US2216997A

US2216997A - Band-pass selector system

Info

Publication number: US2216997A
Application number: US70172A
Authority: US
Inventors: Harold M Lewis
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1936-03-21
Filing date: 1936-03-21
Publication date: 1940-10-08
Anticipated expiration: 1957-10-08

Description

BMD-mss sELn-c'ron jsxsfru I Filed uarcn 21. 195s 2 shuts-sheet 2 INVENTOR. HAROLD M. LEWIS BY a@ t ATTORNEY.

Patented Oct. 8, 1940 PATENT OFFICE BAND-PASS SELECTOR SYSTEM Harold M. Lewis, Douglaston, Long Island, N. Y.,

asaignor to Haleltine tion of Delaware Corporation, a corpora- Application March 21. 1936, Serial No. 70,172 6 Claims. (Cl. 179-171) This invention relates to high-frequency bandpass selector systems and. more particularly, to such systems in which the width of the frequency band transmitted through the system is adjustable. While band-pass selector systems constructed in accordance fwith the present invention are of general utility, they are particularly suitable for use in radio receivers of the wellknown superheterodyne type` for controlling the selectivity and iidelity oi responseL of the receiver.

Band-pass selector systems of conventional design usually comprise a pair of resonant circuits tuned to the same orV different frequencies and suitably coupled inductively, capacitively, or by a combination of these individual couplings. The two circuits are commonly referredi to as input and output, or primary and secondary, circuits, depending upon which circuit is connected to the source of current of the high frequencies comprising the band to be transmitted. 'Ihe responsiveness of this type of selector system is substantially constant over a band of .frequencies in the vicinity of resonance of the individual circuits, while currents of all other frequencies are sharply discriminated against `and are attenuated to a substantial degree through the system. In general, the width of the frequency band passed by such a system may be varied either by changing the coupling between the two cir- Acuits or by adjusting the resonant frequencies of the two circuits to diiferent values. The coupling referred to is non-directive in nature, that is, either circuit may be made the input circuit and the other the output circuit without substantially affecting the characteristics of the system. Such a system is to beV contrasted with the vacuumtube coupling between the input and output circuits of a vacuum tube repeater, whereinthe coupling is primarily unidirective. c Band-pass selectors, wherein the non-directive Aform of coupling is employed, are, in general, open to the criticism that only mechanical or relatively complicated non-mechanical expedients are known for adjusting thewidth'of the frequency band to be transmitted. Further, the type of coupling referred to is inherently incapable of producing amplification of the transmitted frequencies in the coupling path between the input and output circuits'of the system.

It is an object of the present invention to provide an improved band-pass selector system of simple arrangement, in which the coupling between the input and output circuits of the system is directive in nature and which is capable of transmitting a .desired band oi frequencies' against au other rrevide an improved band-pass selector system, of

the type described, which is easily adjustable to vary the width of the frequency band transmitted by the system. n

More particularly, it is anobject of the invention to provide an improved band-pass selector system, of the type described, in which the width of the frequency band passed by the system may be varied by adjusting vthe value of a biasing potential applied to a control electrode of a vacuum tube connected in the system. Briefly stated, the above objects are attained in accordance with the present invention by providing a band-pass selector system comprising input and output circuits, resonant at frequencies within the pass band, coupled by directive coupling means in both the forward and the backward directions. 'I'he two coupling means together tend to provide a feed-back voltage in phase with the voltage across the input circuit at a frequency in .the vicinity of such resonant frequencies. Nondissipative phase-reversing means are included between one of the circuits and one of the coupling means for reversing the phase of the feed-back voltage. The coupling means provided are substantially less frequencyselective than the above-mentioned input and output circuits and cooperate with the input and output circuits to provide a coupling reaction between the circuits which is degenerative at frequencies in the vicinity of the resonant frequency and regenerative at frequencies above and below the resonant frequency. The coupling between the above-mentioned input and output circuits in either direction may be independently adjustable if desired. To secure this form of coupling, l elements having directive transconductance are included in both the forward and the backward coupling means. A vacuum tube having at least a'cathode, a control grid, and an anode is one well-known form of element having this characteristic. With this form of coupling between the circuits, either the forward or the backward coupling may be varied at will by varying the value 'of the transconductance of the element included in the associated coupling means. 'I'he independence of control of the two couplings makes it possible to adjust the width of the frequency band transmitted by adjusting the transconductance of only one of the coupling means, for example, that of the backward coupling means. The bandpass characteristic is symmetrically varied with respect to the mean resonant frequency of the system by adjusting the transconductance of the backward coupling means. characteristic obtained is similar in all respects to that resulting from symmetrical detuning of the input and output tuned circuits, as described in the paper entitled high-Fidelity Receivers with Expanding Selectors, by Wheeler and Johnson, published in the I. R. E. Proceedings, June 5, 1935, at pages 594-609.

Further in accordance with the invention, the resonant input and output circuits of the selector system are coupled together in the forward direction through a vacuum tube amplifier and are coupled in the backward direction by means of a feed-back path which includes a vacuum tube repeater having a controllable transconductance. The transconductance of the forward coupling path is substantially greater than that of the backward coupling path so that amplification of the transmitted frequencies is secured.

The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation; together with further objects and advantages thereof, will best be understood by reference to the following speciiication, taken in connection with the accompanying drawings, in which Fig. l is a circuit diagram, partially schematic, illustrating a radio broadcast receiver of the superheterodyne type embodying a band-pass selector constructed in accordance With the invention; Fig. 2 and 3 illustrate certain operating characteristics of the band-pass selector shown in Fig. 1; and Figs. 4 and 5 illustrate modified forms of the invention.

Referring more particularly to Fig. 1 of the drawings, there is illustrated one form of the irnproved band-pass selector system, embodied in a multi-band radio broadcast receiver of the wellknown superheterodyne type, for controlling the selectivity of the intermediate-frequency channel of the receiver. Briefly described, the receiver comprises a radio-frequency amplifier, indicated schematically at Ill, connected to antenna-ground circuit II and coupled to a frequency vchanger unit, indicated schematically at I2. The radio-frequency selector and amplifier I0 may be of any conventional type and includes for each stage a variable tuning condenser I3 and band-selecting switching means I4. Similarly, the frequency changer unit I2 may be of any .conventional type and includes in both its radio-frequency selector circuit and its local oscillator circuit a variable tuning condenser I5 and band-'selecting switching means I1. The various tuning condensers may be ganged for unicontrol by a single control member, in the manner indicated by the dot-dash line U. Unicontrol means may also be provided for operating the band-selector switches, as indicated by the dotted lines S. Connected in cascade with the output circuit of the frequency changer unit I2 are a band-pass selector and intermediatefrequency amplifier I8, an additional intermediate-frquency amplifier I9, a detector and automatic amplification control 20, an audio-frequency amplifier 2I, and a sound reproducer 22. The bias potential derived from the automatic amplification control source 20 may be applied The form of to one or more of the stages of the radio-frequency selector and amplifier I0, the modulator included in the frequency changer unit I2, and the various stages of the intermediate-frequency amplifier I9.

Considering first the operation of the receiver as a whole, Without regard to the details of the band-pass selector and amplifier I8, per se, the desired received signal waveis selected and ampliiied in the radio-frequency selector and amplifier I0 and is converted into a modulated intermediate frequency by the frequency changer I2. The signal as thus converted is further selected in the band-pass selector I8, amplified in the intermediate-ferquency amplifier I9, rectified by the detector 20, thereby producing the audio frequencies of modulation which are, in turn, amplified in the audio-frequency amplifier 2l and reproduced by the sound reproducer 22. The amplification of the received signal is subject to automatic control by the control bias potential derived from the source 20, according to the manner well understood in the art.

Referring now more particularly to the details of the band-pass selector system, indicated generally at I8 and constructed in accordance with this invention, the system comprises a resonant input or terminal circuit 23 and a resonant output or terminal circuit 24 which are coupled, respectively, to the output circuit of the frequency changer I2 and the input circuit of the intermediate-frequency amplifier I9, together with adjustable directive coupling means between the two

circuits

23 and 24 coupling the two circuits in both the forward and the backward directions. Preferably very loose inductive .coupling is provided between the input circuit 23 and the output circuit of the frequency changer, and between the output circuit 24 and the input circuit of the intermediate-frequency amplifier I9. The purpose of loosely coupling the indicated circuits is to minimize the effect on the

circuits

23 and 24 of the circuits coupled respectively therewith.

For the purpose of coupling the input and

output circuits

23 and 24 in the forward direction, there is provided a pair of cascaded vacuum tube repeaters or amplifiers 25 and 26 which are resistance-coupled by means of the resistor 28. Each of the tubes 25 and 26 is preferably of the well-known pentode type having no appreciable plate-to-grid coupling, due to the presence of the screen grid therebetween, and having adjustable directive transconductance. Biasing resistors 29 and 3D, shunted respectively by bypass condensers, are provided for biasing the respective control grids ofthe tubes 25 and 26 to the correct potentials relative to the cathodes thereof. It will be noted that the resistance coupling between the tubes 25 and 26 renders the forward coupling means between the input circuit 23 and the output circuit 24 substantially non-selective with respect to frequency, this coupling means being so proportioned as to be substantially less frequency-selective than the resonant .

circuits

23 and 24.

For the purpose of varying the forward coupling between the

circuits

23 and 24, there is provided means for varying the values of transconductance of the tubes 25 and 26. This means comprises a control circuit including a source of potential 3l shunted by a voltage divider 32 having an adjustable arm 33 connected to the control grids of the tubes 25 and 26, in the manner indicated.

In order to couple the input circuit23and the output circuit 24 in the backward direction; there is provided feed-back meanswhich `in cludes a vacuum tube repeater" 34r having its in-l put electrodes coupled to the outputcircuit` 24 through. a coupling condenser'35 and its` output electrodes coupled to the inputl I`circuit L23 through a coupling condenser `36.' "I'he vacuum tube 34 is also preferably of the 'well-known pentode type having no appreciable plate-to-grid` coupling but having adjustable directive trans` conductance. A biasing resistor 31, shuntd by" a by-pass condenser, is connectedinl the ,cathode circuit of the tube 34` for properly biasing the control grid of the tube. In order toadjust the bias on the control electrode of the tube 34, "to vary the transconductance of the tubefand there'f by to change the value of the backward coupling between the

circuits

24 and 23,` thereI is; provided a source of biasing potential Q33 anda voltagej divider 39 having an adjustablel contact v40 cone` nected to the grid of the tube 34 through agridf" leak resistor 4l. Plate and screen voltages" are supplied to the tubes 25,26 and 34 through the several leads indicated at +B and +Sc, 1respecf tively.

In order to minimize distortioncfjthejsignal currents in the selector system,` thetubes `215,` and 34 may be of the gradual anode-current crut` off, or variable-mu type. 2'

The backward coupling mearisfihcliidirlgh tube 34 is also substantially nonselective with respect to frequency, the circuit constants ofthe? backward coupling circuitbing so.`p ropcuftioned".` as to render this means substantiallylessdire?"` quency-selective than the resonantl circuitsi'23` and 24. The purpose of makingbothgth'eyorff,` Ward and backward coupling rri'eanssubstanm1 tially less selective than the

input afQricLoutputfJ circuits

23 and 24 is toprevent the seleclztorsys@` tem from oscillating and to 'insurefstability of;` operation at all frequencies withinjthebandtof be transmitted through the system. l,

`In considering the operation o`f{the}selector;` system, it'will be seen that, by` utilizing the tubes 1. 25 and 26 in cascade, the alternating yoltageshimf pressed on the input circuit 23 are twice reversed in phase so that thealternating voltages 4across g2 the output circuit 24 are substantially inffvphase with the voltages across the inputcircuit 23,at frequencies in the vicinity of the resonantiifreg.; quencies of the circuits 23 and.24,at,which fre-. quencies these circuitsare substantiallyQresisf.r tive. By including the single` tube: 3224 ,in .the backward coupling path, however,` vthe feed-"i back voltages impressed on the.` input circuit". 23 through this path `are substantiallyk .re-W. versed in phase, at` the frequencies indicated,y with respect to the input voltages impressedwdim rectly upon this circuit. If thecircuits23and, i 24 have the same resonant frequenciegwhichgis-i. the preferred arrangement, the :input and .feed back voltages will, under these` conditions,l be al most exactly in phase opposition.` `-`Atthese fre quencies the

circuits

23 and 24 also havevltheir maximum impedance, so that thefgains` ofther tubes 26 and 34 are a maximumand` `lie-vpltages developed across the circuit 24`fancl`the feedback voltages are both a maximum. rIjliereffore the system is degenerative to th'elinaxinum4 de gree at these frequencies.

At frequencies substantially above` r e nant frequencies of the circuits 23 anidn24`, these circuits are capacitively reactive that tli` giace? i misteri/eater; diagramfitwill:

` at frequencies:substantiallyfabov and i resonant frequencies wardcouplingfcan beobtainedib ad i negative potential applied to me 9 to. redeeiil trasconductaaf .h negligible-velvetWhenitlieibasfmtenvoltage across circuit 24 lags the'iput voltage" by e value 'avarbhin 90"; L. voltage impressedlurn fh] retardedv by an additional arno ing .so that the feed-bac y stantially in phase with the 1ripii" However. at .these frequencies, the circuits 23` and .2.41m Subs. atresonance; reduclngfthe gai including tubesr 26 andf34; an'dbtlry feed-back voltages. "Thereiore and phase of the feed-back .yo age that) 'at these frequencies, the ages" are subject toieading instead phase shift. ,l .r M The` above.`described` relat onshw ps L"re, graphically `in Fis .2, which .11S alpolar inA (which, the Vangular; 1@Ordinate.s gewesen x, plicitlythe" phase angle(and citlyatli :fr (allger)l 01C: the feedrback. Yeltaeetelativ f input.` voltage, for any `given frequency radial coordinatesrepresent.. `h tude r ofv :2 the y i; Lfeed-back l voltages. this-5dias grarnlutheE vector 43 `may con de` d. astfthw reference vector 1911 each @frequentati @Qmnnenti In this `ilgurelvalsp, .the veCQriZ feedfback voltage at( the mea quencyI of `the system. .flhefvoltage-@

vectors

45 and 46 represent the relative feed-back p ages of` frequencies lincreasing-lv highemthanutlie mean resonant `frequency. ofgthewsystem, .Ivvliileff1 u the .vectors 41, `Lanci.silarepres ntathefreiat' fednbal VQlageSgof, frequencie 11i-.mcreasiiuglyLU lower l.than :the` mean resonant 4equ quencies 11m the; .viciriitv` of.. 4 ...m frequency ofi the system.- `the vectorsvrepresentingm45 feed-back.; .voltages are. opposite; phaseftofzthefx reference vector representingtthe :imputa` voltage,': and therefore the feed-backvgiisa degenerativ However. at. `frequencies displaced'imore tha certain Aamount aboveiand.bb1o uchsinea resonant :frequency-fthe resultan th vectors representing..feedbackyvoltages mi heA -efer ence inputvoltagevector .are'great than th input "vector,` and therefore. th feed back generative. .y

other words, `.thebackward .coupling be tween the` circuits,` 23 .and l24 operatesttoidecreaset theresponsiveness lof the system at requenci'e within the g band inuthe -vicinityEicfathe fresonan frequencieslof A.tlfietwo circutsgwnditoci the responsiveness-"of the system 'Ihefveiiectl` which may be procd the transconductance of the tube 34 .to

i .,-.:i, justedto increase the transconductance of tube 34 at an intermediate value, the characteristic curve lof the seletor system is modied to approach the shape shown by curve E. As can be observed from this curve, with small feed-back voltages the degenerative action is appreciable at frequencies near the meaniresonant frequency of the system, while thepli and magnitude of the feed-back voltage comp ,s of frequencies above and below the mean Mrfafscifiiant frequency are such that the regenerative action 'is of no appreciable effect. A further increase in the transconductance of the tube 34 to increase the feed-back voltage .causes an increase in the amplitude of the feed-back components to modify the response characteristic of the system to that in which the feed-back is considerably regenerative at frequencies substantially displaced from resonance, shown by curve C, accentuating the double peaks on either side of the mean resonant freqeuncy.

From an inspection of the family of curves illustrated in Fig. 3, it will be appreciated that the improved selector system described provides a simple means for easily adjusting the selectivity of a receiver such as that shown in Fig. 1,`by'ad justing the width of the frequency band transmitted through the intermediate-frequency chan'- nel of the receiver. Thus, by adjusting the bias potential on the control electrode of the 34 tube in the manner described above, the Width of the band transmitted through the intermediate-frequency channel of the receiver may easily be varied. Adjustment of the Width of the frequency band passed by the system may also be obtained by adjusting the contact arm 33 of the voltage divider 32 to vary the trans-conductance of the tubes 25 and 26, thereby to vary the forward coupling.

One advantage of the adjustable coupling described above resides in the fact that the gain through the selector system for all frequencies within the band is lowered as the band is widened. Ordinarily, a wide band-pass characteristic is desired with strong signals. and the corresponding gain of the selector system is preferably relatively low. However, with weak signals a relatively narrow band and a higher gain is desirable. The family of curves described above shows that these conditions are satisfied when the band width is adjusted by adjusting the backward coupling between the

circuits

23 and 24.

If it be desired that all frequencies within the band passed by the intermediate-frequency channel be amplified with substantially uniform gain, the excess attenuation of frequencies in the vicinity of the resonant frequency of the input and output circuits 24, exemplified in curve C, may be compensated for in succeeding amplifier stages included in the amplifier i9. This can readily be done by properly proportioning the interstage coupling circuits between the several stages in the intermediate-frequency channel, in the manner disclosed in United States Letters Patent No. 1,921,088 to William A. MacDonald, and further ltreated in the above-identified paper by Wheeler and Johnson.

Referring now to Fig. 4 of the drawings, there is illustrated a modified form of band-pass selector system embodying the present invention, in which a single vacuum tube 5| is included in the forward coupling means between the input circuit 23 and the output circuit 24 and a pair of resistance-coupled cascade-connected vacuum tube 52 and 53 are included in the means coupling the two circuits in the backward direction.

The coupling circuit between the two tubes 52 and 53 includes a condenser 53' and a resistor 54.

Biasing resistors

55, 56 and 51, each shunted by a by-pass condenser. are provided in the cathode leads of the tubes 5i, 52 and 53, respectively. Condensers 6i and 62 are provided for coupling the input electrodes of the tube 52 to the output circuit 24 and the output electrodes of the tube 53 to the input circuit 23, respectively. In order to vary the transconductance of the tube 52, thereby to vary the backward coupling in the manner described above, there is provided a direct-voltage source B0 shunted by a voltage divider 6|' having an adjustable contact arm 62 connected through a grid-leak resistor 63 to the control electrode of the tube 52. Similarly. adjustable biasing means for the control electrode of the tube 5i are provided for varying the transconductance of the tube 5|, thereby to vary the value of the coupling between the

circuits

23 and 24 in the forward direction. This means includes a direct-voltage source 64 shunted by a voltage divider 65 having an adjustable contact arm 68 connected across a high-frequency by-pass condenser 61 and to the control electrode of the tube Suitable anode and screen voltages are applied through the leads indicated at +B and +Sc to the respective a'node and screen electrodes of the tubes 5l, 52 and 53.

The operation of the system shown in Fig. 4 is fundamentally the same as that described above in connection with the arrangement of Fig. 1,

and accordingly a detailed description thereof is deemed to be unnecessary. The desiredvphase re lation between the feed-back voltage and the directly impressed voltage is preserved by including the three tubes connected in the manner shown. With this arrangement a single phase reversal occurs in the forward coupling means and a double phase reversal occurs in the backward coupling means, so that the components of the input and feed-back voltages at frequencies in the vicinity of the mean resonant frequency of the system are substantially in phase opposition. It will be observed that, if an even number of coupling tubes is included in one coupling path and an odd number of tubes is included in the other, this desired phase relation between the feed-back and input voltages is secured.

The circuit of Fig. l has an advantage over that of Fig. 4 in that additional amplification is obtained by including two tubes in the forward coupling path rather than in the backward coupling path.

If desired, transformer means may be employed in either or both of the forward and backward coupling paths for obtaining the desired polarity and amount of the forward and backward coupling. An arrangement constructed in accordance with the invention and including such means is illustrated in Fig. 5, the circuit shown being simliar to that of Fig. 4 but differing therefrom in that the tube 53 is omitted and transformer means, indicated at 68 and 69 are provided for coupling the backward coupling means between the

circuits

23 and 24, respectively. 'I'he transformer 69 comprises a primary winding 'I0 connected between the output electrodes of the tube 5I and inductively coupled to the winding 1|, forming the inductive branch of the resonant output circuit 24. The winding 1I is coupled to a winding 12 which serves to couple the output circuit 24 to a succeeding utilizing circuit (not shown). A fourth winding 'I3 is provided for coupling the output circuit 24 to the input electrodes 75 of the tube l2. Reversal of the polarity of the voltage applied between the input electrodes of the tube 52'relative to the output voltage of the tube 5I is securedby so poling the inductance 'I3 with respect to the inductance 1I. The transformer means 68 comprises a primarywinding 14 connected to the preceding stagerof the system and coupled to a secondary winding 15 which comprises the inductance of the resonant input circuit 23. This transformer is provided also with a tertiary winding 16 included in the output circuit of the tube 52 and coupled mainly to the secondary winding 15. The couplingv between the windings 'I5 and 'I6 serves to impress the feedback `voltage on the input circuit with the correct polarity. 'I'he additional tube 26 of the circuit of Fig. 1, the additional tube 53 of the circuit of Fig. 4, and the transformer windings 13, ll al1 perform equivalent functions; that is, each is effective to reverse the polarity of the feedback to the input circuit of the selector without introducing appreciable dissipation into the tuned input and output circuits of the system. As shown. the winding 'I6 of the transformer 68 and the winding 13 of the transformer 69 are untuned.

The operation of this circuit is fundamentally the same as that described in connection with the circuit shown in Fig. 1, and it is believed that no further amplification of the previous descriptive matter is necessary to a ready understanding thereof. It may be noted, however, that the use of transformer means for obtaining the correct relation between the phase and amplitude of the input and feed-back voltages substantially facilitates the design of the system to secure the best operating conditions.

While the invention has been described with particular reference to its use as an adjustable band-pass selector system in a radio receiver, it will be understood that it is equally applicable for use as a non-adjustable band-pass selector. By properly designing the circuit and properly proportioning the values of transconductances included in the forward and backward coupling paths, a non-adjustable band-pass selector may be constructed which is capable of passing any desired band of frequencies with sharp discrimination against unwanted frequencies outside the band. y, While there have been described what at present are considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein Without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A band-pass selector system comprising input and output circuits resonant at frequencies within said band, directive coupling means coupling said circuits in one direction, separate coupling means coupling said circuits in the other direction,` said coupling means together tending to provide a feed-back voltage substantially in phase with said input voltage across said input circuit at a frequency in the vicinity. of said resonant frequencies, and transformer means included between one of said circuits and one of said coupling means for reversing the phase of said feed-back voltage, said coupling means being substantially less frequency-selective than said terminal circuits and co-operating with said input and output circuits to provide a coupling reaction between said circuits which is degenerative at frequencies in the vicinity of said resonant frequencies and regenerative at frequencies above and below said resonant frequencies.

2. An adjustable band-pass selector system for passing a band of frequencies adjustable in width comprising, input and output circuits resonant at frequencies within said band, amplifier means including directive coupling means coupling said circuits in the forward direction and separate coupling means having an adjustable transconductance coupling said circuits in the backward direction. the transconductance of said forward coupling means being substantially greater than that of said backward coupling means, said coupling means being substantially less frequencyselective than said input and output circuits and co-operating with said input and output circuits to provide a coupling reaction between said circuits which is degenerative at frequencies in the vicinity. of the mean frequency of said band and regenerative at frequencies above and below said mean frequency, and means for adjusting said adjustable transconductance to vary the width of the frequency band passed by said system and simultaneously to vary inversely the amplification of the predetermined frequencies within said band by said amplifying means.

3. An adjustable band-pass selector system for passing a band of frequencies adjustable in width comprising, input and output circuits resonant at frequencies within said band, amplifier means including a vacuum tube amplifier for coupling said circuits in the forward direction and separate coupling means coupling said circuits in the backward direction, the transconductance of said forward coupling means being substantially greater than that of said backward coupling means, and including a vacuum tube having input electrodes coupled to said output circuit and output electrodes coupled to said input circuit, the transconductance of said tube being included in the path provided by said backward coupling means, said coupling means being substantially less frequency-selective than said input and output circuits and co-operatively with said input and output circuits to provide a coupling reaction between said circuits which is degenerative at frequencies in the vicinity of said resonant frequencies and regenerative at frequencies above and below said resonant frequencies and which depends upon the value of said transconductance, and means for adjusting said transconductance to vary thewidth of the frequency band passed by said system and simultaneously to vary inversely the amplification at predetermined frequencies Within said band.

4. An adjustable band-pass selector system for passing a band of frequencies adjustable in width comprising, input and output circuits resonant at frequencies within said band, amplifier means including means coupling said circuits in the forward direction and comprising a vacuum tube amplifier having an even number of stages, said amplifier including input electrodes coupled to said input circuit and output electrodes coupled to said output circuit, separate coupling means coupling said circuits in the backward direction and comprising a vacuum tube repeater having an odd number of stages, the transconductance of said forward coupling means being substantially greater than that of said backward coupling means, said repeater including input electrodes coupled to said output circuit and output electrodes coupled to said input circuit, said coupling means being substantially less frequency-selective than said input and output circuits and cooperating with said input and output circuits to provide a coupling reaction between said circuits which isdegenerative at frequencies in the vicinity of said resonant frequencies and regenerative at frequencies above and below said resonant frequencies, and means for adjusting the transconductance of at least one of the tubes included in\ said backward coupling means to vary the width of the frequency band passed by said system and simultaneously to vary inversely the amplication at predetermined frequencies within said band.

5. A band-pass selector system comprising input and output circuits resonant at frequencies within said band, directive coupling means coupling said circuits in the forward direction, directive coupling means coupling said circuits in the backward direction, said coupling means together tending to provide a feed-back voltage in p phase with said input voltage across said input circuit at a frequency in the vicinity of said resonant frequencies, and nondissipative phasereversing means included between one of said circuits and one of said coupling means for reversing the phase of said feed-back voltage, said coupling means being substantially less frequency-selective than said input and output circuits and co-operating with said input and output circuits to provide a coupling reaction between said circuits which is degenerative at frequencies in the vicinity of said resonant frequencies and regenerative at frequencies above and below said resonant frequencies.

6. lA band-pass selector system comprising, input and output circuits parallel resonant at frequencies within said band, a vacuum-tube repeater 'coupling said circuits in one direction having its input and output terminals individually coupled across said circuits and being eective to couple said circuits with one polarity, and a vacuum-tube repeater coupling said circuits in the other direction having its input and output terminals individually coupled across said circuits and being eifective to couple said circuits with a polarity opposite said one polarity, said repeaters thereby providing a coupling reaction between said circuits which is degenerative at frequencies in the vicinity of said resonant frequencies and regenerative at frequencies above and below said resonant frequencies.

HAROLD M. LEWIS.