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July 16g-1946, w. É. @äADLEY l . `4 2,404,270 BAND PAssfwAvE FILTERy Filed .my 24, 1942 ' „Jus-me45 ZÉ /MPEo/mc: OUTPUT Patented July 1_6, 1946 v2,404,270 " UNITED STATESA PATENT 'oFFics BAND PAss WAVE FILTER William E. Bradley„ Northampton, Pa.,4 assignor to Philco Radio and Television Corporation, Phila-. _ delphia, Pa.„ a corporation of Delaware .Application July 24,1942-, serial No. 452,229 6> Claims. (o1. 17a-44) 1 networks’ or wave filters, and more particularly to a band pass coupling network having a plurality of substantially non-interacting coupling adjust ments. ' 2 and Without any sacrifice in the gain of the stage. It is a further object of the invention to‘pro vide a Wide-band coupling network capable of giving substantially the maximum gain obtain This invention relates to` band pass coupling able in a coupling network between tWo vacuum ’ An important class of band pass wave ñlters frequently employed in the field of communica tions isthat which utilizes transformers of the y a wide-band coupling network having a novel double-tuned, over-coupled type. Transformers combination of impedance and admittance cou of this character are commonly employed in the 10 pling. intermediate frequency stages of conventional superheterodyne receivers adapted for use in the reception of amplitude modulated broadcast sig nals. While these transformer coupled systems tubes. _ ` It is another object of the invention to provide - The invention itself, as well as other objects thereof` will -be understood from the following description and accompanying drawing, in which: Fig. 1 is a generalized schematic diagram of the have been generally satisfactory in certain limited 15 coupling system which comprises the present in and conventional applications, including that just referred to, there are other applications in which the transformer coupled system leaves much to vention; ' 1 Fig. 2 is a schematic diagram of a specific ern lbodi'ment of the invention; and Figs. 3 and 4 are explanatory diagrams which be desired. Included in these applications are cascaded wide-band systems wherein it is de 20 illustrate the operation and performance ofthe coupling circuit ofthe invention. sired to obtain a particular one of a wide variety Reference is now made to the generalized sche of response characteristics simply and reliably. matic of Fig. 1 in which a double-tuned com In systems such as these it is difficult, and some pound-coupled filternetwork is illustrated hav times impossible, for the: designer to obtain de sired response and phase characteristics with 25 ing an impedance coupling element Zr and an conventional over-coupled transformers, and this admittance coupling element Yh. The primary circuit of the network Lcomprises an inductance L1 tuned by a tuning condenser C1 and damped acteristic is asymmetrical. All of these difficulties by a damping resistor R1. The secondary circuit are aggravated in production where it is impor tant that all tuning operations be capable of 30 comprises the inductance L2 tuned by a tuning condenser Cz and »damped by a damping resistor quick, simple, and effective adjustment. Rz. The primary and secondary circuits are pref By the present invention a compound coupling erably isochronous, i. e., tuned to thesame fre system is provided which permits of substan is especially so where the desired response char tially individual control and adjustment of both the high frequency and low frequency halves of the response characteristic. In over-coupled cir cuits exhibiting two resonant peaks, the inven quency. tion permits of individual» peak adjustments R1 and R2 are the tuning con densers C1 and C2 respectively, it is to be under stood that, alternatively, they may be connected t which are- substantially independent of each in shunt with the inductances L1 and Lz, or if , other, and which can be made as readily as the adjustments of single-tuned circuits. Circuits of this character are especially adapted for use in wide-band systems, such. as the intermediate fre' quency stages of television and frequency ymodu lati-on receivers, and in special. applications which may require response characteristics of an asym metrical character. It is a principalobject of this invention to pro vide a band pass interstage coupling circuit with which a wide variety of response characteristicsä may be obtained simply and reliably. It is another object of the invention to provide an over-coupled interstage coupling circuit in which it is possible to adjust either peak of the While the damping resistors se shown connected in shunt with desired the damping resistors may >be eliminated entirely, the required dissipation of energy asso ciatecl with the condensers being achieved, for example, by winding the inductancesv L1 and L2 >of a suitable resistance Wire. , Coupling between the primary and secondary circuits is provided, firstly, by means of an imped ance Zr connected between the common junction ofi-.11 and Leon the one hand'andthe low poten, tial ends of the condensers C1 and C2 on the other hand, and secondly'by `means of an admittance Ye connected between the high potential ter minals of the primary and secondary circuits. Although it is within the general purview of the invention tol make all of ’the elements of. Fig. 1 two-peak response characteristic independently, 'I‘ 65 adjustable, it is within the special province of the 2,404,270 , 3 Y . invention that at least one, and preferably both, ofthe coupling devices Yn and Zk be adjustable. If the admittance coupling element Yu and the impedance coupling element Zk are properly Aad justed, a symmetrical frequency response char-l ' acteristic can be obtained which is similar to that provided by a conventional double-tuned over coupled transformer-system, for. example, sim the frequency of the high-frequency peak as il lustrated in Fig. 3. MoreoverV the height of the high frequency peak may be varied byV adjust ment of the shunt damping resistor Rh to pro duce various response characteristics such as thoseV illustrated in Fig. 4. Adjustments of the induct 'ance Lk and resistor Rk produce similar changes in the position and height ofthe low frequency ilar to thatf'illustrated‘by the curve (v_-a ‘of Fig._'3. peak. Throughthe adjustmentgofthe >elements .The novelty of the'present invention resides in 10 which Ycomprise the impedance coupling and ad the fact that the compound coupling provided by _ lmittance coupling members, an extremely wide> the two coupling elements Yh and Zk permits the. ».»S-.variety of response characteristics may be seindividual peaks of the double-humped response < ' acured, anda desired response characteristic may characteristic to be individually shifted in fre _readily bev duplicated in production by means of quency and varied in height or relative response,V4 1s. tuning adjustments which are individually as sim Yeach rof the adjustments being substantially in-v y pleas those` normally made in single-tuned cir dependent of the other, and having substantially cuits of the simplest character. no eiïectupon the shape of the other peak. If, In coupling systems designed to have a sym metrical response characteristic, i. e., in which justable inductive admittance, then adjustment 20 the heights of the two resonant peaks are equal, of the imaginary part ofthe admittance, i. e., the the real components of Yh andZk, the elements susceptance, will shift the frequency of the` high Rh and Rk respectively, may be omitted.v In such for example, the element Y1;` comprises an ad- - ' frequency peak as shownffor example, by the ì curves-a-b and a-d in Fig. 3. applications only the input damping resistor VR1 On the other and the output dampingy resistor R2 need be em hand if the rea1 part (the conductance) of the 25 ployed. In this case the circuit should be designed admittance Yh is varied, the response or height of the high frequency peak will be affected as is il so that the ratio of R1 to R2 is equal to the ratio lustrated in Fig. 4 by the‘curves e-f, e--y, .and of C2 to C1. Thisarrangement will keep the peak heights substantially equal,»even when C1 and C2 c-h. Adjustment of the imaginary and the real are unequal, for all settings of `L11 and Lk.l parts, the reactance and resistance respectively,r of the impedance coupling element Zk will have In the schematic diagram of Fig; 2 adjustable .damping resistors have been associated vwith both similar effects upon the shape of the low-fre quencyresponse peak. generality,»and suchan arrangement may be used The invention also contemplates the use of ca pacitive-coupling elements, and if this change is ~ in practice where it is necessary frequently to vary the relative heights of the'two response peaks. In made the coupling element Yh will control the position- and height of the low frequency peak, whilethe coupling,y element Zk will control the position and’height of the high frequency re most practical applications,- however, it is pre- ' Lh and Lk. This has been done for the sake of ferred to employ only one of the adjustable damp ing resistors, and then only in conjunction- with a predetermined one of the vtwo coupling> ele sponse peak.h It has been found however that the 40 ments to produce the required asymmetry. V use of inductivef‘coupling elements provides a - Although the _invention is not limited toany higher gain than can be obtained with the use of particular method of tuning the inductances Ln capacitive coupling elements, and for this reason and Lk, it is preferred that this be accomplished ¿ inductive `coupling elements are preferred. ‘ by means of adjustable copper or powdered iron Reference is now made `to Fig. 2 in whichthere Y, 45 cores in a mannerwhich is well understood in is illustrated a specific embodiment of the in vention employed as a coupling network between ‘two amplifier tubes V1 and V2. In this partic the art. ' , - The damping resistors Rh and Rk »may be as sociated with their respective inductances in any desired manner to produce the required damping ular instance the inductive `rform of coupling has been chosen for illustration. In Fig. 2 the ad 50 effect, but it is preferred that the damping re justable inductance L11 and the vadjustable re sistor R11 be connected in parallel with the cou sistor-R11 comprise the imaginary and real parts, pling inductance L11, and that the damping re respectively, of the coupling element Yh of Fi'g..1. sistor Rk be connected in series with its induct Similarly'the adjustable inductance Lk andthe ance Lk. Likewise, if desired, the coupling in adjustable resistor Rk comprise the imaginary and 55 ductances Lk and'Lk may be replaced `by adjust- . real parts, respectively, of the coupling element able condensers of suitable size. i f Zk. In the circuit'chosen for illustration, the It has' been found that the frequency range of primary and second tuning capacities, C1 and C2, the resonant peak controlled bythe- inductance may be provided in whole, or in part, by the out Lk is larger, ordinarily,qthan that controlledby put capacity of the amplifier V1 and the input 00 the inductance Lk, becauseV the inductance of L11 capacity of the ampliñer V2, respectively.. Plate in practice is usually larger than the inductance voltage for the ampliñer V1 may be supplied from Lk. In consequence the inductance L11 tends to 'a highpotential source B+, through a ñlter com resonate with its >distributed capacity, making the prising a series resistor R3 and a shunt condenser apparent inductance of this l coupling element C3, and thence through the damping resistor R1, 65 larger than the actual inductance of the coil. ' to the anode of V1. In order to isolate the'input Because of Vthis self ¿resonantl action of Ln it is grid of the tube V2 from the high voltage applied possible to control the high frequency Vpeakfover to the plate of V1 a direct current blocking con a considerable frequency range. "It is not so easy denser C4` may be provided as shown. VA suitablev t0 obtain wide control of the' :low frequency peak, bias, or automatic volume control voltage, 70 because the value of Lk is not` lonly much smaller maybe supplied to the grid circuit of V2 through than rthe other inductances involved, -but its leak . -a filter comprising the series resistor R5 and the age inductance is `relatively large. This situa `shunt condenser C5 as illustrated. ’ tion can be relieved by the Vprovision of some' With the form of circuit shown in Fig. 2„ ad mutualy inductancezbetween4 L1 and L2 so phased justment of the inductance L11 effects changes in 76 :as to produce the opposite signof inductive cou / 2,404,270 5 6 , cumstances Lk can be increased to a larger value, with the result that its tuning range is greatly extended. Y ary circuits, the degree of over-coupling being such that a double-peaked frequency-response characteristic is achieved; one of said coupling elements including a real component, the magni tude of which determines the height of one of said peaks vwithout substantial effect upon the pling to that produced by Lk. Under these cir ' The coupling network of the present invention is especially well suited for use in multi-stage amplifier systems, in which a plurality of indi height of the other of said, peaks. K . I 2. A band pass coupling network, as claimed vidual stages (e. g. five or six), each constructed in claim l, characterized further in that, in addi in accordance with the invention, may be con nected in cascade. In such a system it is often 10 tion to the coupling provided between said pri mary and secondary circuits, said primary and desirable to “stagger” the tuning of the various secondary windings are coupled by means includ stages, i. e., to peak the individual circuits at pre ing mutual inductance, said mutual inductance being of such sign as to oppose the coupling ef a large number of stages, each having charac teristics similar to that of the curve a-a of Fig. 15 fect of said impedance coupling element. 3. A -band pass coupling network comprising: 3, but each with its characteristic displaced in a primary circuit including a primary Winding frequency a predetermined amount, may be con and a primary tuning capacitance; a secondary nected in cascade to provide a wide-band, sub circuit including a secondary winding and a sec stantially iiat-top, characteristic having greater determined different frequencies. For example, overall gain and a more‘desirable overall phase 20 characteristic than could be obtained’by means of an equal number of identically-tuned ñat topped circuits connected in casca/de. In other instances it may be desirable to combine ampli iiers having characteristics similar 'to the one 25 designated e-g in Fig. 4 with amplifiers having characteristics of the type designated e-h. In all of these multistage applications, the feature ondary tuning capacitance; a first variable-in ductance coupling coil; means for connecting said primary winding, said primary tuning ca pacitance and- said coupling coil in a closed series circuit; means for connecting said secondary winding, said secondary tuning condenser and saidcoupling coil in a second closed series cir cuit; and ra second variable-inductance coupling coil connected between predetermined high p0 tential points on said primary and secondary of the network which makes it possible to adjust independently either peak of the two-peak re 30 windings; the inductance of said coupling coils Y « being of a magnitude such that an over-coupled sponse is of great importance.> This makes pos condition is effected Abetween said primary and sible simplified production adjustment of the secondary circuits, the degree of over-coupling stages, the various trimmers being “peaked” at being such that a double-peaked frequency-re definite frequencies with a signal generator and sponse characteristic is achieved, the position of output indicator, the correct response character said peaks along the frequency axis being sub istic then following automatically. , stantially independent functions of the adjust While the invention has been described with ment of said coupling coils, ` particular reference to a speciñc embodiment and 4. A band pass coupling network as claimed in ' ‘to a particular mode of operation, it will be un derstood that ythe kinvention is capable of general 40 claim 3, characterized in the provision of resistive means in damping relation to at least one of said application and is adapted to other forms of phys coupling coils, the resistance of which determines ical expression and is, therefore, not to be limited the relative height of said peaks. ` 5. A band pass coupling network as claimed in to the speciiic disclosure, but only rto the scope of the appended claims I claim: 45 claim 3, characterized in that, in addition to the 1. A band- pass coupling network comprising: coupling provided between said primary and sec ondary circuits, said primary and secondary windings are coupled by means including mutual inductance, said mutual inductance being of such ondary tuning capacitance; and an impedance 50 sign as to oppose the coupling effect of said iirst variable-inductance coupling coil, and thus to coupling element connected in .a circuit which is permit the use of a larger-than-normal ñrst cou common to both said windings, said impedance pling inductance, thereby to increase the fre coupling element having an adjustable imaginary quency range obtainable through adjustment of component; an admittance coupling element con a primary circuit including a primary winding and a primary tuning capacitance; a secondary circuit including a secondary winding and a sec nected between a high potential point on said primary winding and a high potential point on said secondary winding, said admittance cou pling element having an adjustable imaginary component; said coupling elements being charac terized in that they are non-resonant, and of 60 such magnitude that an over-coupled condition is effected between said primary and second ' said coupling coil. 6. A band pass coupling network as claimed in claim 3, characterized in the provision of resis tive means for damping said primary circuit, and independent resistive vmeans for damping said secondary circuit. - WILLIAM E. BRADLEY.