US2098757A - Electric circuits - Google Patents

Description

Nov. 9, 1937. J. PLEBANSKl ELECTRIC CIRCUITS Original Filed Jan. 4, 1935 INVENTOR JOZEF PLEBANSKI BY ATTORNEY Patented Nov. 9, 1937 UNITED STATES PAT-ENE? or FiCE ELECTRIC CIRCUITS I Application January 4, 1935, SerialNo. 451. Re-

newed- April 23, 1937. 1934 In Poland August 18,

19 Claims. (Cl; 178-44) My invention relates to frequency selective circuits or filters, and a method of operating the same, more particularly to band pass selectors as used. in the communication art such as for the reception of radio signals and the like.

The invention has for its main object to provide a new filter circuit which permits of an accurate adjustment and variation-of the frequency response or resonance characteristic while si- "multaneously'insuring a sharp cut-off effect by the filtercircuit with regard to aselected re sponse frequency band.

Electric filters of the type known in the art, suchas band selectors or band pass filters used for receivingmadio. and similar signals usually comprise a pluralityof loosely coupled resonant circuitstuned substantially to the same operating frequency and coupled by a small mutual reactance such as a small mutual inductance or a large mutual-capacitance. In such circuits reaction from; one circuit upon the other is unstable resulting in a modification of the resonant characteristics of the separate circuits and a spreading of the frequency response band (doublehumped resonance curve) as is well known to those skilled in the art. In other words, the mutual. reaction between the-individual circuit units of the filter causes a slight relative detuning in sucha manner that the resultant resonance 'curve of the circuits obtained from superimposition of the individual resonance curves of the circuits assumes a broadened or fiat top shape known as a band pass or band selector characteristic.

A disadvantage of filter-circuits of this type isthe fact that mutual reaction takes place between the individual filter units; that is, one unit reactslupon the'other'whichin. turn reacts upon the former, making it extremely difficult to adjust the-circuit or resonant constants to secure a band pass characteristic of. apredetermined desired; width.' Still further, if it is desired to shift the response frequency band to another place in the frequency scale, as in the case of changing thetuning of a radio receiver from one Wave length to another, great difficulties are encountered in keeping the individual filter units exactly.=-inxline so as to maintain a constant band width over the entireoperating range.

This dimculty is still further increased by the fact thatthe mutual reactance between the individualiunits in. filter circuits of thetype heretofore known in the art (inductive or capacitative reactance) is dependent-on the frequency asiis ;understood resulting in a variation of the characteristics for different frequencies and making itstill further extremely difiicult to design 'a tunable filter circuit for constant band width and for use within an extended range of operating frequencies."

Accordingly it is a further object of my'in-' characteristicmay be easily and effi'ciently shift- 7 ed within a predetermined operating frequency range While maintaining the resonant character-1 istics ofthe filter substantialiy constant for the various operating frequencies. p

As pointedout; the drawbacks inherent in electrical filters of the type heretofore known in the prior'art is primarily due to the mutual reaction between the individual filter circuits or units making it highly difiicult to adjust each unit independently of the other unit to secure a desiredresultant resonant characteristic.

Accordingly it is a further object of my invention toprovide an electric filter circuit especially of the band-selector type inwhich the individual circuit units may be adjusted independntlyflof each other and without any substantial mutualcouplingexcept as regards the operating current'fiow from the input circuit to the output circuit of the filter. In other words, the invention provides a new arrangement in which the coupling between the individual filter units is unilateral; that is, in the direction from the input to the output and in which there is substantially no coup-ling effect between an out- I put unitand an input unit in such a manner as topermit of an independent and accurate adjustment of the'characteristics of the separate units, resulting in a highly efiicient and favorable resultant response characteristic of any desired predetermined band width. with a'sharp cut-01f efiect' Band pass or band selectors or 'filters as used at present have an unfavorable shape of the resonant characteristic deviating substantially from the ideal rectangular shape, unless a pro:

hibitiye number of separate filter circuits or unitsa are used. This results in a substantial transmis sion of higher harmonics or side frequencies beyond thedesired'cut-offirequencies for a given bandtwith such as the standard 10 kilocycle' band required for reception in broadcast receivers. In this manner a large amount of noise specially of the hissing and crackling nature picked up by the receiver from the supply power line or directly from neighboring disturbing magnetic or electric fields and causing a modulation of. the carrier wave with relatively high harmonics or resulting side frequencies, is allowed to enter the receiving or translating device, such as a loudspeaker, resulting in a low signal to noise ratio and consequent impairment of the sensitivity and efliciency of the reception, as is well understood.

Accordingly it is a further object of the invention to provide a selective circuit, more par-. ticularly for receiving modulated high frequency signals such as broadcasting signals which is simple and which insures a favorable band pass effect with a sharp frequency cut-off effect by means of a minimum of parts and circuits re quired, resulting in a substantial increase of the signal to noise level and consequent increase of the efficiency and sensitivity of. the receiver.

The invention has further objects in View which will appear hereinafter as the following description proceeds taken with reference to the accompanying drawing wherein -I have shown by Way of example a few circuits, embodying the novel'features of the invention. V

Figure 1 illustrates a basic circuit of a filter of the type according to the invention.

Figure 2 shows a resonance curve illustrative of the new effect of the filter circuit according to Figure 1. e

Figure 3 shows a further circuit comprising a pair of. circuits as shown by Figure 1 in cascade to secure a band pass or band selector characteristic.

Figure 4 shows the resonant characteristic as obtained by a filter according to Figure 3, and

Figure 5 illustrates a modified form of a filter for'securing a characteristic shown by Figured.

Similar reference numerals identify similar parts throughout the diiferent views of the drawing. 7

With the aboveobjects in view, the invention generally involves the provision of a filter system using multi-electrode electronic valves whereby'the individual units of the filter are connected to separate electrodes such as grid electrodes of a valve and the mutual coupling between the separate filter units is effected by the electron stream through the valve as distinguished from the usual capacitative or inductive coupling used in filter arrangements according to the prior art. The use of such a type of filter which 'I have chosen to appropriately term electronic filter to characterize 'the use of pure electron coupling between the filter units, involves various advantages and novel effects as will appear in more detail hereinafter.

Thus forinstance, in one embodiment according to the invention, I may use an electronic discharge valve provided with a plurality of grid electrodes such as are used for various other pure poses (screen grid valves, pentode valves, etc.). The valve may be provided with the usual input grid and output anode circuits and in addition with one or more rejector or absorbing circuits constituting the various circuit units of the combined filter arrangement and each connected with a different grid electrode. Preferably these absorbing circuits are slightly ,detuned relative to the tuning of the input and o-utputcircuit that is relative to the operating or receiving frequency. These circuits then operate as,

rejector circuits for definite frequencies by presenting a high impedance to the passage of these frequencies by the discharge stream to the output circuit. In other words, the invention in this embodiment broadly relates to a filter arrangement comprising a plurality of separate resonant circuits coupled to each other electronically in such a manner as to permit reaction from an input to an output unit only but substantially preventing reaction and coupling in the reverse direction, thus making it possible to design and adjust eachunit independently and to secure a highly favorable resultant frequency response characteristic in the output circuit with a sharp cut-off effect substantially independent of the frequency or wave length to which the filter is to be adjusted.

Referring to Figure 1 of the drawing, I have shown at 5 an electronic valve or similar discharge device, in the example given of the pentode type comprising a cathode 4, control grid 5, screen grid 6, suppressor grid 1, and a plate 8. The input frequency currents supplied from an input circuit shown at a-b which maybe a radio receiving circuit, such as antenna circuit, preselectoror intermediate frequency circuit or the like are applied in the usual manner to the grid circuit of the tube by means of an input transformer l tuned by means of a capacity 2 connected across its secondary in the usual manner. The output or anode circuit of the tube connected to the high potential source indicated by the plus and minus signs includes a transformer II] with tuned primary and secondary by -means of capacities 9 and II respectively and serves for supplying the signalling'or operating frequency to an output or translating circuit to be connected to points 0 and d, which may be a similar selector circuit, detector stage or the like, as is readily understood. The arrangement thus far described corresponds to a standard tuned amplifying circuit. The input grid 5 is biased negatively relative to the cathode 4 by means of a biasing resistance H in the cathode lead shunted by a by-pass condenser l8 in a manner well known in the art..

The resonance curve of this type of circuit substantially has a shape as shown by the curve I inFigure 2 in which the abscissae represent the frequency and the ordinates represent the current or. voltage or the signal response such as for example in the well known decibel units. The resonance frequency is shown'as FD.

I have furthermore shown in accordance with my invention acircuit comprising the inductance coil l3 in parallel with a condenser l4 and connected between the positive terminal of the high tension source and the screen grid 6 as shown. Item 5 represents a drop resistance and I6 a.

by-pass condenser for supplying the biasingpotential for the screen grid 6 in a well known manner. The circuit l3, [4 acts in conjunction with the output circuit of the tube as a filter whereby the coupling between the circuits is purely electronic; that is, through the electron the input and output circuits are tuned. In the example shown, it is assumed that the circuit l3, I4 is tuned to a frequency F1 slightly above the resonant frequency F of the system, thus acting as a rejector or absorption'c'ircuit for frequencies slightly abovethe resonant frequency and producing a sharp cut-01f effect for frequen-- cies in close vicinity and above the resonant frequency as shown by the curve II in Fig. 2.'

Since the coupling between the circuits l3, l4 and 9, ID or it, H respectively is purely electronic and uni-lateral that is, through the electron stream only, as pointed out, it is readily seen that both circuits may be accurately adjusted independently without substantial reaction, resulting in a very sharply defined cut-off effect of the resultant resonance characteristic with regard for the currents in the output circuit of the system. In addition, the electron. coupling as used in accordance with the invention between the separate filter units, has the efi'ect that the system becomes highly independent of frequency so that the adjustments made equally apply for different operating frequencies. In this manner it is possible to shift the receiving frequency band from one frequency to another after the circuits have once been aligned and properly tracked by means of a common actuating element while maintaining substantially constant band width within extended operating frequency range. The above effect has been observed both with screen grid tubes and with both high frequency and low frequency pentcde tubes of known design.

Due to the coupling of the separate circuits, it is possible that one of the circuits becomes. a selfexcited' oscillator due to the inter-electrode capacitative coupling. In order to prevent this drawback I have found it advisable in accordance with a further feature of the invention to provide means for neutralizing the inter-electrode feedback effect with a view to stabilizing the operation and preventing the product-ion of oscillations in the circuits. For this purpose any one of the known neutralizing schemes may be used in connection with the invention. Thus in Figure 1,

I have shown a neutralizing condenser l9, con

nected between the lower end of the inductance coil l3 and the input grid 5 while the screen grid 6 serving for electronically coupling the circuit 13, M with the output circuit of the'valve is connected to a suitable tap point on the inductance E3 in such a manner that proper neutralizing potential is applied to the input grid 5 substantially preventing the setting up of undesired oscillations in the circuits. In accordance with a further scheme according to the invention, the production of oscillations'may be readily prevented by properly screening the electrodes in a well known manner such as described in more detail by Figure 5.

An arrangement as described by Figures 1 and 2 for securing a sharp cut-off at one side of the resonance frequency (low pass or high pass filter) may be easily modified to secure a cut-off at both sides as the resonance or ope-rating frequency that is, to obtain a true band pass or band selector eifect for passing a predetermined band of signalling or receiving frequencies such as the standard 10 kilocycle modulation band in broadcasting transmission. Thus, referring to Figure 3, I have shown a system comprising substantially two circuit arrangements of theftype described by Figure 1 connected in cascadewhereby the former is adjusted to'produce a cut-offclosely below the resonant'frequency and the latter is designed to produce a sharp cut ofi immediately above the resonance frequency, resulting in a resultant flat top type frequency characteristic known as 'a band pass or band selector filter as shown in Figure 4. Referring to Figure 3, I have shown in addition to the circuitarran'gement according to- Figure l, a further vacuum valve 20, of the screen grid type having. a cathode 2i, control grid 22, screen grid 23, and anode 24. The input of this valve 'is controlled by the output circuit H), H of the preceding valve 3, as shown. The circuits con posted to the valve 26, are similar to'those asscciated with the valve 3 comprising an output transformer 255, tuned by means of condensers and 27, an absorption circuit comprising the inductance coil 33 and condenser 3| and connected to the screen grid 23. and an extension.

phase of the neutralizing potentials upon the grids 5 and 26 respectively. The grid of the tube 26 is biased negatively by means of a biasing resistance 35 in the cathode lead by-passed by the condenser 36 in a manner well known. The screen grids of both valves are positively biased by connection to the high potential source through the high resistance i5 shunted to ground by the by-pass condensers l6 and 34.

The output circuit cd is connected to the secondary of the tuned transformer 26 in the oute put of the tube 29. a

By using a circuit of this type and by properly adjusting the resonant characteristics of the absorption circuit l3, M of the tube 3 and of resonance characteristic when using only the first circuit associated with the tube 3 having a sharp cut-off above the resonance frequency (F1) and curve III represents the resonance characteristic when using a further filter circuit associated with the tube 29 producing a sharp cut-off below (F2) the resonance frequency F0 While both circuits combined yield a resultant curve as shown at IV with improved selectivity and increased filtering effects as is readily seen.

Practical experiments and tests have shown that a circuit arrangement of the type described, while somewhat reducing the amplification due to power'absorbed by the circuits l3, l4, or'30, 3! respectively, presents the further advantageous effect of a substantial increase of the signal to'noise level which is one of the major sources and troubles in multi-stage superheterodyne receivers. This reduction of the noises in a circuit of the type as described, is due to the sharp cut-off filter effect whereby the higher and most troublesome noise side frequencies are suppressed resulting in a considerably increased signal to noise ratio of a receiver using a filter according to the invention. Thus by using filters of this type in the high frequency'or'prefsuperheterodyne receiver, the all-over noise level can be substantially reduced, resulting in greatly increased efficiency, sensitivity and improved quality of the receiver.

Instead of using a cascade arrangement of two valve circuits as shown, the rejector or filter eifect on both sides of the resonance curve may I 'be obtained by means of .a single valve with a plurality of appropriately screened grid electrodes such as shown by the embodiment according to Figure 5. Referring to this figure, I have shown a vacuum valve 40 comprising a cathode 41, control grid 42, first screen grid 43, a first reaction grid 44 connected to one of the filter or rejector circuits shown at 56, 51, a second reaction grid 45 connected to the other filter or rejector circuit shown at 53, 54 and a pair of further screen grids 46 and 41 for screening the grids 44 and 45 in a manner well known.

The latter may be positively biased in acccrdance with well known practice in the design and operation of radio circuits, and serve to pre vent self oscillations in the tuned circuits connected with the valve. The input currents supplied from the terminals ab are'applied to the control grid 42 by means of a transformer 5| with secondary tuned by means of condenser 52. The control grid is biased negatively relative to its cathode by the provision of a grid biasing resistance L59 in the cathode lead by-passed by a condenser'fifi in the usual manner. I have furthermore shown grounding condenser G! for grounding. the high frequency potentials of the grid 43 for stabilization of the circuit and the operation. In a similar manner the screen grids 46 and 41, may be placed at ground potential with regard to high frequency by 'means of grounding condensers of sufiicient capacity similar to the screen grid 43 as pointed out. The

positive potentials to the grids 44 and 45 are.

directly supplied from the high potential source through drop resistors 58 and 55 by-passed to ground by condensers 66 and 65 respectively. The tuned outp t circuit is comprised of the transformer 55 in parallel with a tuning condenser i and the output terminals are shown at cd similar as described in the previous figures. In this manner a band pass selector effect may be obtained by slightly detuning circuits 53, 54 and 56, 57 one above and the other below the resonance frequency F in a manner similar as described by Figure 3 resulting in a resultant band pass characteristic with extremely sharp cut-ofi" frequencies and presenting the advantages and novel effects of my invention as heretofore described. The rejector effect of the electron coupled filter circuits as described and accordingly the sharpness of the cut-off effect of the resulting frequency response characteristic, depends upon the damping factor of these circuits. The lower the damping, the higher the rejector action as is well known and the greater the filtering eifect of the system. On the other hand, the flat top shape of the all-over resonance curve such as the curve 3 shown by Figure 4, depends on the damping of the input and output circuits. The greater the damping of these circuits, the greater the fiat top or band spread effect. order therefore to get an ideal band pass filter, I have found it advantageous to use output circuits which are adequately damped and to use rejector circuits which have a low damping coefficient. The latter may be obtained by any one of the well known means, such as by using low loss coils or condensers; or also by providing regeneration in any well known manner for obtaining a damping reduction, such as by feedback coupling with the input circuit, such ,as a, coupling between the coils 53 and 56 and a winding of transformer 5|. In thismanner the damp ing of the circuits may be substantially reduced 7 by regeneration while inductive reaction with V the output circuit constituting the remaining portions of the filter is thereby eliminated. If neutralization is used, the regeneration can be obtained by a slight out-of-balance adjustment of the neutralizing condenser or other neutralizing elements provided for this purpose.

It is readily understood that it is possible to use morethan two valves and associated rejector circuits or a'single valve with any desired number of grids to increase the number of filtering units and improve the shape of the resultant characteristic as may be desired. Since the rejector circuits and their associated grids are at a higher direct current potential than the cathode of the valves they are, unless regeneration is provided, sufiiciently clamped to prevent osciilations between them and the anode circuit. However by providing screening or neutralizing arrangements as described as an essential feature of the invention between these circuits and the input or controlling grid circuit different figures.

It is understood that the invention as described is not limited to band pass filters or band selectors but may be easily employed in connection with any type filter such as band stop filters, low pass or high pass filters of known design whereby in the latter case only a single absorption circuit is necessary as is understood.

Furthermore, the invention as described is not limited to electric filters of the special type as illustrated but in general may be used in connection with any type of composite circuit arrangement in which a plurality of separate circuits are arranged to react one upon the other in such a manner as to secure a modified frequency response characteristic of the system. Thus for instance, the inventive idea may be embodied in a selective circuit arrangement adapted for passing separate individual frequencies within a predetermined band with substantially equal response thus securing a type of band spread circuit as distinguished from a specific band pass filter designed to simultaneously pass a band of signalling frequencies. From the above, it follows that the novel idea underlying the invention permits of many uses and applications and I do not wish to be limited to the specific arrangements and embodiments short of the broader and general scope of the invention as expressed in the appended claims.

I claim:

1. An electrical network comprising a first tuned circuit adapted to transmit oscillating energy; a plurality of auxiliary circuits; means for producing a space discharge .current, said first circuit and said auxiliary circuits being connected through portions of said space current forming a coupling path therebetween, said auxiliary circuits being partly detuned below and partly detuned above the resonance frequency of saidfirst circuit to broaden the width of the frequency response characteristic of said first circuit; and means for substantially suppressing any coupling between said first circuit and said auxiliary circuits except through the medium of said space current.

2. An electric filter comprising a space discharge device having main electrodes and a plurality of control electrodes, input and output circuits connected to one of said control electrodes and said main electrodes, respectively, tuning means for said circuits for selectively passing oscillating energy from said input to said output circuit and at least a further circuit detuned relative to said first circuits and connected to another of said control electrodes .to act as a rejector circuit absorbing energy from the discharge current of said device for modifying the frequency response characteristic of the energy transferred from the input to the output circuit, and means for substantially preventing any further coupling effect between said output circuit and said further circuits except through the medium of the space current of said device passing said control electrodes in succession.

3. An electric filter comprising a discharge device having main electrodes and a plurality of grid electrodes, input and output circuits connected to one of said control electrodes and said main electrodes respectively, tuning means in said circuits for selectively passing oscillating energy from said input to said output circuit and a pmrality of further circuits connected to separate control electrodes of said device and each tuned to a frequency different from the tuning of said first circuits to act as absorption circuits absorbing energy passed through said device for modifying the frequency response characteristic of said first circuits, and means for substantially preventing any further coupling effect between said output circuit and said further circuits except through the medium of the space current of said device passing said grid electrodes in succession.

4. An electric filter comprisingan electron discharge device having cathode, control grid, anode and a plurality of further grid electrodes, tuned input and output circuits associated with said control grid and said anode, respectively, for selectively passing high frequency energy through said device and a pair of further circuits one being tuned slightly above and the other being tuned slightly below the resonant frequency of said first circuits and connected each to one of said further grids with substantially no coupling with said first circuits except through the medium of the electron stream in said device, said further circuits acting as rejector circuits absorbing energy from the discharge path through said device to secure a'band pass filter effect for said first circuits.

5. An electric filter comprising a discharge device having a cathode, control grid, anode and plurality of further grid electrodes, tuned input and output circuits connected with said control grid and anode electrodes, respectively, and a plurality of further circuits detuned reiative to the resonant frequency of said first circuits and each connected with a separate grid elecgrid and suitable tap points of said further circuits.

'7. An electric filter as claimed in claim 5 in which said last means is comprised of screening electrodes for said last mentioned grids to prevent internal feedback upon the control grid and production of self-excited oscillations.

8. An electric filter comprising a discharge device having a cathode, a control grid, an anode and a plurality of further grid electrodes, input and output circuits connected with said control grid and said anode, respectively, and tuned to pass a predetermined operating frequency, and a plurality of absorption circuits detuned relative to said predetermined frequency, each connected with a separate grid electrode, means whereby said first circuits are highly damped and said absorption circuits have a substantially low damping, and further means for preventing the production of self-excited oscillations in said absorption circuits.

9. An eiectric filter comprising an electron discharge device having a cathode, control grid, anode, and at least one further grid electrode, tuned input and output circuits connected to said control grid and saidranode, respectively, for selectively passing oscillating energy through said circuits and an absorption circuit connected to said further grid electrode and de-tuned relative to the tuning of said input and output circuits to react upon the discharge stream and modify the frequency response characteristic of said circuits, and means forpreventing the production of selfexcited oscillations in said second circuit.

10. An electric filter as claimed in claim 9 in which said further grid electrode is positively biased relative to said cathode and a neutralizing condenser between a point of said second circuit and said control grid to prevent the production of self-excited oscillations. V

11. An electric filter as claimed in claim 9 in which said further grid is positively biased relative to said cathode and screening means for eliminating internal feedback from said further grid to said control grid'to prevent the production of self-excited oscillations in said further circuit.

12. An electric filter circuit comprising an electron discharge device having a cathode, an: ode, control grid and at least one further grid electrode, tuned input and output operating circuits connected to said control grid and said anode, respectively, for selectively passing oscillating energy through said device, afurther circuit slightly detuned relative to the tuning of said first circuits and connected to said further grid electrode and substantially decoupled from said input and output circuits except through the medium of the electron stream of said device, said further circuit acting as a rejector circuit for absorbing energy from the discharge stream for modifying the frequency response characteristic of said input and output circuits, respectively.

13. In combination, ment comprising an electron discharge device a first circuit arrange-- having a cathode, an anode, an input grid and predetermined 'frequency'through said device, a

tunable absorption circuit connected to said further grid electrode, said absorption circuit being substantially decoupled from said input and output circuits except through the common electron stream of said device to act as a rejector circuit by absorbing energy irom the discharge stream of said device, and a second circuit arrangement similar to said first arrangement and connected therewith in cascade, the absorption circuit of said first circuit arrangement being tuned to a frequency slightly below said predetermined frequency and the absorption circuit of said second circuit arrangement being tuned slightly above said predetermined frequency to secure a band filter effect for oscillating energy transmitted from said input to said output circuits.

14. In combination, a first circuit arrangement comprising an electron discharge device having a cathode and anode, an input grid and at least one further grid electrode, input and output circuits connected to said input grid and said anode for transmitting oscillating energy of predetermined frequency through said device, a tunable absorption circuit connected to said further grid, means for preventing coupling effects between said absorption circuit and said input and output circuits except through the electron stream of said device whereby said ab" sorption circuit acts as a rejector by absorbing energy from the discharge stream of said device, a second circuit arrangement similar to said first arrangement and connected therewith in cascade, the absorption circuit of said first circuit arrangement being tuned to a frequency slightly below said predetermined frequency andrthe absorption circuit of said second circuit arrangement being tuned slightly above said predetermined frequency to secure a band filter effect for oscillating energy transmitted from said input to said output'circuits, and further means for preventing self-excited oscillations in said absorption circuits. V

15. An electric filter circuit comprising an electron: discharge device having a cathode, an anode, a control grid and at least one further grid electrode, input and output circuits connected to said control grid and said anode and tuned for selectively passing high frequency currents of predetermined frequency, an absorption circuit slightly detuned relative to said predetermined frequency connected to said further grid electrode, said absorption circuit being substantially decoupled from said input and output circuits except through the electron stream of said device and serving to modify the frequency response characteristic of said input and output circuits, and means whereby said input and output circuits have a high damping and said absorption circuit has a relatively low damping.

16. A filter circuit'comprising a main resonant circuit; an auxiliary resonant circuit being detuned relatively to said main circuit; means for producing a space discharge currentjsaid'main having a predetermined frequency response,

characteristic; a plurality of auxiliary circuits having response characteristics differing from said main circuit means; means for producing a space discharge current; said main circuit means and said auxiliary circuits being connected through portions of said space current forming a coupling path therebetween, whereby said auxiliary circuits act as absorption circuits modifying the frequency response characteristic of said main circuit means; and means for substantially suppressing any coupling effect between said auxiliary circuit and said main circuit except as afforded through the medium of said space current.

18. In an electric filter, a main circuit having a predetermined frequency response characteristic; an auxiliary circuit having a frequency characteristic differing from said main circuit; means for producing a space discharge current; said main and auxiliary circuits being connected through said space current forming a coupling path therebetween, whereby said auxiliary circuit acts as an absorption circuit modifying the frequency response characteristic of said main circuit; and means for'substantially suppressingany coupling effect between said circuits except as afforded through the medium of said space current. i

19. In an amplifier circuit, an electron discharge device having a cathode, a control grid, at least one further grid electrode and one or more shielding electrodes; means for producing a space discharge current between said cathode and said anode; a main circuit comprising input and output circuits associated with said control grid and said anode respectively, said main circuit having a predetermined frequency response characteristic, auxiliary circuits connected to the further grid electrodes in nonfilters thereby effecting the coupling path be-' tween said input and output circuits and'modifying the frequency response characteristic of said main circuit, said shielding electrodes sub-. stantially suppressing any coupling effect between the auxiliary circuits and said main cir- V cuit except as afforded through the medium of said space current; I

JOZEF PLEBANSKL

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