US2372701A - Modulation system - Google Patents

Description

April 3, 1945. J. J. ANTALEK Y 2,372,701

MODULATION SYSTEM Filed May 8, 1942 3 Sheets-Sheet l HNVENTOR .fo/awJAWZeZu BY imga' ATTORNEY 'April 3, 1945. J ANTALEK 2,372,701 MODULATION SYSTEM Filed May 8, 1942 3 Sheets-Sheet? flcs. offre.sa/7ance April 1945. J; J. ANTALEK 2,372,701

MODULAT ION SYS TEM Filed May 8, 1942 s Sheets-Sheet s INVENTOR Joln JAnZ'QZe/ Y fl Md ATTORNEY lationof' a transmitter of provide a method of .tively narrow band at relatively low -duced carrier current by lation may be secured LPatented Apr. 3, i945 J I I nronuun'ron SYSTEM John J.Antalck, Chicago, Ill., assignor to The Rauland Corporation,

on ot- Illinois Chicago, Ill., a corpora- Application May a, 1942, Serial No. 442,185

3 Claims.

This invention relates to modulation systems. especially as employed in connection with radio telegraph and telephone transmitters. More particularly it relates to a system oi 'amplitude modulation;

One purpose of my invention is to provide a modulation system of comparatively great simplicity and one capable of modulating a carrier frequency by the use of an extremely small amount of power, the amount of power required being comparable to that required for' the moduthe frequency modulation type.

Another purposeof the instant invention is to modulation adapted for use in the low frequency spectrum, due to the relaof side frequencies produced thereby.

A further a modulation system giving high fidelity transmission with a minimum amount of apparatus cost and adapted to produce a transmitted signal which can be received upon apparatus of the type used for the reception of ordinary amplitude modulated signals.

My invention modulates an independently provarying the impedance of a circuit or circuits connected either to the grid or the anode of a tube which acts as the .modulated amplifier tube. Alternatively, moduaccording to my invention by varying the impedance or resonant-frequency of a secondary circuit, arranged so as to energy from either the grid orthe anode circuit of the modulated amplifier tube, to which it is coupled, in accordance with such changes of impedance, thus bringing about absorption modulation.

In order to explain my invention, reference is made to the accompanying drawings, where:

Fig. 1 shows one embodiment of my invention using a microphone of the variable resistance type, connected so as to secure modulation of the variable impedance type;

Fig. 2 shows another embodiment, wherein a variable inductance type of microphone is so connected as to bring about absorption modulation:

Figl 2a shows an alternative and simple method of using a variable impedance microphone to secure impedance modulation;

Fig' 3 shows another method of securing absorption modulation by an arrangement permitting the use of any conventional source of audiofrequency modulating energy;

- Fig. 4 is a graphical representationof the res-,

object of my invention to'providev withdrawonance curve of typical tuned circuit with which my modulation system may be employed;

Fig.5 shows still another method ofabsorption modulation according tomy invention, using a series resonant circuit; and

Fig. 6 shows yet another impedance modulation method according to my invention, utilizing a series resonant circuit and'an alternative method of coupling the tube producing the carrier frequency to the modulated amplifier tube.

Referring now to Fig.1, electron tubes l and 2 are tetrodes and tube 3 is shown as a hexode of conventional type. Tube I has a piezo-electric crystal 4 connected to the grid thereof and having a suitable grid leak resistor i shunting the crystal. The tuned anode or output circuit 1 of this tube is by-pas'sed to ground by condenser I and tuned either to the fundamental frequency of crystal 4 or to someharmonic thereof, according to the design of circuit employed. The screen electrode 9 is connected through dropping resistonce I. to a source of suitable positive potential and by-passed to ground through condenser ll, while the tube cathode is directly grounded.

Electron tube 2 acts as the modulated amplifier tube, having its cathode is grounded and its screen electrode l4 supplied with a suitable positive potential through dropping resistance l5, and bypassedto ground through condenser ll. The output circuit connected to the anode of this'tube may take the form of the tuned circuit II. The

grid I8 is connected through input coil i9 and by-pass condenser 20 to the ground, while a source of suitable negativebias potential is connected to the junction point of this coil and condenser. The input circuit of tube 2 is electromagnetically coupled to the output circuit of tube I, by means of coil is, critical.

Electron tube '3 and its associated elements constitute an electron controlled variable capacity which is-eiiectively connected in parallel with coupling coil 19 of the input circuit of tube 2, these two elements thus constituting a circuit having a variable resonant period. In order to secure good stability and maximum efilciency, it is preferred that coil l9 have its inductance determined by so-called permeability tuning, but this is not essential. The input circuit of tube 2 is so tuned as to have, in its quiescent or nonmodulated condition, a frequency which will, be removed from the frequency of tuned circuit 1 by an amount corresponding approximately to that indicated at C in Fig. 4. .It will be noted that point C is the middle point of the linear'portion this coupling being quite of the selectivity or resonance curve, embraced between points A and 1B. This point C is usually about one percent away from the frequency of and are icy-passed to ground through condenser.

25. The control grid 26 of tube 3 is supphed with audio-frequency modulating energy through transformer 27, the primary of which is excited by microphone circuit 28.

The elements acting to give the variable capacity shoot of the circuits associated with tube 55 comprise condenser is connected between the non-grounded end of coil 89 and anode 3b, the latter of which is fed from a suitable sourcaci positive potential through choke coil 36, and variable condenser 32' and resistance 33 connected in parallel with one another and between grid 34 of the tube and the ground. Phase coupling of grid 3 to the connection between coil aerator through conductor 39c, already referred to, to the input circuit of tube to, and is tuned to a ire-- ill uuency corresponding to the point C of Fig. c, for the reasons already explained in connection with Fig. 1. When (its is varied in inductance by the sound input to did, the energy absorbed iii it and condenser 29 is made via series connected condenser 35 and resistance When modulation signals are impressed upon grid 26 of tube 3, the resonant frequency or the circuit comprised by coil it and the efiective capacity of tube 3 and its associated elements, is varied approximately one-half percent plus or minus the resonant frequency which this same circuit has when non-modulated. In other words,

the resonant period shifts. from point C as far as points A or B; thus traversing the linear portion of the resonance curve shown in Fig. 4.

This results in a linear variation of the radio-ireducncy energy applied to input grid is of tube 3, which tube is preferably so constructed and biased as to function as a class B amplifier,

whereby the power output in anode circuit ii will be proportional to the square of the grid excitation voltage.

While I have described and shown in Fig. i a selectivity curve upon which the point C is removed about one percent from resonance irequency, it is to be understood that the shape of curve and the location of point C will vary with different circuits, the elements of which have different constants or magnitudes.

In the system of modulation just described, the modulation is brought about by varying the effective impedance of the input circuit of the modulated amplifier tube, so that the amount of energy transferred thereto'from' the oscillator tube will vary in accordance with the modulating signals.

In the system disclosed in Fig. 2, on the other hand, modulation is brought about by va the effective, impedance of an absorption circuit coupled to the tuned input circuit of the modulated amplifier tube. In Fi 2 the circuits associated with oscillator tube la are identical with those already described in connection with Fig. 1

-hi kewlse, modulator tube 2a and its output circult are identical with the same elements in Fig.

- l,'so that-corresponding reference numerals are employed in connection with these two tubes.

bill

iroin input circuit Zita, 38c will be varied corrc spondingly, thus causing absorption modulation of the input energy to tube 20..

Referring now to Fig. 2d, there is here shown one comparatively simple form which my invention may assume. In this term the modulating device is physically, as well as electrically, incorporated into the circuits coupling the oscillator to the modulated amplifier. The variable inductence microphone of his. 2 ma be used to secure impedance modulation ead of absorption modulation, by connec the microphone directly in place of circuit constituted in Fig. 2 by coil 3%; and condenser 83a. The system will then function similarly to that of Fig. 1.

With the system of Figs. 2 and 2s, a condenser type of microphone may be employed in lieu of the variable impedance type, sound impingement upon the microphone causing the same shift of resonant period as in the case of the use of a variable impedance type microphone.

In the system shown in Fig. 3, the circuits of tubes lb and 2b are similar to those shown in Fig. 2, except that an alternative method, a separate output circuit is shown coupled to the tuned anode circultilb of tube 21). Moduiation is secured by inductively coupling coil 03b to coil 3% and tuning the former coil by connecting it effectively in shunt with the variable capacity presented by tube 3b and its associated elements, these latter being connected in the same manner indicated in Fig. l. The resonant circuit formed by coil 3% and tube 8b is tuned to a point corresponding to C of Fig. 4, as previously described, and the variation of the efiective capacity of tube 312 by the modulating energy supplied to input grid 2%, thereof, causes'a varying absorption of energy by coil 46b from the input circuit of tube 2b, thus modulating the radio-frequency energy supplied to grid 68b thereof.

In Fig 5 oscillator tube to and modulator tube to are connected in a fashion similar to that @own in Fig. 3. likewise tube to functions in the same as in Figs. 1 and 3. The transier of energy between tube to and the input circult of tube 20 is by a connection to the midpoint ly coupled to input grid 98d of modulator tube by conductor 39a, enclosed in grounded conductor Ala so as to form a flexible low impedance trans- 2d, through coil 48d. Anode supply is through choke coil 53d. Coil 48d and condenser @911 form a series resonant circuit conoiactlvely coupled to tuned output circuitisd of tube id. Tube 36 and its associated elements are connected and act in the same fashion as indicated in Figs. 1, 3

I 9,872,701 and 5. The effective capacity of tube 3d is modpedance of circuit 48d, 49d, which is tuned to a point corresponding to C of Fig. 4, as previously described. This causes the radio frequency energy supplied to grid ml to be modulated in accordance with the modulating energy supplied to transformer 21d.

It is preferred that my modulation systems be connected to the radio-frequency tuned circuits carrying low power in a transmitter, for the reason that it is then much easier to modulate the lower energy level there present. However, my systems are applicable for tuned circuits carrying any amount of energy. For example the modulator tube 3 may be coupled to the anode circuit of tube 2, instead of being coupled to the input circuit.

The selectivity of the, circuits involved, the sir. cult constants, and the proper adjustment of all the resonant circuits are some of the factors which determine the'percentage of modulation to The positive peaks mayreach a value which is three or four times the value of the carrier with out modulation. Thus a controlled carrier system is produced.

What I claim is:

1. A high level modulation system including a source of carrier frequency energy, a modulated amplifier excited by said energy, primary tuned circuit means coupling said source and said modulated amplifier and tuned to said carrier frequency, and a secondary resonant circuit discrete from but coupled directly to said primary coupling means and tuned to a frequency slightly be attained by the employment of my invention.

While the resonant circuit including I! (Fig. 1)

. and the corresponding resonant circuits of the other figures are preferably tuned to a frequency which is of the order of one or one-half percent removed from the carrier frequency, the difference may be higher. For instance, with a. carrier frequency of 1000 k. c. the resonant circuit could be tuned to 1080 k..c. or'920 k. c., i. e., 8%

removed. This depends on the coeflicient of the coupling and the emciency of the resonant circuits, e. g., I and I! in Fig. 1, and on the result desired.

In Figs. 1, 3 and 5 the resonant circuits, e. g., l9 in Fig. 1, could be tuned to the exact carrier. frequency and tube 3 used as a loading device to vary the impedance of the circuit thereby obtaining radio frequency modulation of the grid l0. In Fla. 3 the corresponding resonant circuit is the one including 48b and in Fig. 5 it is llc.

An important advantage of my system of different from said carrier frequency, so that said secondary circuit quiescently absorbs only a portion of the energy of said carrier frequency, said secondary circuit including means for vary ing the resonant period thereof at audio-frequency, whereby said secondary circuit absorbs energy from said primary circuit in accordance with the audio-frequency variations of resonant period and thereby modulates the carrier frequency energy in said primary circuit, but at the same time said resonant period is kept at alltlmes different from said carrier. frequency.

2. A high level modulation system including an oscillator providing energy and a modulated ampllfler coupled by at least one tuned circuit resonant to the frequency of said oscillator, and an audio-frequency modulating element of the variable impedance type including asecond tuned circuit physically discrete from said tuned coupling circuit, but conductively coupled thereto and quiescently resonant to a frequency depart.- ing by about one per cent from the frequency of said oscillator, whereby alteration of the quiescent state of said modulating element will alter the resonant period of said second tuned circuit so as to absorb varying amounts of oscillator energy from said first mentioned tuned circuit and thereby will modulate said oscillator energy.

3. The method of modulating at high level a first circuit. carrying energy of a predetermined by biasing tube 8 to a suitable value on its grid voltage plate current characteristic curve so that during the modulation cycle the negative modulation peaks will be limited tov a value never to exceed a point where the carrier would be cut-off.

. 'radio frequency with a second circuit coupled to said first circuit, which includes the steps of tuning said second circuit to a quiescent frequency slightly different from said predetermined radio frequency and varying the tuning ofsaid second circuit at the modulation frequency, but keeping the tuning thereof at all times different from said predetermined radio frequency, whereby a carrier wave determined by said radio frequency is never wholly Suppressed and whereby said second tuned circuit acts to variably absorb energy from said first circuit at said modulation frequency.

JOHN J. ANTALEK.

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