US2382014A - Universal demodulator for frequency and amplitude modulation - Google Patents

Description

UNIVERSAL DEMODULATOR FOR FREQUENCY AND AMPLITUDE MODULATION Filed Feb. 2, .1943

INVENTOR.

E. H. LANGE 2,382,014

Patented Aug. 14, 1945 UNITED STATES PATENT OFFICE UNIVERSAL DEMODULATOR FOR FRE- QUENCY AND AMPLITUDE MODULA- TION 12 Claims.

This invention pertains to thermionic demodulator devices for converting the pattern of frequency-modulation of a high-frequency carriervoltage into corresponding output-voltages, and more particularly to apparatus for compensating undesirable modifications introduced or superposed upon these output-voltages by extraneous amplitude-modulation of the carrier-voltage, either at random, or by desired'amplitude-modulated signal intelligence; and for utilizing the identical component parts and operating structure of the demodulator device, for also demodulating amplitude-modulation, to accomplish economy of necessary component parts, and of space to accommodate these parts.

The principal object of this invention, is to provide such a universal demodulator, having both high sensitivity of bonversion, and economy of componentparts.

Another object of this invention, is to provide such a demodulator capable of converting frequency deviations above and below a centre frequency into respective positive and negative voltages, directly proportional to the magnitude of the frequency deviations, without the necessity of regulating the impressed voltage upon the demodulator to a minimum determined by the weakest impressed signal, to eliminate amplitude-modulation; to provide operation of the frequency-demodulator from a variable norm of impressed voltage upon the demodulator, by providing apparatus for compensating the effect upon the sensitivity of the frequency-demodulator, of variations of magnitude of the impressed frequency-modulated voltage, and' by providing a selection of operating-magnitudes of impressed voltage.

Another object of this invention, is to provide in a frequency-demodulator, a useful unidirectional control-voltage, which varies in direct proportion to amplitudes of the alternating currents supplied to the input-side of the demodulator,

and which is independent of the frequency deviations from centre-frequency 01' these alternating currents, over the entire useful range of. such frequency deviations.

A further object of this invention, i to provide a balanced frequency-demodulator, and capable of operation with a push-pull amplifier connected to the output-side of the demodulator, and to provide selector means for utilizing the same amplifier and the same demodulator for the reception of intelligence impressed upon the demodulator in the form. of amplitude-modulation.

Still another object of this invention, is to provide a universal demodulator, requiring no thermionic tube or tubes for amplitude-limitation ahead of the frequency-demodulator, and to provide a useful bridge-connection between the output-sides of the demodulator, for separating the component demodulations, resulting from frequency-modulation and from amplitude-modulation.

These objects, and others, are hereafter set' ing capacitively coupled with impressed modulated alternating currents, and the cathode-anode conductance of the compensator-tube being ca- I pacitively coupled with the resonant circuit.

Fig. 2 illustrates a universal demodulator in accordance with the principles of this invention,

, having a thermionic compensator-tube, a doublediode, a resonant circuit and choke coil connected to the double-diode, output resistors for reception of either demodulated amplitude-modulation or demodulated frequency-modulation; the resonant circuit being inductively coupled with the impressed modulated alternating currents, the connected choke coil being capacitively coupled with Y the impressed modulated alternating currents,

and the cathode-anode conductance of the compensator-tube being inductively coupled with the resonant circuit.

Fig. 3 illustrates in diagram form, variations of cathode-anode conductance of thecompensatortube in relation to bias-voltage upon the controlgrid of this tube, and variations of total conductance with the bias-voltage, employed in this erence. to voltage and current magnitudes in the cut of frequency deviations from the centre frequency. v

Fig. illustrates a push-pull resistance-coupled thermionic amplifier, for amplifying the selected output of the demodulators, and illustrates selector means for operation from demodulated signals of either frequency-modulation or amplitude-modulation.

Referring to Fig. 1, the principal inputterminals are shown at 5| and 52, the terminal 52 being connected to the negative terminal 01' a source 29 of unidirectional voltage, the positive terminal of the source 28 being connected through the choke-coil 28 to the terminal 5i. Between the terminals-31 and 38 is a resonant circuit, and at 5 is a choke coil, the resonant circuit 31-38 and choke 5 being connected in series, and capacitively coupled across the terminals 5l52, by the condenser 52b. I

At 38 is illustrated a thermionic tube having an anode 42, a cathode 34, asuppressor-grid 3| directly, connected by connection 33 to the cathode terminal Mai a screen-grid 32 connected to the source 23 by connector 32a, a. control-grid 35a connected to terminal 35, and direct connection of cathode terminal 340 to terminal 52, and to auxiliary input-terminal 36. 38 provide means for impressing amplified modu lated alternating voltages across the terminals 5| and 52. The present invention is not directly concerned with the structures preceding the demodulators of this invention, in the sequence of apparatus utilized from initialreception of signals to final output of intelligence, and the connections of tube 30 to 5I52 are illustrated only for purposes of continuity, there being many well-under- V stood types of linkage with such preceding structures, capable of supplying the required modulated alternating voltages to the principal terminals 5i-52.

At ii is a variable condenser for tuning the Terminals 35 and the ground connection 55, with the terminal 52, and with terminal of choke coil 5, through the voltage-bias resistance 50 connected to 25, or through condenser 60. shunted across the resistance 23, is the condenser 23. The equal resistances 22a and 22 are connected in series, and connected across the cathodes "-41, through the stopping condensers 2| and 2la, and the halfresistance point 22b of, the total series resistance, is connected to ground 55, and to the terminal 0'. Similarly, the resistance 24 is connected across the resistance 23 through a stopping condenser 25, one terminal of 24 being directly connected to the ground connection 55, the other terminal being connected to the terminal Ai. At F1 and F2, are output terminals, F1 being connected directly to the ungrounded terminal of 22, and F: directly to the ungrounded terminal of 22a. The terminals F'i-F: are for demodulated frequencymodulation, and the terminals 0'Ai for demodulated amplitude-modulation.

At i is a thermionic compensator-tube, havin anode 2, cathode 3, and control-grid I. Connected to the common terminal (1, between chokecoil 5 and terminal 38 of the resonant circuit, is the terminal 3a of cathode 3. The anode 2 is connected to the terminal a, which is connected to terminal b by a removable link, the terminals a, b,

resonant circuit between 31 and 38. Between 3l and terminal liib of the condenser ii, is an inductance coil 39, having the parts 8, 8, and i2. For the purpose of this invention, it will be understood that the coil 38 may be a single continuous coil having the connections", 38 and lb thereon. or may be made up of separate coils in series,

8 either without mutual inductance, or with additive mutual inductance between coils, and constituting a single inductance coil between 3I denser shunted across I! and at 23a is a condenser shunted across l8. The anode i5 is connected to the anode l6, through the conductive path i5a, I817, 38, i0, l5a, the lead wires I51: and iiia being directly connected to the respective anodes l5 and I6, and to the included parts i2 and 8, of the inductance of the resonant circuit 3I--38. Theterminal 38 upon inductance coil 39, is located with reference to the terminals Iii and "lb, so that at exact resonance of the resonant circuit 37-38 the alternating voltage across Iii-38 is equal to the alternating voltage across 38-11"), the terminal38 being at the halt-inductance point upon the part or inductance coil 38 included in the conductive path between the anodes as, It; At 23 is a resistance, connected from half-resistance point 53 to the connector'fl, which connects with being terminals of a switch. The terminal b is connected to terminal 21 of the coupling condenser 52b, which terminal is directly connected to the principal terminal 5i. Connected to the resistance 23 is the adjustable connector 1b, which is connected to the control-grid I through the choke-coil 5a. The choke-coil 5a is connected in parallel with choke-coil 5, through the condensers 8, 6b, and 6c, the condenser 60 having one terminal connected to'the choke-coil 5, and the other terminal connected to terminal 52, through conductor 37. The terminal of condenser 50 which is connected to the choke-coil 5, is also connected by an adjustable connector to the biasresistance 50'. The bias-resistance 50 has one terminal connected to terminal 52, at the negativeend of source 29, and'the other terminal con nected to a positive terminal upon. the source 29'.

Referring to Fig. 2, the variable condenser lie is connected across the parts 9a and Ho of the inductance coil 39', to form a resonant circuit. The terminal 38a upon the inductance coil .39 is connected to a terminal of the choke-coil 5b, and the opposite terminal of the choke-coil 5b is connected through the resistance 23 to the halfresistance point. 50 upon the resistor formed by i3 and id, in series.

The principal terminals for application of mod ulated alternating voltages, in the circuit of Fig. 2,,are 5ia and 34a. As discussed with reference to Fig. 1, connections of the tube 30 with the principal. terminals .of this demodulator, are illustrated only for purposes of continuity, and in reference to the utilization of the source 28 of uni- ,directional voltage, of this invention.

inductance coil "4:, in series with the by-pass condenser 4|, and having the tuning condenser 40 shunted across its terminals. Connected to the junction lb, between the by-pass condenser II and coil 88a, is the positive terminal of the source of unidirectional voltage, 28; the negative terminal of 28 being connected to the ground, 55. The terminal lie of the resonant circuit 89'--'l la, is connected by the conductor "a directly to the anode l5, of the double-diode I8, and the terminal id of this resonant circuit is connected directly to anode I, by conductor lid. The choke-coil 5b is energized by the-capacitive coupling with the impressed modulated voltages across 5la-8lq, through the condenser 52a, and connection 55 to ground. The coil 85a has mutual inductance with coil 35', and the resonant circuit 39'--lla is thus energized by the mutual in- 1 ductance with the coil 35a, which coil is shuntconnected to the input-terminals 5la--84a. The junction 38a upon the inductance coil 85', is at the half-inductance pointupon 38, such that the alternating voltages across parts 8a and Ha are equal.

The cathode 8 of the compensator-tube I has its terminal 8a directly connected to the biasresistance 50, by a variable connector, the biasresistance 5c having one terminal grounded at 55, and the other connected to a positive terminal upon source 28. The screen-grid 8 2 of tube in is connected to the source 28 by connector 32a, and the condenser 82b is connected between the screen-grid 82 and cathode terminal 84a. The control-grid 85a is terminated at terminal 85, and terminal 86 is connected to 84s, and to ground 55. The terminals 85 and 88 serve as input terminals for applying amplified modulated alternatin'g voltages across the principal terminals Ila-5hr. The suppressor II is connected directly to the terminal 34a of cathode J4, it being understood that this connection can be within the tube-enclosure,-in a manner commonly used.

Referring to Fig. 5, a resistance-coupled push pull amplifier is shown, having thermionic tubes H and I3; tube 1| having anode l2, cathode l5 and control-grid l1, and tube 18 having anode denser 58 is connected'a cross 81, and by-pass con-' denser 59 is connected across 88. A source of unidirectionalvoltage 280 has its negative terminal nectsgrid ii'to terminal Ar, which terminal is connected by the variable contactor 58 to resistance 8 I, for phase-inversion. The reasons for utilizing the phase-inverting connection, will be subsequently pointed out.

An important feature of this invention is the bridge-means employed to permit interconnection 1 between the output-circuits for frequency-dedouble-diode, to the half-resistance point between connected to ground through conductor I8. Re-

There are two positions in which the switch is used; in position one, as illustrated, the compensator-anode switch 80, connects a and b together, switch 8| connects grid 11 with terminal F1, and switch 82 connects grid I8 with terminal F2 In position two, as shown by 8| and 82, switch 80 disconnects a from b, switch 8| connects grid TI! to terminal A1, and switch 82 conthe output-terminals for frequency-demodulation, F1 and F2. Thus, variation of voltage amplitude across the junction 500' produces no resultant .voltage between F1 and F2, and likewise variation of voltage amplitudes between the oath-' odes l4-ll, across resistances i8, 19, produces no resultant voltage across the terminals Ai05. The utilization of the above-mentioned balanced circuit connections in the universal demodulators V of this invention, will be better'understood upon subsequent consideration of the means of this invention, providing a unidirectional control-voltage in the frequency-demodulator. The frequency-demodulated voltage between the terminals F1 and F: specifically provided to be separate from any superposed effects of amplitudemodulation by the above-disclosed bridge-means, is readily applied to the amplifier of Fig. 5 by any one of manywell-understo'od linkages of conventional usage, for example, by a transformer with primary connected across the terminals F; and F2, and secondary having terminals Fi" and Fr, and a grounded centre-tap O. 7/

In Fig. 2, grid 1 of the tube i is connected by lb to a variable contact on resistance 23, and highfrequency by-pass condenser 25b connected from 1b to cathode terminal 8a serves to keep voltages -'of the high-frequency from being impressed upon the grid 1-. The anode 2, of compensatortube I, is connected to terminal a, which connects to terminal b by a removable link, and terminal b is connected by means of the variable conductor Ila to the inductance coil 39a. When the circuit between a and b is closed, the compensator cathode-anode conductance at is thus coupled with the resonant circuit 39"l la by the mutual inductance between 39a and. 88'. It will be evident that the cathode-anode compensator conductance at can be alternatively coupled with the resonant circuit' 39'l I a by a tertiary coil connected between 5 lb and 40a, instead of by coil 38a.

Having illustrated certain circuit connections embodying this invention, the principles of this invention are hereafter discussed to.point out the important features, and with reference to the above-disclosed circuit connections.

In devices hitherto employed for demodulating frequency-modulated alternating currents, it has been necessary to eliminate any variations in amplitude of the impressed alternating currents, since such devices are also responsive to amplitude variations; without such elimination the resultant response to the frequency modulation would be distorted by -extraneous voltages unrelated to the frequency modulation. Devices hitherto employed, and requiring one or more tubes to accomplish satisfactory elimination of amplitude variations, have employed a saturation principle, wherein alternating currents impressed in excess of a certain minimum value are unable to produce at the output side alternating currents larger than the minimum value. Such devices result in the employment of signals determined by the weakest signal-strength, and but little gain is contributed by the tube or tubes themselves, which are used to so limit the signal amplitudes. It is therefore necessary to provide, by means of the amplifying apparatus preceding such a limiting device, an excess of amplification, such that the weakest signal will upon amplification equal the constant norm fixed by the characteristics of the limiting device, and no benefit is thus derived from the signals which exceed such a fixed minimum. In the devices of this invention, operation of the demodulators' takes place upon a selectable norm of input-signal amplitude,modifications of the amplitude" of input signals being compensated in their effect upon frequency-demodulator, it is necessary to provide a unidirectional control-voltage which is directly proportional to the amplitudes of the alternating currents impressed upon the demodulator, and which is, at the same time, independent of the frequency deviations from centre frequency, for the entire range of such deviations employed upon the frequency-demodulator. The discovery of means for producing such ,a control-voltage in a frequency-demodulator, is an important'feature of this invention, in addition to the apparatus utilizing this control-voltage to compensate for amplitude variations, and permitting operation upon a variable norm.

The principle of operation of the unidirectional control-voltage, will be first pointed out for Fig. 1, wherein a, resonant circuit 31-38 is in series with a substantially pure inductive reactance between the terminals Id and 52, the choke-coils 5 and So being high-inductance chokes, in parallel acrossthese terminals, the impedance of the condensers 6, 6b, and 6c being negligible in relation to the inductive-reactance at the centre-frequency of 5 or 5a.. Also the condenser 526 has a negligible impedance in relation to either 31-38 or the choke-coils 5 and 5a in parallel, and serves. primarily to keep the total potential of source 29 from the anodes l5, it, of the double-diode. When the anode 2, of the compensator-tube I, is disconnected by means of removable connector between a and b, and the resistance 23 is shortcircuited, there is present an uncompensated frequency-demodulator, operable upon frequencymodulated constant-amplitude alternating currents in the input circuit, 21-31-38-411-52, and providing frequency-demodulation across the output. terminals O'F1 and O'-Fz. Theconditions then existing, can be better understood by reference to Fig. 4. At 5c is shown a vector representing the alternating voltage across choke-coil 5; this vector is displaced in phasev by 90 degrees from, the alternating current through the chokes 5, 5a. The resonant circuit 31-38 is sharply resonant to the centre-frequency, the inductance coil 39 having a very high reactance-resistance ratio. Thus, at exact res- .onance for the centre-frequency, the alternating voltage across -38 is in phase with the alternating current passing through chokes 5 and 5a, and is therefore 90 degrees displaced in phase from le. At A is shown the vector voltage across the part 9 of resonant circuit 81-48, and at B is shown the vector voltage across the part I: of the resonant circuit, at enact resonance to centre-frequency. When'the voltage'across 8 is directed toward 18, the voltage across I! is directed away from 38. The voltage impressed across the anode-cathode I 5 ll is determined by the resultant voltage across Nib-31', and likewise the voltage impressed across the anodecathodel 6-l'| is determined by the resultant voltage across ill-31'. At exact resonance, these resultant voltages are equal, and are shown by E1 and E2. The corresponding unidirectional currents through the diodes, are proportional to these resultant voltages, and equal unidirectional currents flow from the 'cathodesthrough l9 and I8 to the'iunction 50, there being no resultant unidirectional voltage across it and IS in series, that is, between the cathodes II and i1.

' Condensers 20 and 20a have a negligible impedance for the high or centre-frequency, in l'relation to the resistance I 8 or II. For a constant alternating current through the resonant circuit, the locus of the voltages across 9 and i2 is given respectively by the circles n and n. For a small percentage change of the frequency from the centre-frequency, the sharply resonant circuit 31- 18 will have the voltage across 9 de-phased by degrees, by this frequency-deviation; the voltage across 9 is thenrepresented by the vector 92,

and likewise the voltage across I! is then repre- .sented by lie. The resultant alternating voltages across the diodes are now represented by El and E2, and the currents flowing to the junction are unbalanced, there being a greater unidirectional current through it than through l9,

' site polarities for deviations above and below cenin general vary over a wide range with frequency deviations employed for frequency-modulation. An important feature of this invention is the means for obtaining independence of this total unidirectional current of frequency deviations. over the entire useful range of such frequency deviations, in the frequency demodulator, and for obtaining direct proportionality between the total unidirectional current and the amplitudes of impressed alternating current.

Thus, it will be seen-from Fig. 4, that while the resultant voltage across H-l| due to frequency deviations is determined by the arithmetic difference of voltages E1 and E: for any particular frequency deviation, that is. by the difference in unidirectional currents through the equal resistances l8 and It, the sum of these unidirectional currents is determinedby the sum or the voltages E1 and E: for any particular frevoltage from resistance 28, and which varies in quency deviation. The variation of this sum, with frequency deviations, can be illustrated from Fig. 4, in reference to the voltage ratio employed, for the ratio of alternating voltage across chokecoil 5 to alternating voltage across the resonant circuit 31-30, at the centre-frequency. Thu the vector A is a measure of the half-voltage across 31-38, when the junction I is at half-voltage across 3l--l8, and comparison can be readily made between the sum E1'+Ez* for resonant frequency, and Ei-l-Ez for frequencies deviated from resonance suillcient to de-phase the voltages across s, and across I I, by 45 degrees, as influenced by the ratio of Se to 4+3, that is, the ratio of voltage across choke-coil 5 to voltage across resonant circuit 31-38, at centre-frequency. Designating this latter ratio by N, then the percentage variation of E1+E2 from E1+Ea' as related to N, is approximately as follows: When N is A,, variation is 29.5 per cent; when N is V2, variation is 19.0 per cent; when N is 1, the variation is 7.6 per cent; when N is 2, the variation is 2.2 per cent;- and .when N is 3, the variation is 1.0 per cent. when the ratio N whichis employed, is suillciently large, the total unidirectional current can thus be made independent of the operating frequency-deviations upon the frequency-demodulator, to the extent. required. When frequency-deviations are less than those sumcient to de phase the resonant circuit voltages by 45 degrees, the percentage variations will be somewhat less, but since frequency-deviations must be a substantial percentage of this magnitude to yield proper operation of the frequencydemodulator, it is apparent that the ratio N=1.0,

defines a minimum ratio, below which the total unidirectional current would be unsatisfactory for controlling unrelated variations in the amplitudes of impressed alternating currents, or controlling the eifect of such amplitude variations upon the input of the frequency-demodulator. The voltages E1" and E2 are, however, directly proportional to the magnitude of alternating current through the input circuit 2|3'|3l--ld- 52, being compounded from is and A or B, each of which is directly proportional to the impressed alternating current. The employment of a ratio N greater than unity, together with the total unidirectional current from the diodes, thus provides an important control means for annulling the eifect of varying alternating current amplitudes upon the output of the frequency-demodulator.

In this invention, the resistance 23 is connected in series with the total unidirectional current from the diodes, to produce a unidirectional control voltage, directly proportional to the amplitudes of the impressed alternating currents, and independent of frequency-deviations over the entire range of such deviations employed upon the frequency-demodulator. Referring to Fig. 3, a graph is shown, illustrating a variation with negative-voltage bias upon the control-grid I of the compensator-tube I, of total conductance, consisting of a circuit conductance and of the cathode-anode conductance of tube l The tube-conductance is indicated by Gt, and circuit conductance by Go. The adjustable negative-voltage bias of tube I, produced by resistance 5c, is indicated by -E-:; at I4 is indicated the various magnitudes of resultant or total conductance across the resonant circuit, e. g. 3I 38, indicated by G. The voltage E1 is the positive voltage introduced upon the control-grid I of compensator-tube I, by the unidirectional controldirect proportion to the magnitudes of the alternating current amplitudes, impressed upon circuit 2l-l'l-3l4d-'-l2. O is the origin of coordinates for the graph of conductance'as related to resultant negative bias upon grid I, and '0" indicates the origin thereon, for the unidirectional control-voltage derived from 23.

The voltage-conversion sensitivity of the frequency-demodulators of this invention, that is.

the voltage output across Il-II, as related to,

the magnitude of frequency-deviations from centre-frequency, designated by M, to the magnitude of impressed alternating currents, designated by I, to the resultant conductance determining the voltage magnitudes of the resonant circuits, designated by G, and to the coefilcient K0, which is fixed by the values of the previously defined ratio N. employed, and by the inductance, capacitance, and resistance values of the alternatingcurrent networks of the demodulators, is:

It will be noted that the demodulated voltageoutput of the frequency-demodulator, Er, varies not only with the frequency deviation Af, but also with the amplitudes of the alternating currents impressed upon the circuits, and upon the inverse square of the resultant conductance, determining the voltage magnitudes of the resonant circuits.

' It is therefore evident, that variations of I are of equal importance with variations of AI,

and that to attain sastisfactory freq'uency-de-f modulation the effect of amplitude-modulation must be annulled. As previously noted herein, this has hitherto been usually accomplished by regulating Iac to constancy, through a saturationtype device, thereby prohibiting usage of any values of Iao other than those identified with the output-characteristic of such a device.

In the demodulators of this invention, the ratio Inc/G is regulated to substantial constancy, thus permitting a very wide range of signal strengths to'be utilized directly upon the demodulator. This is an important feature of this invention. Thus, if this ratio is designated by D, then D=Ia/(;; must be a constant value during reception or demodulation of the signals; howupon the resistor 23, is directly proportional to the value of'La of alternating. currents in the input-circuit of the demodulator, and independent of the variations AI. Designating this ratio by 8.

then Ev'/I= S wherein the factor Sis selective by the adjustable connector lb upon the resistor 23. The manner in which the resultant conductance G must be modified to accomplish this resuit, is therefore:

and this relationship is indicated by the para 1 bolic curze at N, Fig. 3. The conductance of the resonant circuit alone, without loading of the curve 84 and go. It will be noted that this diner ence is substantially a straight line, as indicated by the dotted line 85, and for very large percentage variations of E1, about a mean value between and 0". Such substantially straight-line characteristics of cathode-anode conductance with negative voltage-bias, are attainable in conventional triodes, and also in conventional pentodes, it being understood that for simplicity of illustration tube I is shown as a triode, and that a pentode can also be used with a suppressor connected to the cathode thereof, and a screen-grid connected to the source 29. Also, such characteristics are attainable in conventional tubes, with slight curvature toward the origin 0,. providing even greater conformance with the parabolic curve 84 than indicated by the .line 85. It is therefore apparent, that if the mean value of E1 is selected, a corresponding mean value of In is determined by the selection factor S. Thus amplitude-modulation of Iac causes corresponding amplitude-modulation of E1, and appropriate modification of G, to maintain the ratio E-f/G constant, and thus to maintain the ratio Inc/G constant. If it is desired to operate with a larger signal upon the demodulator, for example, a signal ten times the former signal, in mean value,

' then S is reduced to one tenth its original value by means of variable conductor lb upon 23, providing the same mean value of E1, and thus maintaining a constant value of D ten times as large as the former value.

The impedance of condensers 20, 20a, and 26, is low in relation to the respective resistances shunted by these capacitances, and at the high carrier-frequency, o'r centre-frequencyof the impressed alternating currents; however, these impedances are high in relation to the respective shunted resistances for the low or modulation frequencies. Impedance of condenser 52a is negligible in relation to' the impedance of the chokecoil b, which is essentially pure inductive-reactance. Condenser 26b has a low impedance at the centre-frequency, and operates as a substantial short-circuit for these frequencies, preventing the carrier frequencies from being impressed upon the grid 1, Fig. 2. Condensers 2i and 2h: have low impedance in relation to the resistances 22, 22a, for the low or modulation frequencies of the output-terminals F1, F2, and likewise condenser 28 has low impedance in relation to'resistance 24, for the low or amplitude-modulation frequencies of the output-terminals A1, 0. In Fig. 1, the circuit traversed for impressing upon grid 1 the variation voltages initiated by changes in amplitude of the impressed alternating cur.- rents of the demodulator, is

3a4d--l--8c2123'|b-ia1a---l it will be noted that the equal alternating voltagesacross i and 5a are traversed in opposite directions, and that there is thus no high-frequency carrier voltage impressed upon grid I,

' the condenser 6 also serving as a substantial short-circuit between grid 1 and cathode 2 for such voltages. v

In the demodulators, when it is desired to receive the intelligence impressed upon the alterflcations or in the form of amplitude-modulation containing signal-intelligence, are compensated the terminals A1 and A2, the latter terminal not' being present upon the demodulator, and being connected to the output sideof the amplifier in a manner well understood, to impress upon terminal As an alternating voltage equal to that impressed upon A1 by the demodulator, and of opposite phase, thereby utilizing the same pushpull amplifier for amplifying either typ of demodulation.

The selector means 86-83--808i--82, of Fig. 5, is intended for alternate use with the terminals F1, F2, and with terminals A1, 0', and when connected to the latter terminals the switch disconnects a from b, and thus removes the compensating action of tube i. It will be noted that while this is a preferred usage of the demodulators of this invention, both demodulated frequency-modulation and demodulatedamplitudemodulation can be simultaneously received, each at the respective terminals F1, F2, and A1, 0', with the terminals a and b connected together. For this usage, the employment of larger values of the ratio N previously defined herein, yields an improved correspondence between the amplitudemodulation upon the impressed signal currents and the resultant amplitude-demodulation at Ai-O', and some decrease in the voltage-conversion sensitivity of the frequency-demodulator.

The adjustable negative-voltage bias, E-1, in each instance derived from the resistor 50 connected to the source 29, is employed to set the origin 0", with reference to which the positivevoltage bias E1 operates to modify the total or resultant conductance G. Thus if the positive voltage bias E1 is set to operate about a value of conductance as illustrated at Go Fig. 3, larger values of E1 produce a larger value of G, and smaller values of E1 produce smaller values of G, in conformance with the voltage-conversion 'sensitivity characteristic of the devices. Without any compensation for variation of alternating current amplitudes, the loci n, n, Fig. 4, would be adversely modified by such changing amplitudes of input alternatng current, and in inverse proportion to the constant circuit conductance, e. g., to the loci m, m, the vectors 9e and i2e being respectively increased-,to 09 and cq, by increased alternating current amplitudes. The vectors originating at the point s are proportionately increased, and the voltage-output of the frequency-demodulator is then determined by the diflerence between sp and sq, for the same frequency-deviation, instead of by 31-171. In the devices of this invention, modification of the resultant conductance is eflected in proportion to the square-root of the amplitudes of the impressed alternating currents. In these devices, the output-voltage for any particular frequencydeviation, and any particular alternating current amplitude, is governed by the product of two factors: the size oi voltage A, and the rate of angular deviation of le from A in relation to frequency deviation. Each of these factors varies inversely with the resultant conductance; the output-voltage of the frequency-demodulators is thus governed by the inverse-square of the resultant conductance.

Having thus described several illustrative embodiments of my invention, it will be evident that aaaaou changes can be made in the form and arrangement oi. parts without departing i'rom'the spirit 01' my invention, asset forth in the appended claims, and I do not therefore limit the scope of the invention to such particular embodiments, or otherwise than by the terms of the appended claims. W

What is claimed is:

3. In a frequency-demodulator for producingoutput-voltages in direct proportion to the magni- 1. A universal demodulator, for converting the I envelope of frequency variations and the envelope of amplitude variations, of an impressed high-frequency carrier voltage having both amplitude-modulation and frequency-modulation, to the respective low-frequency alternating voltages corresponding to said envelopes, said demodulator having a double-diode, a first resistance connected between the cathodes of said double-diode, a resonant circuit having an inductance coil, a conductive connection between the anodes oi said double-diode including inductance of said coil, a choke-coil connected in series with a second resistance from the half-resistance point upon said first resistance to the half-inductance point upon said included inductance between said anodes, capable of carrying the total unidirectional current of both diodes, a thermionic compensatortube with anode, cathode, and control-grid, a source of unidirectional voltage connected to said compensator-anode through a second inductance coil and an anode-switch, coupling means coupling said resonant circuit and said choke-coil with voltages of said high-frequency, and coupling the resonant circuit with'the anode-cathode of said compensator-tube, and'circuit connections between said control-grid and said compensatorcathode, including a-negative-voltage bias means and connections to said second resistance, impressing positive voltage from said second resistance upon saidgrid, and preventing application of said high-frequency upon said grid.

2. The combination with a demodulator for demodulating constant-amplitude frequency-modulated alternating currents, of a compensator device, said demodulator having a double-diode, a resistance connected between the cathodes of said tude of frequency deviations above and below a centre frequencyv oi input alternating currents, the means for producing an auxiliary unidirectional control-voltage, directly proportional to-the amplitudes of said input alternating currents and independent of said frequency deviations, said demodulator having a double-diode, a first resistance connected between the cathodes of said d0ublediode, a resonant circuit having an inductance coil, a conductive connection between the anodes of said double-diode including inductance of said coil, a second inductance coil connected mm the halt-resistance point upon said first resistance to the half-inductance point upon said included inductance between said anodes, and coupling means coupling said resonant circuit and said second inductance coil with said input alternating currents, said producing means comprising a second resistance introduced in series with said second inductance coil, connected to said half-resistance point, and carrying the total unidirectional current of both diodes, and the determination of said total unidirectional cur- .rent, by proportioning said inductances to yield a voltage-ratio greater than unity, for the ratio of alternating voltage across said second inductance ant voltages between said half-resistance points double-diode, a resonant circuit having an inductvoltage connected to said compensator-anode through a second inductance coil, coupling means coupling said resonant circuit and said chokecoil withsaid frequency-modulated alternating 1 currents, and coupling said resonant circuit with 'the anode-cathode of said compensator-tube, and

circuit connections from said control-grid to said compensator-cathode, including a negative-volt age bias means and connections to said second resistance, impressing positive voltage from said second resistance upon said grid, and preventing application of voltage at the higher frequencies of said alternating currents upon said grid; whereby voltage is produced across said resistance, and determined by said frequency-modulation, independent'oi' amplitude variations of said frequeasy-modulated alternating currents,

at the terminals of the second resistance, and whereby voltages across the terminals of the second resistance produce no resultant voltage across the terminals of the third resistance, and said junction is maintained at ground potential.

5. A universal demodulator, for converting the envelope of frequency variations and the envelope of amplitude variations, of an impressed highirequency carrier voltage, to the respective output-voltages correspondingto said envelopes, said demodulator having a double-diode, a resonant circuit having an inductance coil, a conductive connection between the anodes of said double-diode including inductance of said coil, a first resistance connected between the cathodes of said double-diode, a choke-coil connected in series with a second resistance, having a terminal of said second resistanceconnected to the half-resistance point upon said first-resistance and a terminal of said choke-coil connected to the halfinductance point upon said included inductance between said anodes, and having the common terminal between said second resistance and chokecoil connected to ground by a negligible-impedance conductive connection, said choke-coil and second resistance being-capable of carrying the total unidirectional current of both diodes, a thermionic compensator-tube with anode, cathode, and control-grid, a source or unidirectional voltage with positive terminal connected to said -compensator-anode through a second inductancecoil and an anode-switch, and with negativeterminal connected to ground, coupling means coupling said resonant circuit and said choke-coil I terminal thereof connected to a terminal of said first, resistance through a condenser, a ground connection at the half-resistance point upon said third resistance, and a fourth resistance with one terminal grounded, and the other terminal connected to the half-resistance point upon said first resistance through a condenser.

6. The combination, with-the structure of claim 5, ct means for selectively amplifying the output voltages across said third resistance, or across said fourth resistance,including a resistancecoupled thermionic push-pull amplifier having cathodes connected to ground, and a two-position 9. A universal demodulator for simultaneous- 1y demodulating irequency-modulation and amplitude-modulation from a high-frequency carrier-voltage impressed upon the demodulator,

switching means, one position coupling a first 1 control-grid of said amplifier and, a second control-grid of said amplifier to theterminais of said third resistance, and closing said anodeswitch; the other position connecting said first control-grid to-the terminal of said fourth resistance opposite said ground connection, connecting said second control-grid to an output-resistance of said amplifier for phase inversion, and opening said anode-switch.

7. The combination with a frequency-demodulator, of a bridge circuit, said frequency-demod ulator having a double-diode, a first resistance connected between the cathodes of said doublediode, a resonant circuit having an inductance coil, a conductive connection between the anodes of said double-diode including. inductance of said coil, a choke-coil connected from the half-resistance point upon said first resistance to the halfinductance point upon said included inductance between said anodes, and coupling means coupling said resonant circuit and said choke-coil with modulated alternating voltages, said bridge circuit including a second resistance connected in series with said choke-coil, and having one terminal connected to the half-resistance point up-' on said first resistance, and the common Junction of the other terminal and choke-coil connected to ground, a third resistance connected to each terminal of said first resistance through a condenser, and having the half-resistance point of said third resistance connected to said common junction, and a fourth resistance with one terminal connected to ground and the other terminal connected through a condenser to said half-resistance point upon said first resistance.

8. The combination'with the structure ofclaim 7, of a thermionic compensator-tube having an anode, a cathode and a control-grid, a source of unidirectional voltage with positive terminal connected to said compensator-anode, and negative terminal connected to ground, and to the com pensator-cathode through a bias resistance, and connections between said compensator controlgrid and compensator-cathode including connections to said second resistance, said coupling means including means coupling the anode-cathoilie gift said compensator-tube with said resonant c to said demodulator having a double-diode, a resonant circuit, balanced-bridge connections to said double-diode including a first resistance connected across the cathodes of said double-diode, a second resistance connected across said first resistance through a condenser connected to each terminal of said second resistance, and having the half-resistance point thereon connected to ground, a third resistance connected from the half-resistance point upon said first-resistance to the half-resistance point upon said second resistance, and a fourth resistance connected to ground and through a condenser to the half-resistance point upon said first resistance, and

thermionic-tube means having a control-grid modifying the conductance of said resonant circuit, including connections from said third resistance to said control-grid.

10. The combination with a frequency-demodulator having a resonant circuit comprising an inductance coil shunted by a condenser, a chokecoil, a voltage-rectifying means including a first diode means in series with a resistance, a second voltage-rectifying means including a second diode means in series with a second resistance, and circuit connections including each diode-anode of said diode means conductively connectedto a separate terminal upon said inductance coil, a common junction between said rectifying means, and the conductive connection of said common Junction to an intermediate terminal upon'said inductance coil between said'separate terminals, through said choke coil, of a control resistance connected in series with said choke-coil, carrying the total thermionic currents of both said first and second diode means, whereby unidirectional'control-voitage is produced thereon, with magnitudes independent of frequency-modulation, and proportionalto the amplitudes of alternating currents impressed upon said resonant circuit.

11. A frequency-demodulator for converting I frequency-modulated alternating currents into amplitude-modulated voltages, said demodulator having resonant circuit means including inductance coil means, a double-diode voltage-rec tifying means connected with said inductance coil means, including a first diode-anode, a second diode-anode, diode-cathode means for said diodeanodes, a conductive connection from each of said diode-anodes to a separate terminal upon said inductance coil means, and conductive circuit connections from said diode-cathode means to an intermediate terminal upon said inductance coil means between said separate terminals, including a circuit through a control-resistance ,carrying the total thermionic currents of both said diode-anodes; a thermionic tube having a cathode, anode, and grid, and means for controlling alternating currents in said resonant circuit means including a coupling between said anode and said resonant circuit means, and connections between said grid and said control-resistance for modifying said alternating currents, to provide a constant ratio of conversion of frequency-deviations into output-voltages, independent of amplitude variations of said alternating currents;

12. The combination with a frequency-demodulator for providing amplitude-modulated output-voltages from impressed frequency-moduiated currents, of compensating means for compensating undesired variations of said outputvoltages due to impressed amplitude variations,

- said demodulator having resonant circuit means ode means serially connected with resistance means, said compensating means including a controi means for aontroliing alternating currents in said resonant circuit means, havmg a thermionic tube with a cathode, anode, and grid, a coupling means coupling said anode with-said resonant circuit means for modifying alternating currents therein, and conductive circuit connections between said grid and said cathode including circult connections with said resistance means s chronousiy applying said envelope-responsive voitages, to provide a constant ratio of conversion of frequency-deviations into output-vol ages, independent of variations of amplitude of said'impressed carrier currents.-

EDWARD H.- LANGE.

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