bypass 61 and conde ' sers 41 and 68 respectively. The anode 64 of va ve 60 is coupled by means of condenser 66 to the grid I2 of valve ID. The screening grids I3 and 63 of valves I0 and 60 are supplied with suitable positive potentials by means of decoupling resistances 38 and 59, in conjunction with condcnsers 40 and I0, which provide low impedance paths for voltages of carrier frequencies between the screening grids and earth. The circuit will normally oscillate at a frequency determined by the tuned circuit IOI.

For the purpose of controlling the frequency of oscillations, a portion of the circuit IOI is damped by means of the cathode impedance of valve 25 to the control grid of which the modulation or control potentials are applied via resistance 9 and the grid leak 21. A small value condenser 8 is connected between the grid of valve 25 and earth in order to efiectively earth the grid of valve 25 at carrier frequencies but not at modulation frequencies. Resistances l8 and 28 provide grid bias for valve 25 and resistance I5 in conjunction with condenser I6 form an anode de-coupling circuit. The screening grid of valve 25 is connected to the anode of that valve and the suppressor grid is connected to the cathode. The cathode of valve 25 is connected by means of condenser 36 to a tapping on the inductance IOI. For the purpose of providing a low cathode impedance to modulation fre-- quencies with a high impedance to carrier frequencies a circuit comprising resistance ll, condenser 26 and high frequency choke 46 is provided in shunt with resistance 28. On applying modulation potentials to the grid of valve 75 the slope of the valve is changed and consequently the damping which valve 25 imposes on the circuit l0| is varied and this varies the eflective re.- actance between the grid of valve 10 and the earth line. Thus, the frequency at which the oscillator comprising valves I0 and 60 oscillates is varied.

By the use of suitable components it is found possible to obtain a frequency variation of from 10 to megacycles per second. Instead of taking the output from circuit I02 it is also possible to derive the output from across the circuit Hll.

It will be understood that although the main object of the invention is to produce large variations of frequency for frequency modulation, the circuits described can be used for other purposes, such as automatic frequency correction, remote control of the oscillator frequency of a superheterodyne radio receiver, etc. One special application is to provide sawtooth frequency variations for the purpose of showing the response curves of filters and networks on the screen of a cathode ray oscillograph.

The circuits also have applications to so-called resistance tuning. If the feed back is reduced to such an extent that the circuits are rendered just short of the oscillatory condition, then by applying signals to a point in the circuit it is possible to tune selectively the arrangement to a signal frequency by applying controlling potentials in the manner already described. For example, with my arrangement, signals from an antenna may be applied to the circuit Illl and the output from circuit )2 may be fed to the mixer stage of a superheterodyne receiver. By

varying the potential on the grid of valve the input circuit of the receiver is'efiectively tuned to a required signal frequency. One way of varying the potential of the grid is to connect the grid of valve 25 to earth and to vary the value of the resistance 28.

What is claimed is:

1. In a frequency modulation system, at least three electron discharge valves each having an anode, a control electrode and a cathode, a tuned circuit interconnecting the control electrode and cathode of one of said valves, 8. tuned circuit coupling the anode to the cathode of said one of said valves, a coupling between the control grid of another of said valves and said last named tuned circuit, a coupling between the anode of said other of said valves and said first named tuned circuit, a source of modulating potentials connected with the control grid and cathode of a third of said valves, a circuit including reactance connected between the cathode of said third valve and ground and a coupling between said last named circuit and said first named tuned circuit.

2. In an electrical system, a plurality of electron discharge valves each having an anode, a control electrode and a cathode, a tuned circuit interconnetcing the control electrode and cath ode of one of said valves, a broadly tuned output circuit coupling the anode to the cathode of said one of said valves, a coupling between the control grid of another of said valves and said last named broadly tuned circuit, a regenerative feedback coupling between the anode of said other of said valves and said first named tuned circuit, said circuits, valves and feedback being arranged to generate wave energy in said valves and circuits, a source of control potentials, a damping impedance coupled in shunt to a portion of the reactance of said first tuned circuit, a third electron discharge valve means having electrodes coupled to said damping impedance, and means coupling said source of control potentials to said third valve means for controlling the impedance between the electrodes thereof to thereby control the effect of said damping impedance on the reactance of said first named tuned circuit and the frequency of the generated wave energy.

3. In a frequency modulation system, a plural- .ity of electron discharge valves each having an anode, a control electrode and a cathode, a tuned circuit interconnecting the control electrode and cathode of one of said valves, a broadly tuned circuit coupling the anode to the cathode of said one of said valves, a damping resistance in said last named circuit, a coupling between the control grid of another of said valves and said last named broadly tuned circuit, a regenerative feedback coupling between the anode of said other of said valves and said first named tuned circuit. said circuits, valves and feedback being arranged to generate wave energy in said valves and circuits, a source of modulating potentials connected with the control grid and cathode of a third of said valves, and a circuit of low impedance to modulating potentials and high impedance to wave energy of the generated frequency coupling said third valve to said first named tuned circuit to control the reactance thereof in accordance with the modulating potentials supplied to the control grid and cathode of said third valve to thereby control the frequency of the generated wave energy.

4. In a frequency modulation system, a plurrality of electron discharge valves each having an anode, a control electrode and a cathode, a tuned circuit interconnecting the control electrode and cathode of one of said valves, a tuned circuit coupling the anode to the cathode of said one of said valves, a coupling between the control grid of another of said valves and said last named tuned circuit, a feedback coupling between the anode of said other of said valves and said first named tuned circuit, a source of modulating potentials connetced with the control grid and cathode of a third of said valves, a circuit including a damping impedance connected between the anode and cathode of said third valve, and connections coupling said damping impedance in shunt to a portion of said first named tuned circuit. WILLIAM SPENCER PERCIVAL.

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