US2185388A - Band-pass selector system - Google Patents

Description

Jan. 2, 11940. H. A. WHEELER ABl-xND-PSS SELECTOR SYSTEM Filed July 29, 1957 www INVENTOR HAROLD A* WHEELER ATTORNEY Patented Jan. 2, 1940 UNITED STATES PATENT GFFICE BAND-PASS SELECTOR SYSTEM poration of Delaware Application July 29, 1937, Serial No. 156,295

19 Claims.

This invention relates to high-frequency bandpass selector systems and, more particularly, to such systems in which the width of the frequency band transmitted by the system is adjustable. While band-pass selector systems constructed in accordance with the present invention are of general utility, they are particularly suitable for use 1n the intermediate-frequency channel of superheterodyne receivers for controlling theselectivity and delity of response of the receiver.

Band-pass selector systems of conventional design usually comprise a pair of suitably coupled resonant circuits tuned to the same or slightly diiferent frequencies. The two circuits are commonly referred 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. The responsiveness of this type of selector system is usually substantially constant over a band of frequencies in the vicinity of resonance of the individual circuits, while currents of all other frequencies are sharply disy, criminated against and are attenuated to a substantial degree. In general, the width of the frequency band passed by such system may be varied either by changing the coupling between the two circuits or by adjusting the resonant frequencies of the two circuits relative to each other. The coupling referred to is nondirective 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 system is to be distinguished from a system including Va vacuum-tube coupling between the input and output circuits, in which the coupling is mainly unidirective.

' Band-pass selector systems wherein only the nondirective form of coupling is employed are,

in general, open to the criticism that only mehanical or relatively complicated nonmechanical expediente are known for adjusting the width of the frequency band to be transmitted. Furtherincre, this type of coupling is inherently incapable pf producing amplification of the transmitted frequencies in the coupling path between the input and output circuits of the system. Band-pass selector systems of the prior art, wherein a vacw uum tube or a undirective coupling is used in both .the forward and backward directions between the input and output circuits, are open to the criticism that a device having a directive transconductance is required for each of the directive coupling paths.

It is an object of the present invention to provide an improved adjustable band-pass selector system of simple arrangement, in which the coupling between the input and output circuits of the system is obtained by a single device having directive transconductance.

It is another object of the invention to provide a band-pass selector system of the type described, in which a single vacuum tube controls the amplication between the terminal circuits and, in addition, controls the resonant frequencies of the terminal circuits to adjust the width of the band of frequencies passed by the system.

It is a further object of the invention to provide an improved band-pass selector system of the type described, which is adjustable, by means of the conventional automatic amplification control of the receiver in which the system is embodied, to procure ideal band-pass characteristics for a wide range of received signal amplitude.

Briefly stated, the above objects are obtained in accordance with the present invention by providing a band-pass selector system comprising resonant input and output circuits coupled by directive coupling means in the forward direction and having means for introducing a reactance component in each of the terminal circuits which varies with the transconductance of the coupling tube effectively to alter the resonant frequencies of the terminal circuits in accordance with the transconductance of the coupling tube. With this form of coupling between the terminal circuits, the dependence of both the amplification between the terminal circuits and the width of the band of frequencies passed by the system on the transconductance of the coupling tube makes it possible simultaneously to adjust the width of the frequency band transmitted and the gain of the system. The band-pass characteristic is symmetrically varied with respect to the mean frequency of the system by adjustment of the transconductance of the coupling tube. The form of characteristic obtained is that resulting from symmetrical detuning of the input and output tuned circuits as described in the paper entitled High fldelity receivers with expanding selectors, by Wheeler and Johnson, published in the I. R. E. Proceedings, June, 1935, at pages 594-609, with the added desirable feature that the gain of the circuit further decreases with decreasing selectivity.

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 description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims. In the drawing, Fig. 1 is a simplified circuit diagram of a band-pass selector system embodying the invention; Fig. 2 is a circuit diagram, partly in block form, of a complete superheterodyne receiver embodying a band-pass selector constructed in accordance with the invention; Fig. 3 illustrates certain operating characteristics of the band-pass selector system shown in Fig. 2; and Figs. 4, 5, 6, and 7 illustrate diierent arrangements for neutralizing the inherent capacitance of the vacuum tube of Figs. 1 and 2.

Referring to Fig. 1 of the drawing, there is illustrated one form of the improved band-pass selector system, comprising a tuned input circuit L1, C1 and a tuned output circuit L2, C2 coupled by a vacuum tube l0 in the forward direction. For the purpose of effectively coupling a reactance into circuit L2, C2 which varies directly in accordance with the transconductance of the tube I0, there is provided a tuned circuit La, C3 Which is included in the input circuit of vacuum tube I0, in series with the tuned circuit L1, C1 and moderately inductively coupled to the circuit L2, C2, as indicated by the dotted line M23. Since the purpose of circuit L3, C3 is not to obtain selectivity, but rather to obtain a current having a phase displacement of one quadrant with respect to the current of circuit La, C2, the tuning of this circuit to the mean resonant frequency of the selector system is broadened by a shunt resistor R3 of relatively small resistance. Similarly, for the purpose of effectively coupling a reactance of opposite type into the primary circuit L1, C1 which varies directly in accordance with the transconductance of the vacuum tube I0, there is provided a tuned circuit L4, C4 which is included in the output circuit of the vacuum tube IIJ in series with the tuned circuit Lz, C2 and moderately inductively coupled to the circuit L1, C1, as indicated by the dotted line M14. Circuit L4, C4 is also very broadly tuned to the mean resonant frequency of the selector system, the tuning being broadened by a shunt resistor R4 of relatively small resistance. The input or primary circuit L1, C1 is sharply tuned to a frequency slightly above or below the mean resonant frequency of the selector system. The output or secondary circuit L2, Cz is sharply tuned to a frequency differing equally and oppositely from circuit L1, C1 with relation to the mean resonant frequency to be passed by the selector system. In order to couple opposite reactance 1 components into circuits L1, C1 and La, C2, the

mutual inductances M14 and M23 are of opposite polarity. The simplest arrangement is one which is symmetrical with respect to input and output circuits, that is, L1, C1, La, C3, and R3 are equal, respectively, to L2, Cz, L4, C4, and R4. In this case, M14 and M23 are equal and opposite. Operating potentials for the vacuum tube I0 may be supplied by any suitable source, such as the batteries II, I2, and I3, while the grid bias may be adjusted by means of a voltage divider II'.

'Ihe adjustment of feed-back circuits Lc, Ca and L4, C4 is made before the terminal circuits are adjusted. If one of the feed-back circuits has a phase displacement more or less than one quadrant with respect to the terminal circuit to which it is inductively coupled, the feedback effects not only the tuning but also the damping of the terminal circuit. The phase of the feedback circuits Ls, Ca and L4, C4 may be adjusted by slight variation of C3 and C4, respectively. The phase of each is adjusted so as to have no eiect on the damping of the sharply tuned circuit with which it is inductively coupled. The

' only effect of circuit La, Ca is then an effect on the resonant frequency of circuit Lz, C2; likewise for L4, C4 and L1, C1. The adjustment of circuits L1, C1 and LzfCz can then easily be effected by tuning these circuits to the mean resonant frequency of the selector system with tube I0 adjusted for maximum transconductance. Reducing the transconductance of the tube III has the effect of letting circuits L1, C1 and Lz, Cz fall out of tune by the proper amount.

In considering the operation of the system described, it will be assumed that, initially, the transconductance of the tube I0 is a minimum, the circuit L1, C1 is resonant at a frequency slightly above, and circuit L2, C2 is resonant at a frequency slightly below, the mean resonant frequency of the band to be passed. Under these conditions the gain and selectivity of the system are minimum, providing the maximum fidelity of reproduction. If the bias on tube I0 is now changed by means of an adjustment of potentiometer II' to increase the transconductance of the tube, the couplings between circuits L1, C1 and L4, C4 andA between circuits L2, C2 and La, C: are correspondingly increased and increasing reactance components of opposite kinds are effectively coupled into the terminal circuits L1, C1 and Lz, C2, tending to bring these terminal circuits into tune with each other and with the mean resonant frequency of the selector system, in this manner narrowing the band of frequencies which is transmitted by the system.

In Fig. 3 there is illustrated a family of characteristic curves which are representative of the selector system of Fig. 1 when the transconductance of the tube is varied over a range of values. It is apparent that the increase in amplification, due to the resonant gain of the terminal circuits and the gain of tube I 0, is cumulative as the transconductance is increased by an adjustment of the grid bias of the tube I0. 'I'he characteristic curves are symmetrical and control of the selectivity is accomplished without complicated mechanical parts and without any additional tubes.

In Fig. 2 there is shown, partly in block form, a complete superheterodyne receiver of a conventional design embodying the present invention in a preferred form. In general, the receiver includes a tunable radio-frequency amplier I4 of one or more stages having its input circuit connected to an antenna I5 and ground I6 and its output circuit connected to a frequency changer I'I. Connected in cascade with the frequency changer I1, in the order named, are a selector system I8, a detector and automatic ampliflcation control rectifier I9, an audio-frequency amplifier 20 of one or more stages, and a sound reproducer 2|. Automatic amplification control is applied in a conventional manner from rectifier I9 to one or more stages of radio-frequency amplifier I4, frequency changer I'I, and selector circuit I8.

It will be understood that the various system components just described, except the selector I8, may be of conventional construction and operation, the details of which are Well known in the art, rendering further description thereof u unnecessary. Considering briey the operation of the receiver as a whole, a desired received signal is selected and amplifiedy by radio-frequency amplier I4, converted to a modulated intermediate-frequency carrier in frequency changer lll, amplied and selected by band-pass selector il, and rectified by the detector I8, thereby deriving the audio frequencies of modulation, these frequencies, in turn, being amplified in the audio-frequency amplifier 20 and reproduced by sound reproducer 2l. 'I'he automatic amplification control bias, supplied to one or more stages of radio-frequency amplier I4, frequency changer Ill, and the band-pass selectoi system i8, serves to maintain the signal output within a relatively narrow range for a wide range of received signal intensities.

The band-pass selector system I8 of Fig. 2 is similar in many respects to the band-pass selector system illustrated in Fig. 1 and corresponding parts are identified by the same reference characters. Selector system I8 comprises an input circuit L1, C1, C"1 inductively coupled to winding 22 in the output circuit of frequency changer il and an output circuit L2, C2 inductively coupled to winding 23 in the input circuit of the unit I9 including the detector and automatic amplification control rectifier. Tuned circuits le, Ca, C3 and L4, C4 differ from those shown in Fig. 1 only in that they are included in the cathode sides of their respective circuits rather than in the grid and plate sides as in Fig. 1, and that condenser C: of Fig. 1 is divided into two portions Ca and Ca for ease of adjustment in aligning the input and output circuits and adjusting the neutralization, as described hereinafter.

lt is believed that the operation of band-pass selector i8 will be apparent from the explanation given with respect to the selector of Fig. 1. Variation of the trans-conductance of tube 4I effected by the automatic amplification control causes the selectivity of the circuit to vary directly with the gain and results in a family of characteristic curves for selector i3, as shown in Fig. 3.

The inherent capacitance across the input leads and terminals of vacuum tube Ill was neglected in Fig. 1, but in Fig. 2 is represented by condenser 2t shown in dotted lines, while the inherent capacitance across the output leads and terminals of the tube is represented by condenser 25, also shown in dotted lines. While there is no appreciable undesirable capacitive coupling between terminal circuits L1, C'1, C1 and L2, C2 due to the grid-anode capacitance of tube lli, because of the interposed screen grid, the inherent capacitances represented by condensers 24 and 25 have an undesirable coupling effect between certain of the circuits, such coupling effect between the terminal circuits being most detrimental since these circuits are sharply tuned. There are two paths by which the terminal circuits are coupled outside of the tube l0. In the first path, terminal circuit L1, C1, C1 is coupled to the tuned circuit La, C's, C3 by the inherent input capacitance of tube lll, represented by condenser 24, while the tuned circuit comprising inductance L3, Cs, is coupled to the tuned circuit La, C2 by the mutual inductance M23 In the other coupling path, the terminal circuit L1, C'1, C1 is coupled to the tuned circuit L4, C4 by the mutual inductance M14, while the latter tuned circuit is coupled to the terminal circuit L2, C2 by the output capacitance of tube il), represented by condenser 2t. Each of these paths involves one capacitive coupling and one inductive coupling. If the stage is symmetrical, the coupling eiects of the two paths are equal except that the mutual inductances are of opposite polarity. That is, the undesirable coupling effects of the two external paths neutralize each other. If the system is not precisely symmetrical, or the inherent capacitances 24 and 25 are not equal, neutralization of the coupling effect can be effected by adjustment of either or both of the auxiliary condensers 26 and 21 connected, respectively, across capacitances 24 and 25.

A neutralizing arrangement of three condensers is shown in the input circuit of tube I0, comprising capacitance C1, capacitance C3, and con- -denser 26 supplementing inherent capacitance 24'.

A similar arrangement may alternatively be used in the output circuit of tube I0 or in both the input and the output circuits. In order to avoid detuning the circuits L1, C1, C"1 and La, Cz, C"a by adjustment of condenser 26, the adjustable condensers C"1 and C"a may be adjusted oppositely to condenser 26 by means of a unicontrol mechanism U to maintain constant the total capacitance effective across L1 and La.

Fig. 4 shows an alternative arrangement by which the effect of capacitance 24 can be neutralized entirely in the input circuit. The arrangement is equally applicable to the output circuit of the tube for neutralizing the inherent capacitance 25. In Fig. 4, a voltage from a neutralizing coil 28, closely inductively coupled to coil L1 is coupled to the circuit La, Ca through a neutralizing .condenser 29, as shown, the values of elements 28 and 29 being such that the coupling provided thereby between circuits L1, C1 and La, Cs is equal and opposite to that provided by capacitance 24. The inductance of 28 may be less than that of L1 and the capacitance of 29 greater than the inherent capacitance 24'. Fig. differs from Fig. 4 only in the inversion with respect to the terminals of the input circuit, Fig. 4 being applicable to Fig. 1 and Fig. 5 to Fig. 2.

Figs. 6 and 7 illustrate two other arrangements for neutralizing the incidental capacitance of the input circuit of vacuum tube I0 and dier only in the manner of connecting the two tuned circuits in the input circuit of tube I0, Fig. 6 being applicable to Fig. l and Fig. 7 to Fig. 2. In these modifications the circuit L1, C1 is effectively connected in series with one-half of circuit La, C3, by means of a connection between inductance L1 and the mid-tap of inductance L3. The values of La, C3, R3 in this case will differ from those of Figs. 1 and 2 because of the change in the method of their connection into the input circuit of tube I0. A condenser 3l), having a capacitance equal to that of condenser 24, couples the circuits L1, C1 and L3, C3 in opposite phase toI condenser 24, as shown, and effects neutralization since the coupling eiects of condensers 24 and 30 between circuits L1, C1 and La, C3 are equal and opposite. As in the case of Figs. 4 and 5, neutralizing arrangements similar to Figs. 6 and 7 may be employed also to neutralize the incidental capacitance in the output circuit of the tube In an alternative form of the invention, the sharply resonant circuits L1, C1 and La, C2 are tuned to the mean frequency of the band with minimum instead of maximum transconductance of tube l0. The resonance curves of Fig. 3 then have the same shape, but the gain at the mean frequency tends to be approximately uniform, being maximum for an intermediate value of transconductance. This results from the fact that the coupling tube I0 tends to increase the gain of the system with an increase in itstransconductance, while the resonant gain or the system decreases with the symmetrical detuning which occurs with increasing transconductance. 'I'he effect of symmetrical detuning of coupled tuned circuits alone is shown in Fig. 3b at page 603 of the above-mentioned Wheeler and Johnson paper. A selector system having such nearly uniform gain can be utilized where it is desired to change the band width of the system with little or no change in amplification in the controlled stage of the system. In this case, the bias on tube I 0 from the rectifier of unit I9 has to be rearranged to provide a less negative value for greatersignal strength in order to expand the pass band of selector i8. l

In another alternative form, the inductive couplings M14 and M23 have like polarity so that the resonant frequencies of the sharply resonant circuits L1,C1 and L2,C2 are shifted in the same sense by varying the transconductance of the tube. The neutralizing arrangements of Fig. 2

then become inapplicable, but those of Figs. 4 to 7 are still applicable.

While there have been described what are at present considered the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modications may be made therein without departing from this invention', and, therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What is claimed is:

1. A band-pass selector system for passing a Vband of frequencies, comprising tuned terminal circuits, repeater means having a variable transconductance coupling said circuits in one direction, and means for effectively coupling into said terminal circuits reactances variable in accordance with said transconductance, thereby to vary the selectivity of said system.

2. A band-pass selector system comprising a repeater having a variable transconductance, input and output tuned circuits connected with said repeater, and two feed-back means exclusive of said tuned circuits but coupled respectively therewith and both including said repeater, whereby said feed-back means couple effectively into said tuned circuits substantial impedance varying in accordance with the transconductance of said repeater.

3. A band-pass selector system comprising a repeater having a variable transconductance, a tuned circuit and an auxiliary circuit connected between the input terminals of said repeater, a tuned circuit and an auxiliary circuit connected between the output terminals of said repeater, feed-back means including said repeater anda coupling between said output auxiliary circuit and said input tuned circuit, and separate feed-back means including said repeater and a coupling between said input auxiliary circuit and said output tuned circuit, said feed-back means being proportioned to couple effectively into the respective tuned circuits substantially pure reactance variable in accordance with the transconductance of said repeater.

4. A band-pass selector system for passing a band of frequencies, comprising tuned terminal circuits normally tuned, respectively, slightly above and below the mean resonant frequency of the band to be passed by said system, repeater means having a variable transconductance coupling said circuits in one directiomaand means for eectively coupling into said terminal circuits reactances of opposite kinds varying directly in accordance with variations of said transconductance, said reactances being effective to tune said terminal circuits toward said mean resonant frequency.

5. A band-pass selector system comprising. a repeater having a variable transconductance, two tuned circuits equally sharply selective and detunedequally on opposite sides of the mean frequency to be passed by said system, an auxiliary circuit and one of said tuned circuits connected between the input terminals of said repeater, an auxiliary circuit and the other of said tuned circuits connected between the output terminals of said repeater, feed-backmeans including said repeater and a coupling between said output auxiliary circuit and said input tuned circuit, and separate feed-back means including said repeater and a coupling between said input auxiliary circuit and said output tuned circuit, said feed-back means being proportioned to couple eilectively into the respective tuned circuits substantially pure reactances of opposite kinds variable in accordance with the transconductance of said repeater and being proportioned to bring said tuned circuits into resonance with each other at said mean frequency when the transconductance of said repeater ls a maximum.

6. A band-pass selector sys'tem for passing a band of frequencies, comprising terminal circuits normally tuned to the mean resonant frequency of the band vto be passed by said system, repeater means having a variable transconductance coupling said circuits in one direction, and means for eiectively coupling into said terminal circuits reactances oi opposite kinds variable in accordance with said transconductance, thereby to vary the selectivity of said system.

7. A band-pass selector comprising a repeater having a variabletransconductance, two tuned circuits equally sharply selective and tuned to the mean frequency'to be passed by said system, an auxiliary circuit and one of said tuned circuits connected between the input terminals of said repeater, an auxiliary circuit and the other of said tuned circuits 'connected' betweenthe output terminals of said repeater, feed-back means including said repeater and a. coupling between said output auxiliary circuit and said input tuned circuit, and separate feed-back means including said repeater and a coupling between said input auxiliary circuit and said output circuit, said feed-back means being proportioned to couple eiectively into the respective tuned circuits substantially pure reactances of opposite kinds variable in accordance with the transconductance of said repeater and being proportioned to detune said tuned circuits equally on opposite sides of 'said mean frequency.

8. A band-pass selector system for passing a band of frequencies, comprising tuned terminal circuitsa first auxiliary circuit coupled with one of said terminal circuits, a second auxiliary circuit coupled with the other of said terminal circuits, repeater means coupling said terminal circuits in theforward direction, a iirst feed-back path including said repeater means coupling said one of said terminal circuits to said first circuit and a second feed-back path including said repeater means coupling said second circuit to the other of said terminal circuits, and means for varying said repeater means, thereby to vary the selectivity of said system.

9. A band-pass selector system for passing a band o'i frequencies comprising a pair of tuned terminal circuits, a first auxiliary circuit in series with one of said terminal circuits and .coupled with the other of said terminal circuits, a second auxiliary circuit in series with said other of said terminalcircuits .and coupled with said one of said terminal circuits, repeater means coupling said terminal circuits in the forward direction, a first w feed-back path including said repeater means coupling said one of said terminal circuits to said second circuit, a second feed-back path including said repeater means coupling said first circuit to the other of said terminal circuits, and means for varying said repeater means, thereby to vary the selectivity of said system.

I10. A band-pass selector system for passing a band of frequencies comprising a pair of sharply tuned terminal circuits, a first broadly tuned .w circuit in series with one of said sharply tuned circuits and coupled with the other of said sharply tuned circuits, a second broadly tuned circuit in series with said other sharply tuned circuit and coupled with said one of said sharply tuned cir- .2u cuits, repeater means coupling said sharply tuned circuits in the forward direction, a feed-back path including said repeater coupling said one of said sharply tuned circuits to said second circuit, a second feed-back path including said repeater am coupling said rst circuit to the other of said sharply tuned circuits, and means for varying said repeater means, .thereby to vary the selectivity of said system.

1i. A band-pass selector system for passing a band of frequencies comprising a pair of tuned terminal circuits, a rst auxiliary circuit in series with one of said terminal circuits and coupled with the other of said terminal circuits, a second auxiliary circuit in series with the other of said terminal circuits and coupled with said one of said terminal circuits, a vacuum-tube coupling means coupling said terminal circuits, coupling said one of said terminal circuits to said second circuit, and coupling said first circuit to the other of said terminal circuits, and means for varying the transconductance of said vacuum tube, thereby to vary the selectivity of said system.

12. A band-pass selector system for passing a band of frequencies comprising tuned terminal circuits, a first auxiliary circuit coupled with one of said terminal circuits. a second auxiliary circuit coupled with the other of said terminal circuits, repeater means coupling said terminal circuits in the forward direction, a feed-back path including said repeater coupling said second circuit to the other of said terminal circuits. said feed-back coupling path introducing incidental capacitive coupling between said second auxiliary circuit and said one of said terminal circuits, means for neutralizing the effect of said incidental coupling,.and means for varying the transconductance of said repeater means, thereby to vary the selectivity of said svstem.

13. A band-pass selector system comprising a to the` other of said terminal clrcuits saidcoupling means introducing incidental capacitive coupling between one-of said terminal circuits and its `series-connected auxiliarycircuit, means for varying the transconductance of said repeater,

thereby `to vary the selectivity of said system, and

means for neutralizing the effect of said incidental capacitive coupling.

14. A band-pass selector system comprising a pair of tuned terminal circuits, a rst auxiliary circuit in series with one of said terminal circuits and coupled to the other of said terminal circuits, a second auxiliary circuit in series with the other of said terminal circuits and coupled to said' one of said terminal circuits, repeater means having a variable transconductance coupling said terminal circuits in the forward direction, a feed-back circuitA including said repeater coupling said one of said terminal circuits to said second circuit, a feed-back path including said repeater coupling said' first circuit to the other of said terminal circuits, said repeater means introducing incidental capacitive coupling between the series-connected circuits of each pair, said first and second circuits being coupled to said terminal circuits with opposite polarity, whereby said vincidental couplings, at least in part, neutralize each other, and means for varying said transconductance; thereby to vary the selectivity of said system.

l5. A band-pass selector system comprising a pair of tuned terminal circuits, a. rst auxiliary circuit in series with one' of said terminal circuits and coupled to the otherof said terminal circuits, a second auxiliary circuit in series with the other of said terminal circuits and coupled to said one of vsaid terminal circuits, said couplings to said terminal circuits being opposite polarity, repeater means having a variable transconductance and input and output terminals coupling said terminal circuits in the forward direction, a feedback circuit including said repeater coupling said one of said terminal circuits to said second circuit, a feed-back circuit including said repeater coupling said first circuit to the other of said terminal circuits, said repeater introducing incidental capacitance across said input and output terminals, means for varying said transconductance, thereby to vary the selectivity of said system,

and means comprising supplemental capacitance in parallel with the lesser of said incidental capacitances and said couplings between said first and second circuits and said terminal circuits to neutralize the incidental capacitive coupling ofl said system.

16. A band-pass selector system comprising tuned terminal circuits, a first auxiliary circuit coupled to one of said terminal circuits, a second auxiliary circuit coupled to the other of said terminal circuits, a vacuum-tube repeater having an incidental capacitance across its input electrodes for coupling said terminal circuits in the forward direction, a feed-back circuit including said repeater coupling said one of said terminal circuits to said first circuit, a feed-back circuit including said repeater coupling said second circuit to the other of said terminal circuits, means to vary the coupling effect of said coupling means, thereby to vary the selectivity of said system, and an auxiliary coupling between said one of` said terminal circuits and said second circuit offjo'pposite phase to the coupling effect of said incidental capacitance for neutralization thereof.

17. A band-pass selector system comprising tuned terminal circuits, a rst auxiliary circuit coupled to one of said terminal circuits, a second l auxiliary circuit ,coupled to the other of .said terminal circuits, a vacuum-tube repeater having incidental capacitance across its output electrodes for coupling said terminal circuits in the forward direction, a feed-back circuit including said repeater coupling said oneof said terminal circuits to said rst circuit, a feed-back `circuit including said repeater for coupling said second circuit to the other of said terminal circuits, means to vary the coupling eiect of said repeater, thereby to vary the selectivity oi said system, and an auxiliary coupling between said other of said terminal circuits and said first circuit oi' opposite phase .to the coupling eiect of said incidental capacitance for neutralization thereof. I

18. A band-pass selector system comprising a pair of tuned terminal circuits. a iirst auxiliary circuit having a-portion in series with said one of saidterminal circuits and coupled to the other of said terminal circuits. a second auxiliary circuit connected in series with said other of said terminal circuits and coupled to said one of said terminal circuits, repeater means having a variable transconductance coupling said terminal circuits in the forward direction| a feed-y back circuit including said repeater coupling said one of said terminal circuits to said second circuit, a feed-back circuit including said repeater coupling' said rst circuit to the other of said terminal circuits, means for varying said trans- 2,1s5,ass

conductance, thereby to vary the selectivity oiy said system, incidental capacitive coupling between said one oi said terminal circuits and Isaid series-connected portion of said first circuit, and an auxiliary coupling of opposite phase between the remaining portion of said iirst circuit and 'said one of said terminal circuits to neutralize said incidental capacitance coupling.

19. A band-pass selector system comprising a pair oi' tuned terminal circuits. a iirst auxiliary circuit in series with one of said terminal circuits and coupled to the other of said terminal circuits to said second circuit. a feed-back circuit including saidrepeater coupling said iirst circuit tothe other of said terminal circuits, means i'or varying said transconductance, thereby to vary the selectivity of said system, inci-l dental capacitive coupling between the other of said terminal circuits and said series-connected portion of said second circuit, and an auxiliary coupling of opposite phase between the remaining portion of said second circuit and said other of said terminal circuits to neutralize said incidental capacitive coupling. v

HAROLD A. WHEELER.

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