US2231854A - Frequency modulation - Google Patents

Description

Feb. 11, 1941. w PERCIVAL 2,231,854

FREQUENCY MODULATION Filed. Feb. 24, 1939 INVENTOR WILL/AM SPENCER PERCIVAL svvggm ATTO R N EY Patented Feb. 11, 1941 f 2,231, 54

FREQUENCY MODULATION William Spencer Percival, Ealing, London, England, assignor to Electric .& Musical Industries Limited, Hayes, jMid-dlesex, England, a company of .Great Britain Application February 24, 1989, Seria'lNo. 258,141 'In Great Britain February '24, .1938

7 Claims. .(ol. ire-171.5)

This invention relates to thermionic valve cirvide improved means for frequency modulation cuits and has particular, but not exclusive, of a carrier frequency which is especially suitreference to circuits for transmitting wide band able where the modulation frequencies comprise frequency modulated signals, the invention being television or other wide hand signals. applicable also for other purposes for which it According'to a feature of theinvention a thers is desired to obtain oscillations of varying fremionic valve circuit is provided for generating quenciesas, for example, for automatic frequency varying frequency oscillations comprising a thercorrection in radio receivers'andtransmitters. mionic valve oscillator including a frequency In the field of radio communication by means determining circuit, a plurality of Valves in oas- 9; carrier waves there are threefundamental cade the :output circuit of the cascade valves 1-0 methods of modulation ofa carrier wave, namely, being effectively connected to the input circuit amplitude, phase and frequency modulation. At f the cascade valves for the generation of oscilthe present time the most commonly employed lations; and means for applying a controlling method of modulation is amplitude modulation, potential for determining the frequency of the wherein the amplitude of the carrier wave is generated oscillations. 1.5 varied in sympathy'with the modulation frequen- According to a further feature of the invencies. Now it is known that by using frequency tion a thermionic valve circuit is provided for modulation instead of amplitude modulation a generating varying frequency oscillations commore efiicient transmission can be efiected, thus prising a thermionic valve oscillatorincluding at 25 resulting in economy, especially in the operation least twcrvalves each associated with va'frequency of .large transmitting stations. In addition, it is determining circuit tuned to a different frealso known that frequency modulation, whereby quency, the outputs of said Valves being effecthe .frequency of the carrier wave is altered in tively connected to the input of a thermionic response to modulation frequencies, provides an valve phase-inverting'circuit having separate outimprovement over amplitude modulation in the puts "which are arranged 'to feed back energy signal-.to-noise ratio of reception, and this is of independently to'the input circuits of said valves great importance in the transmission of 'teleinsuch a manner as to tend -to cause said valves vision and other signals which occupy wide freto generate oscillations atthe different frequenquency bands. cies,:said valves being so connect-ed that they are Known methods of frequency modulation have forced toproduce oscillations of .a resultant fre- 30 been found difficult to apply to the wide pass quency, and means for applying controlling band required for television, and there are two, potentials to said valves so as *to determinethe main reasons for this. In the first place, it is resultant frequency of the generated oscillations. not easy to vary the carrier frequency over a In 'order'that the said invention maybe clearly sufi-lcien-tly d b nd, d condl it i not a understood and readily "carried 'into efiect the 35 simple matter to maintain a wide pass band for same'wi-ll now be'more fullydescribed with referthe side bands, it can be shown that theoretically ence to the accompanying drawing in which the frequency modulation produces an infinite numsingle figure illustrates a thermionic valve cirber of side bands. For example, let it berequired cuit arranged in accordance with the :invention.

40 to vary the frequency of a carrier between 42 In the sin e fi u Ofthe drawing e Screen 40 megacyc-les per second and 48 megacycles per grid-valve III is provided in'its grid circuit with second at a low periodicity to represent a low an inductance 1'01 which, in conjunction With frequency component of the modulation signal. the -stray capacities of the circuit, forms :a fre- -At the same time let there be superimposed a d my determining circuit. Similarly, the-valve high modulation frequency component of say, 3 20 :has an inductance 103 connected to its grid 45 megacycles per second, but of a very small ampli- 22,the resonant frequencies of the two frequency tude. The minimum side bands required for the determining'circuitsfbeing arranged at or beyond higher frequency-component will be 42 3 megathe opposite ends 0f the band of frequencies over cycles/second :and 484-3 megacycles/second dewhich it'isdesiredto vary the frequency of oscil- 50 ending on the instantaneous amplitude of the lation. The anodes M and 24 of the two valves 5 low frequency component. From "this it will be I 0 and 20 are arranged in parallel with a common seen that for television a very wide pass band anode impedance l5 and a suitable de-coupling is required in the radio-frequency circuits even resistance 14 in-conjunction with a de-coupling if the higher sidcbands are :neglected. condenser 16. ,The anode circuit is coupled by :Itjis the objector the present invention-to promeansof ;a condenser 48 to the two grids 52 and which are connected across a part of the inductance I02. Valves 50 and 60 are provided with suitable cathode biasing circuits, namely, resistances 51 and 61 and condensers 58 and 68 respectively. valves 50 and 60 are de-coupled by means of the resistances 59 and 69 in conjunction with a condenser 10'. provided with load resistances, the loadyresistance 55 in the anode of valve 50 being de-coupled from the source of positive potential by means of resistance II and condenser 12. The .same de-coupling circuit is also used to de-couple the anode impedance 65 of valve 60. The anode'54 of valve 50 is connected by means of condenser 56 to the grid circuit of valve 20 whilst the anode 64 of valve 60 is similarly coupled by means of condenser 66 to the grid I2 of valve I0. The screening grids of valves l0 and 20 are supplied with suitable positive potentials by means of resistances 3B and 39, while condensers 40 are for the purpose of providing a low impedance at carrier frequencies between the screening grids and earth. The modulation potentials are applied between the terminals 35 and 45 in phase opposition. The control grids of valves I0 and 20 are biased by using a common cathode im pedance I8 Whilst condensers 31 and 41 are for the purpose of by-passing the impedance I8 at carrier frequencies.

The operation of the circuit can be explained asfollows:

Assume that the valve 20 has a large negativ potential applied to its screning grid 23 so that the anode current of the valve is reduced sub stantially to zero and that the screening grid l3 has a large positive potential applied to'it. Should the grid of valve I0 become slightly positive, then the potential applied to the grid of valve 60 is changed in phase by by means of valve I0 and is consequently negative. The valve 60 then changes the phase of its input signal by 180, or, in other words, the anode 64 becomes positive. This positive signal is fed back to the grid of valve I 0 and since the change of phase of the fed back signals is exactly 360 the circuit I 0| will oscillate at its resonant frequency, that is, at a frequency which is at one end of the band of frequencies over which it is desired to modulate the oscillator. Similarly, if the screening grid 23 of the valve 20 has a large positive potential applied to it and the anode current of valve I0 is reduced to zero, then the circuit I03 will oscillate owing to the feedback produced by valves 20 and 50 at a frequency at the other end of the frequency band. If both valves are passing current and the ratio of the extreme frequencies of the two circuits I0! and I03 is not too great, then oscillation will occur at an intermediate frequency. Thus, the effect of modulation applied to the terminals 35 and 40 is to make the frequency vary in sympathy between the extreme limits determined by the frequency determining circuits IOI and I03.

It is desirable that the valves I0 and 20 be matched. This is not so necessary when a com- The screening grids 53 and 63 of Each of the valves 50 and 60 are, 5

mon cathode impedance I8 is employed which is large at modulation frequencies but is small at the frequencies of the carrier and sidebands. Such an impedance causes the sum of the currents through the valves to vary only slightly at modulation frequencies and the effect of the mis-matching between the valves is reduced to a second order.

The purpose of resistances I9 and 29 is to prevent the circuit oscillating at very high frequencies although any other suitable means may be employed for this purpose.

I claim:

1. A thermionic valve circuit for generating varying frequency oscillations comprising a thermionic valve oscillator at least two valves each associated with a frequency determining circuit tuned to a different frequency, the outputs of said valves being effectively connected to the input of a thermionic valve phase-inverting circuit having separate outputs which are arranged to feed back energy independently to the input circuits of said first two valves in such a manner as to tend to cause said valves to generate oscillations at the different frequencies, said valves. being so connected that they are forced to produce oscillations of a resultant frequency, and means for applying controlling potentials to said valves so as to determine the resultant frequency of the generated oscillations.

2. A thermionic valve circuit according to claim 1, wherein each frequency determining circuit is associated with a pair of cascade-coupled therminonic valves, the output circuit of each pair of cascade valves being effectively connected to their respective input circuits.

3. A thermionic valve circuit according to claim 1, wherein the controlling potentials are applied to a plurality of valves, said valves having a common cathode impedance which is high at the frequency of the controlling potential or potentials but which is low at the frequency of the generated oscillations.

4. In a frequency modulation system a pair of electron discharge valves each having a cathode, a control electrode and an anode, a circuit broadly tuned to the frequency of the oscillations to be generated coupled to the anodes of said valves, a tuned circuit connected between the control electrode and cathode of each of said valves, an additional valve coupling said first named tuned circuit to the control grid of one of said first named valves, a further valve coupling said first named tuned circuit to the control grid of the other of said first valves, said additional and further valves serving to reverse the phase of voltages supplied thereto and apply the same to said control grids, and means for modulating the impedances of said first two valves in phase opposition at signal frequency.

5. A system as recited in claim 4. wherein the cathodes of said first pair of valves are tied directly together and. connected to a source of direct current by means for maintaining the directcurrent supply to said cathodes substantially constant.

' '6. In a frequency modulation system a pair of electron discharge valves each having a cathode, a control grid and an anode, a circuit broadly tuned to the frequency of the oscillations to be generated coupled to the anodes of said valves, circuits tuned to frequencies on opposite sides of the mean frequency to which said first circuit is tuned connected between the control grids and cathodes of said valves, a third valve coupling said first named tuned circuit to the control grid of one of said first named valves, a fourth valve coupling said first named tuned circuit to the control grid of the other of said first valves, said third and fourth valves serving to apply voltage from said first circuit to said control grids, and means for modulating the impedances of said first two valves in phase opposition at signal frequency.

7. A system as recited in claim 6, wherein the cathodes of said first pair of valves are tied directly together and connected to a source of direct current by means for maintaining the directcurrent supply to said cathodes Substantially 5 constant.

WILLIAM SPENCER PERCIVAL.

CERTIFICATE OF CORRECTION. Patent No. 2,251,85h. February 11, 19141.

WILLIAM SPENCER PERCIVAL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, second column, line 16, claim 1, after "oscillator" insert including-; and that the said Letters re ent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 5th day of May, A. D. 1914.2.

1 Henry Van Arsdale (Seal) Acting Commissioner of Patents.

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