US2498689A - Arrangement for modulating continuous waves - Google Patents

Description

Feb. 28, 1950 M. M. LEVY E AL ARRANGEMENT FOR MODULATING CONTINUOUS WAVES Filed Jan. 5, 1946 ATTORNEY Patented Feb. 28, 1950 UNITED STATES, PATENT O-FF'II'CTEJ ARRANGEMENT ronluonuaarmo CONTINUOUS .WAvEs Maurice Moise Levy and Edward Arthur Rattue,

Isondon, England, assignors, by'mesne-assignments, tolntemati'onal Standard Electric Corporation, New York, N

Delaware.

. Yr, a corporation v of Application January 5, 19f6; SerialNo. 639,288 In; Great Britain January'15, 1945 1 I The present invention relates to-improvements in arrangements for modulating continuous: el'ecw t'rlc Waves; with particular referenceto keyed radibtelegraph systems.

It: is common practice in radiotelegraph sys tones to modulate a continuous high frequency" carrier wave with an audible frequency wave or tone, and to interrupt the modulated waves at telegraph speed by means ofa suitable key or switch arrangement. It is also generally known.

that the maximum audio frequency output from:-

a receiver for such a system is produced if the:- audio frequency waves modulating" the carrier" waves areof rectangular form, so thatthe carrier wave is in efiect sharply switched on and ofi. It. has been found that best results are obtained when the length of the on periods are approxi mately times thelength of the off periods;

Various arrangements are known for modulating the carrier wave with rectangular waves. Some of these arrangements involve the use' of a conventional two-valve multivibrator circuit which generates rectangular switching waves,

which are applied to'a valve'ina carrierf-requency eluding the said valve and adapted to generate-- carrier waves, the said circuits being'sod'isposed" that modulation of the carrier-waves by the pulses takes place in the said valve.

According to another aspect, the-invention-provides: an arrangement for a radio telegra-phy transmitter which includes a multivibrator arranged for'generati'ng modulating waves comprising two valves each having an anode; a control; grid-- and a cathode, one of the said valveshavihg an additional anode connected with the" control grid and cathode of the same-valveinsuch manneras t'o-generatemodulated carrier-waves;

The figureof the accompanying drawing shows:- aschematic circuit diagram of an arrangement according to the invention; Itcomprises two thermionicampl-ifying valves i and 2 ;of whiclhther 55 This arrangement saves a valve toreclaims... (01. 250-17) 2 first has two grids- 3 and cathode 5 and. theanode 6 This valvesmay;.for:z example; beatetro'd'eof which 3; and 4. are respectively the control gridiand; the-screen-gridz 12f."

desired: this valve may be a pentode with the supv presser grid (not-shown) connected: to: the oathode in the usual way Valve: 2 needs only one: grid; 1' between the cathode 8 and. tl'ieanodeil, butthis? valve could be a tetrodeor pentodewith the. ad:-- ditional grids (not shown) appropriately polar i'sed.

The electrodes 3; 4"", 5 of the valve 1: and; the electrodes I 8, 9- of the valve 2 are connected in; aconventional multivibrator circuit; While? the: electrodes 3,. 5 and 6 together form a triodavalve connected to an oscillation circuit which. gener ates the-high frequency carrier waves. During the operation ofthe multivibrator, each 'ofi the. valves is alternatelyblocked and unblocked, and.

so the vaIve-- I can only generate oscillations in those alternate periods of" time during which it is unblocked. Thus it will generate equally" spaced rectangular pulses of high frequency" waves. If the frequency of the multivibrator: is within'the audible range, the effect is equivalent to-modulating the high frequency waves" with: an". audible tone of rectangularwave form.

' Thescreen grid 4, which acts as an anodefor the multivibrator; is connected to the grid- 1. through a condenser I 0; and the anode- 9-is con--- nectedto the grid 3" through: a; condenser l*l',, and through an inductancel2 which acts as-uaz. high frequency choke,- but which can be disrea garded" when considering the action of the multi' vibrator: Thegrids 1 and 3 are respectively com-- n ected"- to ground through resistance elements- I3. and" f4.

The electrodes 4 and 9 are connected through== resistances li-an'd l6 and the=normally= open con- 40 tacts of a telegraph'key I1 (or equivalent rela-yor switching device) 1 to the positive terminal' of the high -tensibn source l8=for the valves. The nega--' tive terminal of this source is connected to earth; and a smoothing network -l'il comprising two re-- sistances and-a condenseris-associa-ted with thekey for suppressing-switching clicks, accordingto the-usual practice.

The' grid' a and the anode 6" of thevalve 1 are connected by' a parallel resonant circuit compris ingthe" inductance ZDand condenser 21 which is tuned*to thedesired carrier frequency. A* piezo' electric crystal 2 2 or other equivalent series resonant' circuit" or device-tuned to the same irequenc maybe includedin series-with thegricf Anode current is suppliedto the anode-6 :fro'm tor 4 interposedbetweenthe the anode potential, and a resistance 25 may also.

be included to prevent undesired parasitic oscillations. Condensers 26, 21 and 28 are by-pass condensers for the carrier frequency and will be smal1 compared with the multivibrator con- I densers l and H, so that the will have a negligible susceptance at the pulse repetition frequency. The cathode is connected to earth through a bias network consisting of a resistance 29 and by-pass condenser 30, and the cathode '8 is connected directly to earth. The modulated carrier wave output may be obtained from the terminal 3|, and may be supplied to suitable amjplifiers or other apparatus (not shown) in any convenient way. I

When the key 11 is operated to connect the positive terminal of. the high tension source I8 to the valves, the multivibrator operates in the usual way,.and the time constants of the coupling networks formed by the elements Ill, [3, l5 :and l I, I4, [6 should be so chosen that the valve ll is blocked for a desired fraction of the period, :and unblocked for the remainder. For example, -the period of the multivibrator could be 1.25 milliseconds, and the time constants could be chosen so that the valve I is blocked for .5 millisecond and unblocked for 0.75 millisecond, giving the preferred ratio of 1.5. The constants of the oscillation circuit associated with the anode 6 could be chosen so that the carrier frequency which is generated therein is 500 kilocycles per second, for example.

It will be evident that when the key I! is operated, an output of the carrier Waves modulated by an audible tone at 800 cycles per second, for example, will be obtained at terminal 31, and when the key is released, there will be nooutput. The tone frequency modulated output may therefore be interrupted according to a telegraph code by means of the key IT in the usual way..

It will be understood that the multivibrator circuit associated with both the valves and also the oscillation circuit associated with the valve I are of conventional types and do not need any .more detailed explanation, since their action is;

vibrator circuit. These elements should preftime constant of the corresponding coupling net--' work.

It should also be noted that the functions of the electrodes 4 and 6 of the valve 1, could be interchanged, if desired: in other words the electrode 4 could serve as the anode for the oscillation circuit, and the electrode 6 could serve as the anode for the multivibrator circuit.

What is claimed is:

1. An arrangement for generating modulated electric carrier waves comprising a first and second thermionic amplifying valve each having an output circuit coupled to the input circuit of the other valve through a corresponding reactanceresistance network in such manner as to generate regularly repeated pulses by which the said valves are alternately blocked and unblocked, and an oscillation circuit coupling the input circuit of the first of the said valves to a second output :erably be small enough for their effects on the multivibrator circuit to be negligible. The resistance 29 should be chosen so that the valve I is biased so that oscillations start easily when,

the valve becomes unblocked and the condenser 30 should be large so that the cathode 5 is efiectively earthed at the carrier frequency.

It will be understood that each of the crosscoupling network circuits for the multivibrator consists effectively of a series condenser ,(H! or H) and two shunt resistances (I3, l'5 or l4, l6)

since the resistances l5 and It, are effectivelyv 21. It is to be noted that the inductance l2 could,

be replaced by a resistance which, however, would have tobetakeninto account in considering the,

circuit of the same valve to generate pulses of carrier waves during the periods when the said first valve is unblocked.

2. An arrangement according to claim 1,-in which the said first valve comprises a cathod electrode, anode electrode, and at least a first and second grid electrode intermediate its anode and cathode electrode, the input circuit of said first valve comprises its cathode electrode and the first grid electrode closest thereto, one output circuit of said first valve comprises its cathode electrode and the second grid electrode and the. second output circuit of said first valve comprises its cathode electrode and its anode electrode, said second valve comprising a cathode, anode and grid electrode, the input circuit of said second valve comprises its cathode and grid electrode,

which the anode electrode of the first valve is coupled to said oscillation circuit.

4. An arrangement according to claim 3, wherein one of said reactance-resistance networks comprises a condenser coupling the first grid electrode to the anode electrode of the second valve and:

a resistance coupling the first grid electrode to ground, the other reactance resistance circuit comprises a condenser coupling the grid electrode. of said second valve to the second grid electrode,

of the first valve, and a resistance coupling said grid electrode of said second valve to ground, the

corresponding cathodes being also coupled-to ground. I.

5. An arrangement according to claim 4 in which the oscillation circuit comprises a resonant circuit connecting the anode electrode of the first valve to the first grid electrode and tuned to a,

frequency high compared with the repetition frequency of the pulses.

6. An arrangement according to claim 5 in which the resonant circuit comprises a parallel resonant circuit.

7. An arrangement according to claim 6 com-- prising a piezo-electric crystal connected in series with the parallel resonant circuit and saidfirstgrid electrode, and tuned to the same frequency.

8. An arrangement according to claim 7 comprising means for excluding the carrier waves;

from the circuits in which the pulses are generated.

connected between the first grid of the first valve and said one reactance-resistance network, a condenser shunting the input of the said one network and a condenser shunting the output of said other network, the reactance of the inductance and the susceptance of the condensers being negligible at the frequency of repetition of the pulses.

10. An arrangement according to claim 9 in which the time constants of the reactanceresistance networks are so chosen that the repetition frequency of the pulses is within the audible range.

11. An arrangement according to claim 10 in which the said time constants are also so chosen that the length of each unblocking period of the first valve is substantially equal to 1.5 times the length of each blocking period.

12. An arrangement according to claim 11, comprising means for interrupting the modulated carrier waves for signalling purposes.

13. An arrangement according to claim 12 in which the interrupting means comprises a twoposition switch adapted to disconnect the source whichlprovides the electron current for the said valve or valves in one position, and to reconnect the said source in the other position- I 3 MAURICE MOISE LEVY.

EDWARD ARTHUR RAT'I'UE.

REFERENCES CITED The following references are of record in the fileaof this patent:

UNITED STATES PATENTS Number Date

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