US2281621A - Band pass filter - Google Patents

Description

May 5, 1942.

N. M.'Rusr Erm.

BAND PASS FILTER Filed April 3o, 1941 NUEL M.

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ATTORNEY Ratenied May 5, 1942 BAND-PASS FILTER Nol Meyer Rust, Chelmsford, Joseph Deugias Brailsford, London, and Ernest Frederick Goodenough, Springfield, Chelmsford, England, assignors to Radio Corporation' of America, a

corporation of Delaware Application April 30, 1941, Serial No. 391,2t In Great Britain ctober 5, 1939 7 Claims.

This invention relates to band-pass illters and has for 'its object to provide improved lters which are comparatively exible in design and will facilitate the production of wide band ampliers of high gain andvlarge signal-to-noise ratio, or narrow band amplifiers oi great selectivity and with a at topped response curve such as will give a good signal-to-noise ratio. Though not limited to its application thereto the primary applications of the invention are to television amplifiers, and to intermediate frequency ampliiers for broadcast and like receivers.

The present invention may be regardedv as a development or variation of the invention contained in our co-pending application Serial No. 359,182, iiled October 1, 1940. According to this said invention a band-pass iilter effect is obtained by superimposing upon the frequency-impedance characteristics of one network, the frequencyimpedance characteristic of at least one further network which is coupled to the first in such manner that the impedance elements lin the second network are, in effect, reversed. In other words, there are employed, in accordance with the invention contained in the specification of the above application, two networks coupled together in such manner that the second network acts as though it were directly included in and Vas part 'of the first network but with its imoriginal positive impedance to be operated upon.

tive and in' the other of which the eective parallel resistance is negative the said twotuned circuits being connected together by means of a quarter wave line 'and the values of the' resistances in each case being equal to that of the characteristic impedance of the said line.

The invention is illustrated in and explained y in connection with the drawing accompanying the specification, in which Fig. 1 represents a circuit which will serve to explain the principles underlying the invention, Figs.` 2 and 3 are modifications of an embodiment of the invention, Fig. 4 is a practical circuit arrangement embodying the invention, and Fig. 5 represents the electrical equivalent of the circuit shown in Fig'.` fl.

In order that the invention may be the better understood there will first be given a brief and simplified description of the phenomena upon which the invention is based.

Consider first a circuit as shown in Figure 1 which is an explanatory diagrammatic iigure. In'this circuit a quarter Wave line (conventionally indicated by the dimension M4, i being the wave length) is terminated by impedance ntworks which are similar as regards reactive properties but whose resistive components are equal and opposite. In other words, the equiv-- aient shunt resistance is positive in one impedance networlr and negative in the other. Each. moreover, is numerically equal to the characteristic impedance Zu of the line.

In accordance with the well known properties of a quarter wave line the impedance looking into the line with the near end termination disconnected is Zn at the in-tune frequency, but at frequencies off-tune, this impedance behaves as In other Words, negative resistance effects are used to compensate positive resistance eiects, and negative reactance effects to compensate positive reactance effects. In these arrangements both negative effects are produced by valve circuits.

The present invention diers from the previous invention above referred to in that, although pure negative resistance produced by a valve arrangement is still used to cancel positive resistance, the cancellation of reactive eiects vis produced by a quarter Wave line action. The present invention oiers the advantage of being rather more practical and economical for narrow band working,e. g. for handling ultra short wave television carriers and sidebands.

According to the present invention a band pass lter comprises at least two tuned circuits including similar reactive elements and in oneY of which the effective parallel resistance is positheinverse of that terminating the far end o f the line. Thus a parallel resonance circuit will appear as a series resonant circuit of approximately the same Q as that of the parallel resonant circuit and its reactance will change in the opposite direction and at approximately the same rate over a small band of frequency near resonance.

The change in reactance (or rather susceptvance) of a circuit shunting the near end will will change an increasing amount with increasing departure from this in-'tune frequency and cancellation will no longer be complete.

Measurements taken at point A (to earth) would reveal that at the ,tune frequency of the circuits the` negativey and positive resistance effects would cancel and the impedance would be very high (theoretically it would be infinite). v

` wiring stray capacities.

About the tune frequency for a small frequency band the impedance would remain resistive owing to the quarter wave reactance cancellation,- but the net value would drop, owing to the negative resistance component thrown over by the quarter wave line varying. Finally as the divergence from tune increased, the reactance cancellation would not holdfurther. and the net impedance would not only further decrease in value but also swing in phase angle. A useful but limited compensation effect with an increase of impedance would be obtained, therefore, the limitation be-l opposite sign to those thrown overby the quarter' wave lines, and, (over a small-band .of frequencies) produces reactance cancellation, the resistance component of thiscircuit stabilising the circuit over all.

` Actually the susceptance thrown over by the two lines increases uniformly at first with change of frequency 'from resonance, then more slowly and ilnally decreases. If the initial susceptance -change of the center circuit be made slightly less than the initial susceptance change of the lines,

at some frequency off-tune, the susceptance of the center circuit will equal that of the lines and exact cancellation will take place. This arrangement will then give a treble humped responsel curve, viz., one hump at the tune frequency Fh and two at the frequencies Fi and F2 of susequidistance from Fo. Outside Fi and F: the curve drops sharply because the susceptance of the middle circuit increases much more rapidly than that of the lines, and compensation is increasingly reduced.

The resistance component remains unchanged (expressed as a parallel resistance) since the only resistance that changes is that of the lines. each of which changes to the same amount and cancel one'another out. Between Fi and Fi the impedance is substantially constant and 'is raised very considerably by the action of the negative impedance.

In practice the required conditions can be obtained with simple circuits, as shown in Figure 4.

In Figure 4 actual values are indicated by way of example.

valve has a condenser of 26 mmfds. and the grid load of the output valve one of 16 mmfds.in both cases these can be realised with valve and The arrangement of cathode impedance in the output valve limits the negative impedance applied by the back coupling to 1,000 ohms, or more logically, the negative admittance applied cannot exceed 1/1000 or .001

As .will be seen from Figure 5, lthe coupling system betweenthe circuits is equivalent to quarter wave lines. In this figure the quarter wave lines are conventionally indicated as M4 and the reactance correction circuit is indicated as RC.

The shunt inductance of the quarter wave line systems are, of course, lumped in with the tuning coils of the tuned circuits. just as the4 valve and wiring stray capacities are lumped in with the condensers.

what wqlciaim is:

1. A band pass fllter circuit comprising atleast two tuned circuits including similar reactive elements andin one o'f which the effective parallel lresistance is positive and in the other of which the effective parallel 'resistance is negative, the said two tuned circuits being connected together, by means of a quarter waveline and the values of the resistances in each case being equal to that of the characteristic impedance of the said line.'. f

2. A illter in accordance with claim 1 wherein the two tuned circuits arelconnected together by two quarter wave lines in series, there being connected across the filter atthe junction ofthe two lines a parallel tuned circuit of relatively high Q value.

- ceptance cancellation, spaced logarithmically capacitances of said parallel tuned circuits are Voo The circuit illustrated in Figure 4 is designed to the order of 50 with television pentodes. It will be seen that the anode circuit load of the rst 3. A. filter circuit comprising-a valve having a y tuned anode circuit, a parallel tuned circuit hav- `connected to said anode circuit over a second quarter wave length line.

4. A filter circuit as claimed in claim 3 wherein the said further -tuned circuit is connected to the grid of an output valve.

5. A lter circuit as claimed in claim 3 wherein the. said further tuned circuit is connected to the grid of an output valve and there is included in the cathode leg ol' said output valvea resistance of value equal to said positive effective resistance of the first parallel tuned circuit and wherein the anode of said output valve is coupled back to the inductance of said further tuned circuit.

6. A filter circuit as claimed in claim 3.wherein the said further tuned circuit is connected to the grid of an output valve and wherein the constituted, at least in parts, by the capacities of the valves and wiring.

' '1. A alter circuit as ciaimed in claim 3 wherein the saidfurther tunedl circuit isv connected to the grid of an output valve and wherein the inductances of the quarter wave line are constituted by the inductances of the tuned circuits.

NOEL MEYER RUST. v l JOSEPH LiJOUGLAs BRAILSFORD. .ERNEST FREDERICK GOODENOUGH.

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