US2231996A - Frequency variation response circuit - Google Patents

Description

Feb. 18, 1941. LLA Em 2,231,996

FREQUENCY VARIATION RESPONSE CIRCUIT Filed June 5, 1937 2 Sheets-Sheet l INV ENT OR. gust-ave guanella BY max atl'mann ATTORNEY.

provision of an arrangement-responsive to: devia Patented Feb. 18, 1941 UNITED STAT ES PATENT OFFICE FREQUENCY VARIATION ansrons'n cmcmr Gustave Guanella; and Max Lattmann, Zurich,- Switzerland, assignors to Radio-Patents Corporation, Newfilork, N. Y., a. corporation of New York Application June 5, 1937, Serial No; 146,555 .In: Germany'March 26 .1936

The present invention relates to tuning circuits for radio receivers or the like andLmoreIJparticularly to a novel circuit arrangement for and: a method of controlling the tuning of radio systems so as to increase the sharpness and accuracy of the tuning adjustment.

An object of the invention is the" provision of an arrangement responsive to deviations of thetuning or natural frequency of-ia selective: circuit or network in a radio receiver from the frequency of a radio signal impressed thereon for indicating and/or controlling the tuning of thereceiver'.

A more specific" object of the invention is the tions of the natural or'tuningfrequencyof asei'ective circuit or network in a radio receiver: fromthe frequency of an impressedradio signal wherein the direction of detuning of the circuit whether below or above the signalling frequency is indicated.

Another object is theprovision of an. error'detector or tuning indicator for a radio'ireceiver or the likeoperating in accordance withia null method or compensation principle wherein the indication and/or control at exact resonance is zero as compared with the adjustment to. maximum signal in strength in the case of the known. tuning indicators heretofore provided in. radio: receivers.

A further object of the inventioniisthe provision of an error detectoror' discriminator arrangement embodied in a radio receiver adapted to supply a control potential varying: in accordance with the degree of tuning ofthereceiver' in respectto the frequency of an incoming? radio signal and being of opposite sign dependent on: whether the signal frequency is above or below the tuning frequency of the. receiver whereby such control potential may serve to operate: a'tune adjusting device to automatically correct the tuning: of the receiver to be in exact resonance with aniincomin'g radio signal.

Further objects and features: of the invention will become more apparent from the following detailed description taken with. reference to: the accompanying. drawings formingrpart of this specification and'wherein Figure-1 illustrates diagrammatically a basic circuit constructed and operatediaccordingto the invention,

Figure 2 is a modified arrangement of 7a circuit shown in Figure 1,

Figures- 3- and 4 illustrate practical; circuit arrangements employing rectifying; devices'for*pracdoing the invention,

electronic mixer valve for producing tune responsivepotential variations, Figures'fi' and '7 illustrate circuits comprising a bandpass filter or-netw'ork used as a tune responsive means: to produceindicating and/or control-v potentials according tothe invention,

Figure 8- illustrates arrangement similar to Figure? for producing-both a volume and tune indication in a receivenand Figure 9 illustrates an alternative method of practicing the invention utilizing rectifying debike reference characters identify like parts throughoutthe diiferent views of the drawings.

It'is common: practice in modern radio receivers to provide a station indicator to facilitate exact tuning but despite such provision correct tuning of a receiver especially one equipped with an antifa'ding orvolume control arrangement is by no means an easymatter to be accomplished by un-'- skilledrpersonsl' One of the undesirable results of inexacttuning resides in a non-linear distortion of the demodulated. or low frequency currents caused by an unsymmetrical position of the 'modulatingside bands with respect to the central frequency of the pass band of the selector circuits normally usedin radio receivers of present day construction. It is therefore desirable to provide an arrangement for indicating both the magnitude and direction of detuning ofthe receiver.

It has already beenproposed to provide what may be termed semi-automatietuning systems or systems for automatic fine adjustment of the tuning wherein accurate or fine tuning of a receiver to an incoming. carrier signal is effected automatically after the receiver has been brought approximately in tune-manually with the incoming signalf'requency such asby operating an ordinary ormechanical dialor thelike for roughlyselecti ing the: station which it is desired to receive;

In one typeofsemi-automatic tuning: systems, especially adapted for-superheterodyne receivers, .two resonant circuits are provided at a suitable point -in the intermediate frequency amplifier one .45

supersonic frequency which are selected by tuned 5 designed. The currents in the two circuits are then rectified and differentially combined to pro--. duce a resultant direct current potential whose magnitude and sign will be a measure of the extent and direction of the detuning of the receiver;

This potential may be applied to an indicating in strument thereby to indicate the extent and sense of detuning or it may serve to effect an automatic tuning adjustment by controlling a suitable tune adjuster device such as a thermionic valve suitably associated with the oscillating circuit of the local oscillator to determine the frequency thereof in a manner well known. In this manner the tuning of the receiver will be automatically regulated so as to correspond exactly with the predetermined intermediate frequency for which the receiveris designed, it being necessary for the operator or listener only to bring the tuning'approximately to the point of resonance such as by means of a mechanical dial or the like.

A disadvantage of this type of error detector or discriminator for tuning indication or automatic tune adjustment is due to the necessity of two exactly and sharply tuned resonant circuits provided somewhere within the intermediate frequency section of the receiver and to the fact that it is the difference between the natural frequency of the two resonant circuits and the fixed intermediate frequency of the receiver which determines the magnitude of the control potential. The two circuits in practice do not operate at their resonance points and are detuned relative to the intermediate frequency of the receiver, as by a few kilocycles and accordingly if they are directly coupled with an intermediate frequency stage from which they are to be controlled they will im-' 45 pose a considerable damping effect upon this stage resulting in a substantial impairment of the accuracy and sharpness of tuning. For this reason the circuits are usually energized through a separate coupling valve provided for this purpose 50 which in turn involves increase of parts and bulk as well as cost of the receiver.

By the present invention the above drawbacks and other defects of fixed and slightly detuned resonant circuits acting as a discriminator or error 55 detector are substantially eliminated by a novel arrangement and method utilizing the phase difference of the current in a resonant circuit at predetermined points of the circuit with variations of the frequency of the currents, to derive a potential 60 which will depend upon the extent and direction of detuning of the circuit with respect to the impressed frequency. As is well. known, the total impedance of a resonant circuit is equivalent to a I pure ohmic resistance when the circuit is exactly 65 in tune with the impressed frequency. For a slightly higher frequency the circuit presents a capacitative reactance and for a slightly lower frequency the circuit presents an inductive reactance. If the impressed frequency is in exact 70 resonance with the tuning or natural frequency of the circuit the input current is exactly 90 out of phase or in'phase quadrature with the oscillatory current developed within the circuit. In the case of detuning this phase displacement becomes 75 greater or smaller than 90. 1

In carrying out the invention both series or parallel tuned circuits or selective networks such as band selectors or any other complex circuit arrangement may be used to produce indicating and/or control potentials dependent on both the extent and amount of detuning of the circuit. Thus for example in a bandpass type of filter the mutual phase position of the potentials at the input and the output ends is in one direction, say in a negative direction where the applied frequency is low, in the case of resonance the mutual phase relation becomes zero and then changes to the other direction (positive) when the applied frequency istoo'high. It is thus possible, in carrying out the invention, to use either simple or complex resonant circuits already existent in .the receiver, preferably resonant circuits of the band selector type in order to obtain a desired extent of phase shift for a given amount of detuning for tuning control and/or indicating purposes.' However, if desired special resonant or selector circuits may be provided for the tune indication or adjusting system fed from an incoming RF or IF signal in any suitable manner as is understood.

According to one embodiment and feature of the'invention, a so-called watt meter type circuit or a modulating or mixer device is employed to produce a direct current potential from a pair of alternating currents varying in accordance with the degree of detuning or mutual phase relation of the currents and having a direction dependent on the sense of detuning of one current with respect to the other. In a watt meter or modulating type arrangement there is obtained a direct cur-- rent component or potential proportional to the product of the amplitudes of the two currents or potentials impressed upon the device and to the cosine of the phase angle therebetween, thereby enabling the generation of a tuning error or discriminating potential for tuning indication and/or automatic control for the purpose of the invention.

A basic circuit arrangement of the type according to the invention is illustrated in Figure 1. In the latter there is shown a multiple tuned circuit comprising a condenser l0 shunted by an inductance II in series with a non-reactive impedance such as an ohmic resistance i2. This circuit may be connected anywhere in a radio system and may constitute for instance the output circuit of an amplifying valve in the IF stage of a superheterodyne receiver in which case the upper terminal a of the circuit may be connected to the anode of the valve while the lower terminal is connected to the positive pole of an anode potential source indicated by the plus sign in series with a further non-reactive impedance [3 provided in accordance with the present invention. The voltage drops E1 and E2 developed across the impedances l2 and I3 are ap lied to a watt meter type or modulator device l4 adapted to produce a combined direct output potential E at its output terminals 1) and o varying in accordance with the degree of detuning of the incoming frequency relative tothe natural frequency of the circuit H), H, l2 and having a sense dependent on whether the detuning of the circuit is below or above the impressed signal frequency.

In an arrangement of the above type the oscillatory current is is displaced by 90 with respect to the total current is flowing into the circuit if the impressed frequency is in exact resonance with the natural or tuning frequency of the circuit. If the voltages E1 and E2 set up across the resistances I2 and I3 by the currents in and 7:21, respectively,

gear-sec are combined or mixed;m-the arrangement ll, a direct output potential E is obtained in the'ma'nner described which is zero in thecaseof'resonance when the currents are-in phase quadrature and which varies towards-positive andnegative values if the phase of the currents is less'or more than 90 respectively, i. e., according to the degree of detuning of .the circuit relative-impressedsig nal frequency. Device I4 maybe ofanytknown type such as a watt meter type or balanced modu lator comprising in the example showna pairj'of input transformers I5 and I5", the former serv-' ing to excite the grids of a pair of electron valves I6 and I6" in opposite phase by connecting 'the ends of its secondary each to one of thegrids, and the latter excitingthegrids of valves I6 and 16" in likephase by connecting its secondary between the common cathode of the valves-and the center tap of the secondary of transformer" I5' The anodes of the valves are connectedthrough load resistances I7 and IT to the positive terminal of an anode or B-supplysource on the onehand and to the output terminals b, c on the other hand. There is obtained in this manner by mutualmod'ulation or product formation ofthe impressed inputpotentials E11, E2 an output potential E'at terminals 1), c varying both in sign and magnitude in dependence upon variations of the mutual phase relation between the input potentials in either sense from a normal'90 phase relation as native arrangement similar to Figure 1 wherein the potentialE1 is derived from the drop across the inductance II of the resonant circuit. In this case the potentials E1 and E2 are in-phase in the case of resonance and in order to obtain an output potential E varying in accordance with the phase relation of the currents and being zero in the case of resonance the device I4" must be of the type and operated according to a reactive power watt meter. Alternatively one of the potentials E1 or E2 may be phase shifted by 90 by means of any suitable phase shifting device of well known type in which case an ordinary watt meter or mixer arrangement may be employed. An arrangement of this type is shown in Figure 2 wherein the modulator I4 is of the reactive watt meter type differing from the arrangement according to Figure 1 by the provision of a quadrature phase shifting network in the secondary circuit of the transformer I5. This network in the example shown comprises a resistance I8 and a condenser I9 in series. By proper design ofth'elatter, the potential across condenser I9 will lag the current by 90 in such a mannerthat a tune responsive output potential is obtained at the terminals b, c which is zero or a minimum in the case of resonance and increases in a positive ornegative direction in proportion to the degree of detuning of the resonant circuit in either sense relative to the impressed signal frequency.

In the accompanying Figure 3 there is shown a circuit arrangement for practicing the invention employing rectifying devices which may be metallic rectifiers, valve type rectifiers or the like. For this purpose the inductance II is provided with a secondary I5 and a further transformer is inserted in the lead from the resonantcircuit to the high potential source comprising a primary I6 and a secondary IT. The potential Eris thus developed across the secondary I51and the potential E2 is developed across the secondary I]. The

secondaryI5 islconnected in series. with a pair of rectifiers I8 and" I9 to an ohmic or non-reactive impedance 20; while the center taps or the secondary I5 and the impedance 20 are connected through the secondary I! in the manner shown. Preferably the portions of the impedance 2B are shunted by smoothing condensers Zi'toimprove the steadiness of the controlpotentials obtained at theoutputterminalsb and 0. Assuming that the circuit lll; II presents'a high ohmic resistance as'in the case of resonance. the secondary potentials' Ei and Eamaybe represented by the followingtheoretical equations well known to those versed in the art:

In these expressions, M1 and M2 represent the mutual inductances of the transformers and w=2f1rf.' I Thusit is-see'n thatthephase difierences' are retained'in; thei potentialsEi and E2. The resistance 2I'I' is preferably'given a high value.

Referring-to Figure 4 there is shown a-modification'ofa circuit described by Figure'3. According to Figure 4 the rectifiers I8 and I8 are connected in series and in opposition across the secondary I5 through a pair of condensers 2| and 22 and the secondary I1 is connected between the center point of the secondary I5 and the junction point of the rectifiers. As-isunderstood, the function and operation of this circuit in combining or mixing the currents set up by the potentials E1 and E2 is substantially similar to Figure 3.

Thecircuit so far illustrated and described involves the comparison of the amplitudes of two alternating potentials or of the vectorial sum and difference of the two potentials derived from different' points in an oscillatory circuit and rectified to obtain corresponding direct current potentials for tuning indication and/or control. A great advantage of arrangements of this type is the fact that even if the circuit or deviceenergized by the final resultant. control potential has a relatively small resistance any additional damping imposed upon the oscillatory circuit disappears as soon as the exact tuning is reached; that is, when the output potential is zero.

In place" of a modulating or mixing arrangement utilizing dry rectifiers or the like, any other phase indicator circuit arrangement such a a thermionic: valve or the like may be employed for the. purpose of the invention. Thus, referring toFigure 5 of the drawings there is shown an electronmixer valve 25 having a cathode 26, a first grid 21', a second grid 28 surrounded by shield grid; 29, and an. anode 39'. As is well known in valves of this and similar construction in which two-control grids are provided, shielded from each other by a positively biased screen, the electron current may be controlled separately and independently at two or more points so as to produce a direct modulation or product combination of the impressed potentials in the output circuit of the tube. In the present example the secondary voltage E1 supplied by the winding I5 is impressed upon the electron path between the cathode and the grid. 28 while the potential Eaproduced by the secondary I I isimpressed upon the path between the cathode 26 and the grid 21. In this manner the two potentials are mixed and a steady output potential: E produced varying in accordance with the degree of. detuning orzmutual phase relation of the.-.impressed'potential and having a sign which is. either positive or negative in respect to a normal meamvalue. Item 28. is a source of space ourrent and items 29' and 30' a load resistance and by-pass condenser across the output terminals b-c adapted to develop output potential in substantially the same manner as shown in the remaining diagrams.

In the examples so far illustrated, simple tuned circuits have been shown but as pointed out the invention may be practiced in using any type of selective system or network. This is shown in Figure 6 where a so-called band pass circuit is provided comprising a primary circuit having an inductance I l shunted by the capacity It) and a secondary circuit having an inductance l5 shunted by a capacity 32. The inductances H and 15 are in mutual coupling arrangement thereby forming a band pass or filter circuit of well known type. The circuit may serve as a tuned coupling transformer in the anode circuit of an intermediate frequency amplifier or may be arranged at any other suitable point in a radio receiver. The lower points of the secondaries l I and I5 are shown connected through a coupling condenser 33, while the upper terminal of the input circuit 10 and H is connected to the grid 28 and the grid 21 is controlled by the secondary circuit |5, 32. The phase of the output potentials of such a band pass filter varies substantially in regard to the phase of the input potential, the variation in the vicinity of the predetermined intermediate frequency being from zero to 180. In the case of resonance the phase difference is equal to 90 and the two. potentials may therefore be compared and utilized to produce a resultant control potential in substantially an analogous manner as described by the previous illustrations. In this arrangement the additional damping imposed by the output current is zero when the detuning is zero, an important feature if the band filter is used in a normal manner as a frequency selective circuit in the receiver.

In Figure 7 there is shown another circuit employing a band pass filter and a pair of ordinary rectifiers 31 and 38 as modulating devices. The latter are connected in opposition across the secondary of the band pass filter through the condensers 34 and 36. The junction point between the rectifiers is connected to the terminal a of the primary tuned circuit 10, II through a condenser 35 and the electrical center point of the secondary inductance i5 is connected to the lower terminal of the primary circuit l0, l1 through the condenser 33 which latter also serves as a blocking condenser if terminal 01 is connected to the grid of a secondary amplifier stage. In this manner the circuit for the modulating or watt meter arrangement is completed similarly as in the previous figures.

The output terminals, b, c are connected to the rectifiers through smoothing resistors 39 and 40 with a decoupling condenser 4| being provided across the output terminals.

According to a further feature of the invention, a phase or tune responsive arrangement for indication or control of the tuning may be combined with an automatic volume indicator varying in accordance with the unmodulated or carrier component of a modulated signal for either indication or automatic control of the volume as known in automatic volume control or anti-fading systems. An arrangement of this type is schematically shown in Figure 8 which is otherwise substantially identical to Figure 7. In Figure 8 the operating potential for a station indicator or for automatic volume control is derived from terminals e, connected between the junction point of the rectifiers 31, 38 and the center of the resistance 25] across the output terminals while the potential for thetuning indication or adjustment is supplied from terminals 1), c in a manner as described hereinabove.

A very satisfactory form of optical tuning indication is obtained if points e and f are directly connected and the voltage between points ,f and c is applied to one indicator and the voltage between points I and b is applied to another indicator arranged adjacent thereto. In an arrangement of this type the total deflection or indication of both indicators is a measure of the amplitude of an incoming carrier wave, while the diiference between the two indications or deflections is a measure of the degree of detuning. If the indicators are such as to be responsive to high frequency such as in the case of glow discharge lamps no special rectifiers are required. Thus, referring to Figure 4 the latter may be modified by substituting two glow devices for the rectifiers shown and by connecting the leak resistance 20 directlybetween the junction of the rectifiers and the terminal 17 or between the junction of the rectifiers and the terminal 0. In any one of the arrangements i1- lustrated, one or more of the elements or circuits such as band pass filters or dry or valve rectifiers may be combined with already existing devices and parts provided in the receiver.

Referring to Figure 9 there is shown a further arrangement for practicing the invention comprising a Wheatstone bridge arrangement associated with the oscillatory input circuit. There is shown a bridge comprising rectifiers 4|, 42, 43 and 44. The diagonal points m, n of the bridge are connected to the lower terminals of the condenser lil and the inductance II, respectively, while the remaining diagonal points 0 and p are connected to the output terminals 1) and 0 across which is placed a center tapped resistance comprising equal parts and 46 shunted by condensers 41 and 48.

The operation of an arrangement of this type is as follows: The path for the oscillatory current during one-half cycle is through the rectifiers 4| and 42 as indicated by the full arrows and during the other half cycle through the rectifiers 44 and 43 indicatedby the dashed arrows. The valve circuit in whose anode circuit the tuned circuit I0, I I is assumed to be connected is completed via the bridge points 0 and p and the center tap 1 between the resistances 45 and 36. The rectifiers between 11. and p and m and p are simultaneously non-conducting during the first half cycle of the oscillating currents in which case no current can flow from the outside via the point p to the rectifier bridge. Simultaneously during the first half cycle the point 0 may be regarded as connected electrically with points 111. and n so that a total current flows to the bridge and the oscillatory circuit via this point during the first half cycle. The same applies as regards the point p during the second half cycle. Thus if the total current is. is in phase with the oscillatory circuit is the positive half wave of the total current in each case will flow through 0 and during the negative half cycle through p in such a manner that the condensers 41, 68 are charged to the total direct potential E. If the oscillatory current is out of phase with the total current, the sense of E changes. In the case of 90 a resonance phase displacement between the oscillatory current and the total current the resultant of the total cur rent through b and through c is equal to zero. This circuit arrangement offers the advantage of great simplicity for only one tuned circuit is required and this may be the same circuit as is used for ordinary frequency selection purposes in the stage in question of the intermediate frequency amplifier. This arrangement does result in a slight additional damping of the oscillatory circuit, but this defect is outweighed by its simplicity. It is to be recommended when small tuning indicating potentials are all that is necessary.

Where the present invention is employed for automatic fine tuning, the actual utilization of the tuning control potential to effect tuning variation may be effected in any well known way, e. g. in the case of superheterodyne receivers it may be used to vary the local oscillator frequency by varying the dynamic grid capacity of a thermionic valve connected in parallel with the normal tuning condenser in the circuit determining the local oscillating frequency. In such a case the other tuning circuits of the receiver, i. e. the intermediate frequency circuits and the high frequency preselection tuning circuits, if any, do not require, any control. In some cases, for instance in the case of straight or TRF receivers automatic control of tuning circuits may be effected by means of a motor controlled by the tuning control potential. In such a case automatic tuning correction should be applied to all the resonant circuits of the receiver including the circuit from which the tuning control potential is obtained.

The automatic fine tuning methods of the invention are obviously not limited to their application to ordinary broadcast receivers; for example, they may be applied to television receivers, in which, as is well known, exact tuning to an incoming carrier wave is of great importance.

The invention is well suited for use in connection with short wave and ultra short wave superheterodyne receivers, for her-e great difiiculty is commonly experienced with known receivers, in obtaining and maintaining accurate tuning.

It will be apparent from the above that the invention is not limited to the specific circuits and arrangements of parts shown and described herein for illustration, but that numerous variations and modifications may be made coming within the broad scope and spirit of the underlying novel idea and inventive concept as defined in the appended claims. The specification and drawings are accordingly to be regarded in an illustrative rather than a limiting sense.

We claim:

1. A frequency variation response circuit comprising a resonant network having a predetermined resonating frequency, means for feeding radio frequency signal energy to said network, means for deriving a pair of signal potentials from such points of said network that the phase angle between the derived potentials will vary in either direction from a normal phase relation both according to the sense and in proportion to the amount of detuning of said network relative to v the signal frequency, an electron tube having a cathode and an anode to produce an electron space current, a pair of control grids located in the path of said space current, a further grid located between said control grids, means for maintaining said further grid at a positive potential with respect to said cathode, means for impressing each of said signal potentials upon one of said control grids, an output circuit for said tube, and load means operatively associated with said output circuit to develop energy having an amplitude varying proportionately to the phase angle between said impressed potentials.

2. A frequency variation response circuit comprising a resonant network having a predetermined resonating frequency, means for feeding radio frequency signal energy to said network,

' means for deriving a pair of signal potentials from such points of said network that the relative phase angle between the derived potentials will vary in either direction from a normal phase relation between the derived potentials according to the sense of and proportionately to the amount of detuning of said network relative to the signal frequency, an electron tube having a cathode and an anode to produce an electron space current, a pair of control grids located in the path of said space current, a further grid located between said control grids, means for maintaining said further grid at a positive potential with respect to said cathode, means for impressing each of said potentials upon one of said control grids, an output circuit for said tube, and load means operatively associated with said output circuit to derive energy having an amplitude varying proportionately to the phase angle between said impressed potentials.

3. A system as claimed in claim 2, wherein said network is a parallel tuned'resonant circuit and said potentials are derived from the circulatory current flowing within said circuit and from the total input current to said circuit.

4. A system as claimed in claim 2, wherein said network is a band-pass filter comprising a primary and secondary tuned circuit inductively coupled with each other, said potentials being derived from points of said primary and secondary circuit carrying potentials at 90 phase difference when said network is in tune with the impressed signal frequency.

GUSTAVE GUANELLA.

MAX LA'ITMANN.

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