US2329877A - Demodulation system - Google Patents

Description

se t,-z1,1943. M. CAWEIN 2,329,877

DEMODULATION SYSTEM Filed Sept. 29, 1941 INVENTOR MADSON CAWEW TTORNEY Patented Sept. 21, 1943 UNITED sTArEs PATENT oFFicE 2,329,817; j j

7 Madison Oawein, Marion, worth Television and R poration of Delaware V 7 Application September 29, 1941, Serial No. 412,732

'4 Claims. (01. 250521) t I v fore, to provide a new and improved controllable This invention relates to a demodulation system and particularly to a demodulation system having a controllable output.

Demodulation systems conventionally used in modern receivers of modulated carrier Waves generally comprise a diode rectifier tube connected in series relation with a load circuit comprising a fixed resistor shunted by a fixed condenser. Since theintensity of received carrier Waves can vary greatly, the intensity of the demodulated signal developed across the load circuit of the diode rectifier also varies accordingly. However, as the reproducing device, to which the demodulated signal is finally applied, sometimes after amplification thereof, operates best within a limited range of input signal intensities, it is necessary to provide a control between the demodulation system and the repro-' ducing device in order to apply to the reproducing device an input signal extending over a desired range only.

In television receivers, it has been customary heretofore to provide a potentiometer between the demodulation system and the cathode ray picture reproducing tube, by means of which the intensity of the control signal applied to the cathode ray tube can be adjusted for the purpose of controlling the contrast of the reproducedtelevision image. Such a potentiometer is usually connected in the load circuit of the diode rectifier tube or in the load circuit of an amplifier tube coupled to the diode rectifier and has its sliding contact connected to thecontrol grid of an amplifier tube or of the cathode ray picture reproducing tube. Since there always exists a certain amount of distributed capacity across the resistance winding of such a potentiometer, a varying amount thereof according to the potentiometer adjustment is connected in shunt relation to the control grid-cathode space of the tube, to whose control grid the sliding contact is connected. For different settings of the sliding contact of the potentiometer therefore, the input admittance of the tube following the potentiometer is varied, which gives rise to changes in the frequency response characteristics of the channel transmitting the demodulated signal. Such changes are highly undesirable, particularly in a television signal channel which is usually corrected for substantially uniform frequency response over a very wide band of frequencies, because any changes in circuit constants in the channel tend to impair the frequency responsecharacteristics.

It is an object of the present.invention,"therepedance comprising a Ind., asjsignor to Farn sadio Corporation, acordemodulation system; free from the disadvansofthe arrangements hitherto used. I'

In accordance with. the present invention, there is provided a -controllable" demodulation system having a pair ofinput terminals adapted to have a modulated carrier signal applied there'- to, having half-waves of opposite polaritiesi- A first circuit is provided comprising a first uni lateral impedance means and a load impedance connected between the input terminals. The first unilateral impedance me nal half-waves of one polarity only,

ans is responsive to sigacross the load impedance. There-is also provided a second circuit "co'niprisinga second unilateral impedance means, a variable impedance means, and the load-circuit,"connected between the inputterminals, The second unilateral impedancemeans is responsive to signal half waves of the opposite polarityonly, thereby'to develop across the load impedance a demodulated signal of'opposite polarity and'of an intensity dep ending upon the value ofthe variable impedance.

Finally, there is provided apair of output terminals coupled to'the' load impedance for deriving a demodulated signal of controllable intensity therefrom;

For a better understanding of the invention; together with other and further objects thereof," following description, the accompanying I impedance means, such as the diode rectifier tube 3, and a load im resistor '4; connected in shunt relation with a condenser li." Asecondj cir cuit is also connected between the'input terminals I and 2 comprising a second unilateral impedance means, such as the diode 6, connected inserie's relation with a variable a variable resistor 1 shunted and also including terminals 9 and I0 pedance comprising thereby to develop a demodulated'signal of one polarity demodulation system em there is shown a con and 2 is a first circuit" unpedance comprising by a condensen'fl, 1 the I load v impedance comprising-resistor4 and condenser 5. A'pair of output" are coupled to the load im'-' a resistor 4 and condenser" the applied carrier signal only. The voltage def veloped across the condenser 5 is of such polarity that. the cathode of the diode 3 176116831205 become.

increasingly positive. The ratev at which this voltageincreases is determined'by theinipeda nce of the charging circuit of the condenser 5; which comprises the signal source (not shown) connected to the input terminalsv l' and 2' and the diode 3. Obviously, the voltage developed across the condenser 5 at the end of each positive half wavciofrthe carrier signal denendsoni the im.- p danceot its charging: circuit. Durin r thcme ative hal -wave he conden er: 5 disch r zacross.

the resistor 4;, as is well ls-ncwnitzo those skille im he art. In this-mamien a. demodulated; sign l:

corncsmndingto, the modulation envelope orthop sit-liv carrier halt-waves; is developed. across the load: impedance comprising condenser; E; and

resistor 41';

Thedi d 6., i not. responsive to positive can rier 'halfi-w a ssince t: isccnn ct d; withl polartr opn siteto thatiof the; diode 3;. and it conuc saonlyduring the ne tive hafrWEYCS-Of the carri r" signal; nd closes, a. secondcircuit. betweenv thc'inputz rmina s l nd 2; whichialsoi d s the: condenser Again, a, current is caused; to flow through-1 the; condenser 5, whose directiom. however; is: opposite. tothe. condenser current, flowin -throu h thedicde 3 durinszm itive half- The voltage developed; acro s: the condenser 5. bythe; current, flowing through, the; diode: l5:- isof pposite; p la i y t the v l a e developed; by the; positive carrier signal half-waves and tends tomake! the: anodeof. the diode; 65 negative, condenser. currentand the voltage developed, across. condenser 5 during; each: negative, hal-f wave again depends on the impedance; of the,

ondenser char ing, c rcuit wh chnow inc udes the: source of carrier, signal (not shown-ls. the;

diode 6;and thevar-iable impcdan egelement; comprising, the variable resistor T; shuntedby; the

condenser 8. If the resistor-l iscompletelx-shert.

circuited, the voltage developed across the con.- denser 5,, due tothe negative halfewaveawill; be equal and;oppositetorthevoltage developed there--v across by the positive half-wavese If, however, the resistor I has a finite-value, the'chargingcurrent, during the negative halt-waves and, therefore. the. voltage developed acrossthecondenser 5, is. decreased. Again, obviously, the condenser charges, due to the negative hali wavesrleak off;

through shunt resistor t duringrtheipositive halt waves. It; is evident, therefore, that a. demodulated: signal is developed across; the load imped ance, comprising resistor 4. and condenser 5. The polarity of this signal is opposite to that of the demodulated signal produced bythe'positive carrier halt-waves, whereby the resulting; demodu, lated signal is ofcontrollable intensity ranging between zero andayalue determlnediby the;val-ue of theresistor 1-. Thecondenser-8 has1beenproe vided tor by -passing the. carrier iltcduency but is nctgessentiai tor atisfa tory operation; of; the; rranssmcnt. l

The resulting demodulated signal of controllable intensity is developed across the output terminals 9 and H! which can be coupled to the control grid and cathode, respectively, of an amplifier tube or a reproducing device, such as a cathode ray picture reproducing tube. Since the output of the demodulation system is controllable, i1; is obvious that no other co ,trolsare required between the; demodulation system and a reproducing device, so that the circuit constants of the modulation signal frequency channel remain unchanged when the magnitude of the demodulated nal ischa e d.

Whilethere has been described what is at Dresent' considered the preferred embodiment of the inventionit willibe obvious to those skilled in the art that various changes and modifications may be. madetherein without departing from' the invcnticmand it is, therefore, aimed in the appended' claims to cover all such changes and modifications as. fall; within the true spirit and scope, of theinvention,

What is claimed is;

1.. controllable demodula ion sys em adapted tolhave amodulated arrier signalapplied thereto hev n rhalf Wavesof different polarities comprise ing: rectifying: means and-- a fixed time constant circuit; connected in,- series-therewith for develop? n a. d modulat d; si nal of? e p arity a oss 1 said fixedtime constant circuit, a second rectify-- ing moans anda variable impedance means connected: in; series with each other across said first rectifying means for normally developing a demodulated signal of opposite polarity across said time; constant circuit of an; intensity depending upon the value of said variable impedance means and controlling; the-intensity of thewhole demodlatedsisna 2. A- controllable demodulation system com prising a. pair ofinput terminals adapted to-have a modulated carrier signal applied thereto; having; half-waves. of opposite polarity, a pair of output terminals, a load impedance connected between said output terminals and a. demodula- ,tion circuit connected between the inputterminals and the output terminals comprising a.

means.- responsive to signal half-waves of one polarity only for developing across said load impedance a, demodulated signal of one polarity, and means connected to .said input terminals and said; load impedance responsive to signal half- Waves, of the opposite polarity only for developing; a, demodulated signal across said load impedance ofopposite polarity and of an intensity dependent uponthe valueof said variable im-.

pedance means to thereby control the intensity of, thewhole; demodulated sign al-.

3, A. controllable demodulation; system comprising a pair of input terminals adapted to. havea modulated carrier signal applied theretohaving. half-waves oioppositev polarity, a pair of output. terminals, a load impedance connectedbetween said output terminals and a demodulas tion, circuit connected between one of; said input.

erminals and one of said output terminals com prising, a first unilateral, impedance means re sponsive to signali half-waives of one polarity only for" developing across; said load; impedance a demod lated signal of one polarity, anda series including a, second: unilateral impe ance means: responsive to. signal hair-waves oi the pposite polar ty' only. and a variable impedance means. connected across said first unilateral in, pedance means for developing a demodulated 2 ignal: across.- said; load impedance of pposite polarity and of an in'tensity'dependent upon the value of said variable impedance means thereby to control the amplitude lated signal.

4. A. controllable demodulation system comprising a pair of input terminals adapted to have a modulated carrier signal applied thereto having half-waves of opposite polarity, an output terminal connected to one of said input terminals, another output terminal, a load impedance connected between said output terminals and a demodulation circuit connected between the other of said input terminals and the other of said output terminals comprising a first uniof the Whole demodulateral impedance means responsive to signal impedance means thereby to control the amplitude of the Whole demodulated signal.

- MADISON CAWEIN.

and a series circuit including a

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