US2489297A - Electronic pulse filtering system - Google Patents

Description

Nov. 29, 1949 Filed May 24, 1945 E. LABlN El' AL 4 shet's-sheet 1 @Zyl DOA/@LD D. GP/EG ELECTRONIC PULSE FILTERING ySYSTEM Filed May 24, 1943 4 sheets-sheet 2 @@5934 ggg 1 51 a l-/ /52 :r 41a Il 42 /434 A 171 I Wm Vlbl/JzfzbKgg :155 (48 ,3 i {an} JJM 55% g2 J4@ 1/415 W42@ Lijm kf 'zm l l l l JIJ I r I il Ll l l Y y zas 5 kan' TURN Nov. 29, 1949 E. LABIN ET AL ELECTRONIC PULSE FILTERING SYSTEM Filed May 24, 1943 4 Sheets-Sheet 3 IN VEN TORJ' Vey/05% Nov. 29, 1949 E. LABIN ET AL 2,489,297

ELECTRONIC PULSE FILTERING SYSTEM Filed May 24, 1943 4 Sheets-Sheet 4 w/m'H aisee/Minerali 103 HMPL l TUDE DISCR/M/NHME ATTRAZY THPESHULD Patented Nov. 29, 1949 UNTED ST'EE TENT @Fifii ELECTBONEC PLSE FILTERING SYSTEM Appealing May 24, 1943, serial No. 488,180

16 Claims.

This invention relates to communication systems utilizing trains of pulses for conveyance of intelligence and more particularly to a selective ltering system therefor.

In the case of pulse communication, either radio or telegraphy, Where the pulses are time modulated for transmission of intelligence, interference pulses or pulses of other communicating channels present will cause interference and jamming for the usual receiver. In the case of radio communication an enemy may, for example, attempt to jam the communication by utilizing a similar pulse transmission. This imposes on the same carrier unwanted pulses of one or more shapes. Then again, additional pulses dffering in character such as amplitude, Width, the slopes of leading and trailing edges and/or pulse repetition rate may be added to the train of pulses as separate channels of communication or merely to confuse the enemy and render it diflicult for him to determine which of the pulses of the wave carries the intelligence which he Wants to jam.

It is one of the objects of our invention to provide a method and means to eliminate from a train of pulses those pulses which differ in one or more shape characteristics from a given pulse shape.

Another object ci our invention is to provide a selective pulse filtering system capable of eliminating those pulses diiiering in amplitude, width, slope (build-up and decay characteristics of leading and trailing edges) and/or the repetition rate from wanted pulses of a given shape and given repetition rate.

Assuming, for example, that a train of pulses is made up of pulses differing in shape characteristics such as amplitude, width and slope and also in repetition rates, a pulse :filtering system in accordance with our invention is capable of selecting those pulses of a given pulse shape and given repetition rate from the train of pulses and to eliminate all other pulse shapes and pulses of said given pulse shape occurring outside of the given repetition rate. In one embodiment of the invention energy of the train of pulses is subjected separately in parallel circuits to a plurality of iiltering operations and the resultant thereof mixed and clipped, While in another embodiment the train of pulses is subjected in succession to a series of filtering operations. Each of the iiltering operations operate on a different shape characteristic or repetition rate to assist in the elimination of those pulses diiiering from a given pulse shape or in given repetition rate.

One of the ltering operations, for example,

2 eliminates those pulses differing in amplitude with respect to a given amplitude characteristic, a second filtering operation eliminates those pulses differing in slope from a given slope characteristic, a third operation eliminates those pulses differing in width from a given Width characteristic and a fourth operation eliminates odd pulses not included in a given pulse repetition rate. The unwanted pulses, of course, may differ in one, two or three shape characteristics or only in pulse repetition rate from a Wanted pulse and its :repetition rate. Regardless of these differences our system operates to eiiectively eliminate the unwanted pulses.

For a further understanding of the invention, reference may be had to the following detailed description to be read in connection with the accompanying drawings, in which:

Fig. 1 is a block diagram of one embodiment of our invention;

Figs. 2A, 2B, 2C and 2D are graphical illustrations representing the ltering operation of the parts of the system illustrated in Fig. 1;

Fig. 3 is a graphical illustration representing the end result of the combined filtering operation of the parts of Fig. 1;

Fig. l is a block diagram of a second embodiment of our invention;

Fig. 5 is a graphical illustration of the operating steps of the system of Fig. 4; and

Fig. 6 is a schematic diagram of the amplitude discriminator, slope selector and Width discriminator of Fig. 4.

Referring to Fig. 1, the filtering system therein shown is divided into four branches A, B, C and D connected together in parallel. Each of these branches performs a diierent filtering operation. The branch A discriminates in amplitude between the pulses applied thereto, branch B discriminates as to diierences in slope, that is, diierences in the build-up and decay character of the leading and trailing edges of the pulses, branch C discriminates as to diierences in width, and branch D discriminates according to a given pulse repetition rate.

Referring to the branch A of Fig. l and. to Fig. 2A, assume that a train of pulses il, i2 and i3 such as shown in curve Ai is applied to the branch. Pulse l2 is a desired pulse of a given amplitude, while pulse il is of greater amplitude and pulse i3 is of less amplitude. amplitude discriminating means of this branch may comprise any suitable amplitude discriminating means, such for example as disclosed in the copending applications of D. D. Grieg Serial No.

561,553, filed November 2, 1944, now United States Patent No. 2,434,921, issued January 27, 1948, and Serial No. 487,071 filed May 15, 1943, now United States Patent No. 2,419,548, issued April 29, 1947, or the system disclosed in U. S. Patent No. 2,406,882. For purposes of illustration, however, We choose to show the amplitude discriminator disclosed by Grieg in application Serial No. 487,071, the circuit details of which are shown in Fig. 6.

When the pulses of curve A1 are applied to the branch A through input 9, the pulses are first subjected to a threshold clipping action by the clipper I whereby the smaller pulse I3 is eliminated by the threshold clipping level Ill. If the larger pulse were desired, the clipper would then be biased to clip at level |4a.

The output of the threshold clipper I0 retains the pulses I and I2 as shown by curve A2. These pulses are applied to an amplitude selector I5, Figs. 1 and 6, adjustable at |5a to threshold clip the pulses at a level I6 thereby clipping the upper portion of the larger pulse I. This clipped portion is amplified and inverted as shown by curve A3 as pulse la. The output of the amplitude selector I5 is applied to a mixer Il, Fig. 6, together with the output of threshold clipper I0 which is supplied to the mixer Il through connection I8. The pulse energy Ila thus cancels pulse leaving the wanted pulse I2 as indicated by curve A4.

The output of the mixer Il, if desired, may be applied to a differentiator I9, Fig. 1, thereby translating the pulse I2 into positive and negative pulses |2a and |2b corresponding respectively to the leading and trailing edges of the pulse. The mixer |'I may be biased to provide a threshold clipping operation at a level lla to remove noise fluctuations and other disturbances such as might occur due to the cancelling operation of the larger pulses. As is clear in Fig. 2A, the amplitude discrimination takes place regardless of the shape or width of the pulses.

Referring to branch B of Fig. l and Fig. 2B, A.

vwhile pulse 2| has a width corresponding to the base of pulse 22, its leading and trailing edges are steeper than those of pulse 22. Pulse 23 has the leading and trailing edges of greater slope and the base thereof is wider than the base of pulse 22.

The pulses 2|, 22 and 23 are applied to a limit clipper 24 to limit the amplitude of the pulses as indicated by the clipping level 25. The output of the clipper 24 is applied to a differentiator 26 whereby positive and negative pulses are produced corresponding respectively to the leading and trailing edges of each of the pulses. These differentiation pulses, as shown by curve B2, are of amplitudes corresponding to the steepness of the 'corresponding edges of the pulses 2|, 22 and 23. Thus, the pulses 2 Ia and 2lb are of greater amplitude than the pulses 22a and 22|), and the latter are of greater amplitude than the pulses 23a and 23h. In order to discriminate between these pulses of different amplitude, the output of the differentiator 26 is applied to an amplitude discriminator 2'| which may be identical with the amplitude discriminator of branch A (see circuit parts I5 and of Fig. 6). It follows, therefore,

that by proper clipping of the pulses 2|a or 2lb, as the case may be, a pulse output is obtained corresponding to the larger pulse 2 I.

Since it be desirable, however, to obtain a pulse corresponding to the wanted pulse 22 and to eliminate those corresponding to pulses 2| or 23, a clipping operation will be performed at level 29 corresponding to the clipping level I4 in Fig. 2A. This results in carrying forward the pulses according to curve B3. By clipping and inverting the peak of the greater pulse 2Ib as indicated by the pulse 2|c (curve B4) and mixing it with the pulses of curve B3, the pulse 2Ib will be eliminated leaving pulse 22h (curve B5) which corresponds to the trailing edge of the pulse 22. If desired, the clipping operation may be made on pulses 2|a and 22a thereby resulting in an output pulse corresponding to the leading edge of the pulse 22.

Referring to Fig. 2C and branch C of Fig. 1, let lt be assumed that the width discriminator of branch C receives a train of pulses illustrated by curve C1. This curve is shown provided with pulses 3 I 32 and 33 in which pulse 32 is the wanted pulse of a given width. Pulse 3| is of less width than pulse 32 while pulse 33 is of greater width. Pulse 3| is also shown of greater amplitude than pulse 32 while pulse 33 also differs in the slope of the leading and trailing edges thereof.

The width discriminator may be any suitable width filtering circuit and preferably is of the character disclosed in the copending application of E. Labin, Serial No. 467,509 filed December 1, 1942, now United States Patent No. 2,418,127, issued April 1, 1947. The pulses of curve C1 are applied to a differentiator 34 thereby translating the pulses 3|, 32 and 33 into positive and negative pulses illustrated by curve C2. The pulses produced by the differentiation of input pulses 3| and 32 are of the same amplitude since the build-up and decay slopes of these input pulses are the same. The pulses 33a and 33b of input pulse 33, however, are of less amplitude. It follows that a clipping operation could be used to eliminate the pulses 33a and 33h similarly as in the case of the pulses illustrated in Fig. 2B. This, however, would leave pulses 3|a, 3|b and pulses 32a, 32h without any other diierenceexcept the timing therebetween.

According to the method selected for purposes of illustration, width discrimination is made by applying the output of the differentiator 34 to an invertor 35 and then through a delay device 36 f which is adjusted to retard the pulses an amount t1 corresponding to the width of the wanted pulse 32. This inversion and retardation effect is illustrated by curve C3. The pulse output of the delay device 36 (curve C3) together with the pulse output of the differentiator 34 (curve C2) are applied to a mixer and clipper stage 38. Curve C4. illustrates the mixed relationship of the pulses. It will be noted that in this mixing operation only those pulses 32a and 32h are in alignment and therefore are the only pulses of the two curves that add together as indicated at 32e. The stage 38 is preferably biased to clip the pulses at a level 39, thereby selecting pulse 32e and eliminating all the other pulses. It therefore follows that by this method all pulses differing in width from a given pulse shape are eliminated and a new pulse 32e is produced corresponding to the trailing edge of the pulse of given width.

If an output pulse corresponding to the leadrngedgeio the wanted pulse, is desired, thisV may llaaccornplished` by inverting the pulses of curve G2. and mixing same with p ulses according to curve C2 delayed a period equal to the period of the Wanted pulse. This will result in a large pulse corresponding to the leading edge of the wanted pulse and by clipping all other pulses canbeeliminated.

Referring now to Fig. 2D and branch D of Fig. l-let it be assumed that the pulses fila, Mb, 42 and; 42a ofcurve D are applied to the input of 'lllllch` D. The pulses ela, 4I?) and 42a are the wanted pulses and they are shown to occur at a given repetition rate. The blocking feature of the branch D is ofA special utility for eliminating otherI pulses. of shape identical to the wanted pulses, Vwhichoccur at a repetition rate differing from the repetition,- rate of the wanted pulses. The method followed in carrying out this blocking operation may be in accordance with known method or those disclosed in the copending ap plication of H. G. Busignies Serial No. 380,186, led February 24, 1941, now United States Patent No. 2,423,082 issued July 1, 1947, and U. S. Batent No. 2,406,019. The pulses of curve D1 are first applied to the coupling stage 44. The output of the coupling stage de is applied to a selector circuit 45. The selector circuit is tuned tov a period corresponding to the repetition rate of the wanted pulses so as to produce a suitable harmonic. By suitable adjustment, the selector means 45. and the square wave generator 46 may be caused to produce from this harmonic a blocking potential occurring only when no wanted pulses are due for reception. This blocking potential is illustrated by the potential line 41. When the blocking potential drops below the zero axis of curve D1, as `indicated at 48, a, n egative blocking potential is produced at the output of the square wave generator which is applied by connection 49 to the coupling stage 44. This blocking potential will thereby eliminate pulses such asthe pulse 42 occurring between the wanted pulses. The wanted pulses thus passed by the coupling stage 44 are applied over connection 50 to a diierentiator 5l by which the pulses are translated into positive and negative pulses shown by curve D3.

In Fig. 3, curve rc, we have shown an input train of pulses which, let it be assumed, is applied at theinput 9 to theltering system of Fig. 1. The p ulses of thistrain are selected of various shape characteristics to illustrate the filtering function ofthe several branches of the liltering system. The train comprises pulses Si, 62, 63, 64, 65 and 6,6: Assume that pulse 62 is the wanted pulse havinga given amplitude, given build-up and decay characteristics and a given width. The p ulse 6I differs both in amplitude and width from the wanted pulse 62. Pulse 63 diiers in build-uprand decay slopes from pulse 52 and pulse64 diers therefrom in width. The pulse 65- is identical to the shape of pulse E32 but is outsideof a given repetition rate of pulse E52. Pulse 66l differs from pulse 52 in that it is of smaller amplitude and of greater width.

Curve sa represents the output of the branch A after the pulses are filtered according to amplitude (see Fig. 2A). Curve 3b represents the output of the branch B after the pulses have been filtered according to the buildup and decay slope vcharacteristics of the wanted pulse 62 (Fig. 2B).

It Awill be seen'that only those pulses having the corresponding slopes remain at the output of thebranch B., Curve 3c represents the output.

UT u',

1 lector of branch C in accordance with the width dscrimination (see Fig. 2C). Curve 3c, therefore, has pulses corresponding only to the input pulses of a width equal tothe width of pulse 6.2. Curve represents the output of branch D in accordence with the given repetition rate of pulse 52 (see Fig. 2D). This blocking effect is indicated by the blocking potential 68 and it will be seen that pulse 65' is thereby eliminated. This blocking feature may be adjusted to eliminate the other pulses 5l, 63., t4 and 66 in branch D, although not necessarily so.

Curve 3e represents the output o the several branches as they appear in mixed relation in the mixer and threshold clipper l0. It will be seen from the alignment of pulsations of curves 3a, 3b, te' and iid that the pulsationsV corresponding to the trailing edge of wanted pulses 52 are greater in number than for` any of the pulsations corresponding to the unwanted pulses. The threshold clipping function of the mixer l0 may therefore be adjusted to clip only the largest pulsations which in this case are pulses 52e. The final output is represented by curve 3f.

It is tnus clear that according to this embodiment of our invention all pulses diiering in shape characteristics as Well as repetition rate from a given pulse shape and a given repetition rate are eliminated thereby reducing interference to only pulses or disturbances that are in superposition of a wanted pulse. Since this superpositioning of interfering pulses is infrequent such interference be disregarded or atleast easily overcome by repeating signals.

In the embodiment shown in Fig. 1, the several shape discriminating stages are arranged in parallel and the outputs thereof are combined and clipped to obtain the wanted signal increments. In Figs. 4, 5 and 6, a second embodiment is illustrated in which the several pulse discriminating stages are arranged in tandem. The consecutive stages of this system comprise an amplitude discriminator lili, a slope selector E02, a width discriminator |03 and a repetition rate seltlf. Each of these stages is adjustable so that a pulse of given shape, that is of given amplitude, given slope characteristics, given width and the repetition rate of which is known may be selected. For a discussion of the application of the pulse amplitude, slope and width discriminating features of this embodiment as an interference limiter in radio receivers reference may be had to our copending application Serial No, 488,182, filed May 24, 1943, now United States Patent No. 2,434,937, issued January 27, 1948.

Let it be assumed, for example, that a train of pulses lll. H2, H3, H4 and H5 such as shown, by curve 5a of Fig. 5 is applied to the input l ifi of the amplitude discriminator lill. Also yassume that pulse l l2 is the given pulse shape of the wanted pulses. The amplitude discriminator is preferably selected of the character disclosed in branch A of Fig. l. According to the clipping features of the discriminator of branch A.- the pulses are rst clipped at a level HS to eliminate the smaller pulses such as pulse l i3 and second. at a level to clip the larger pulses for inversion and mixing whereby the larger pulses such as pulse lll are eliminated. Curve 5b, therefore, represents the output of the amplitude discriminator lill after the larger and smaller pulses Ill and H3 of curve 5a are eliminated. It will be noted that -by this amplitude discrimination, pulse !|5.has been re-shaped increasing 7 the steepness of the edges thereof las indicated at ||'5b.

The slope selecting stage |02 may be of any known gate clipping character capable of clipping the pulses between two selected levels. For illustration purpose-s we show in Fig. 6 a clipper gate of this character which is also disclosed in our copending application Serial No. 488,182. Assuming that pulse ||2 has the given slope characteristics, the stage |02 will be adjusted at |02a and |2b to slice the pulses between limits such as indicated by lines |31 and |32 Where pulse llb, for example, differs in width from pulse ||2.

The width discriminator |03 is of the adjustable L-C damped circuit character disclosed in our copending application Serial No. 488,182. As described in detail in our aforesaid copending application, the width discriminator first inverts and amplies at lila the pulse energy to provide negative pulses as shown by curve 5c. It will be noted that by slicing pulse ||5b and amplifying, 'a pulse l |50 substantially rectangular in shape s produced. These negative pulses are then applied to the L-C circuit to produce undulations according to curve 5d. Where the L-C circuit is tuned to a period twice the time interval between the leading and trailing edges of pulse I2, the shock excitation caused by the leading and trailing edges will produce oscillations 12| and |22 Which Iare in step and therefore produce a maximum undulation ll2d. The pulses Ill and l l5, however, are of larger and smaller periods respectively than the period of this tuning adjustment. The larger period of pulse H13 produces oscillations which are out of step and therefore form an undulation Hdd which is of amplitude less than the amplitude of undulations ||2d. The period of pulse shape l |519 being smaller than the period of pulse H2 also produces an undulation ||5d which is of smaller amplitude. By suitably clipping at a level |25 by clipper tube |03b, a pulsation |2e is obtained corresponding to the wanted pulse l I2. The damper tube |030 is maintained inoperative by the input pulse energy applied to the control grid |0301 After the duration of the input pulse energy the tube ||J3c is permitted to conduct when oscillations in the L-C circuit swing to negative polarity thereby damping out the oscillatory energy following undulations llZd, Hdd and ll'd as indicated in 'curve 5d.

A Should it be desired to obtain a pulsation corresponding to the pulse l5 and to eliminate the other pulses of the curve 5a., the slope selecting stage EQ2 will be adjusted substantially as above to gate clip the pulses of curve 5b between limits |3| and |32. By width discrimination of the resulting pulse shapes of curve 5c with the L-C circuit for stage H53 tuned at a period which is twice the period of pulse l lc, a series of undulations l iZf, l Ulf and l 55j different from the corresponding undulations of curve 5d is produced. It will be observed that therundulation |5f is of greater amplitude than the undulations |21' and Hilf. By a clipping operation the pulsation |I5g may be segregated from the other undulations.

The additional undulation |34 adjacent undulation Hilf is the oscillationproduced by the leading edge of the pulse.

The repetition rate selector stage Ill@ is shown yin tandem relation with respect to the shape discriminator stages of Fig. 4.and may be used to eliminate pulses identical in shape with the 'Wanted pulse Where they occur outside of the 8 given repetition rate of the Wanted pulse. Since this is illustrated graphically in Figs. 2D and 3, a further illustration yand description of the function thereof is believed unnecessary.

It will be observed that the width discriminator feature of Fig. 4 is considerably different from the width discriminator C of Fig. 1. Should it be desirable, the width selector |03 may be substituted for the width discriminator C. Should the substitution be made, it will be necessary, however, to provide a delay device for the output from branches A and B` so as to retard the pulse output of these branches an amount corresponding to the retardation of the undulations produced by the width discriminator in response to the input pulses. The repetition rate discriminator could, in that case, be used in series with the miXer stage 'it similarly as in the series arrangement of Fig. 4.

While we have shown and described the principles of our invention in connection with specific embodiments, we recognize that various changes and modifications may be made therein without departing from the invention. For example, any two or more of the discriminating stages may be used together in the manner illustrated where a lesser number of shape characteristics are involved and Where pulse repetition rate is not an important factor. It will be understood, there'- fore, that these embodiments are given by way of example only and not as limiting the objects of our invention and the appended claims.

We claim:

1. A method of selectively ltering a train o pulses to eliminate those pulses differing in shape characteristics from a given pulse shape comprising subjecting energy of the train of pulses independently to a plurality of filtering operations, each of said filtering operations being such as to eliminate those pulses differing in one of said characteristics from said given pulse shape, mixing the remaining pulses from each of the filtering operations whereby the pulse energy passed by the filtering operations is greater for pulses of the given pulse shape than for pulses of other shapes, and clipping the greater pulse energy thus obtained thereby eliminating the energy corresponding to those pulses differing from said given pulse shape.

2. The method defined in claim 1 wherein said given pulse shape has given amplitude and slope characteristics, and the filtering operations are such as to eliminate those pulses differing in at least a selected one of said characteristics.

3. The method defined in claim 1 wherein said given pulse shape has given amplitude and width characteristics, and the filtering operations are such as to eliminate those pulses differing in at least a selected one of said given characteristics.

4. The method dened in claim l wherein the given pulse shape has a given base width characteristic and the edges thereof have given slope characteristics, and the filtering operations are such as to eliminate those pulses differing in at 65 least a selected one of these characteristics.

5. The method defined in claim 1 wherein the given pulse shape has given amplitude, width and slope characteristics, and the filtering operations are such as to eliminate those pulses differing 70 from said given pulse shape in at least a selected one of said characteristics.

6. A system for selectively filtering a train of pulses to eliminate those pulses diiiering in shape characteristics from a given pulse shape com- 75 prising a plurality of pulses filtering means,

means connecting said filtering means in parallel so that each operates independently on input energy of the train of pulses, each of said iiltering means being such as to eliminate those pulses diiering in one of said characteristics from said given pulse shape, a mixer, means to supply the output of said iiltering means to said mixer, whereby pulse energy corresponding to the characteristics of said given pulse shape passed by the ltering operations of said ltering means combine, the pulses corresponding in all of the given shape characteristics producing a combined energy greater than those pulses differing from one or more of the given shape characteristics, and means for clipping the greater pulse energy thus obtained thereby eliminating the energy corresponding to those pulses diiering from said given pulse shape.

7. The system deiined in claim 6' wherein the filtering means comprise an amplitude discriminator stage, a slope discriminator stage and a Width discriminator stage.

8. The system defined in claim 6 in combination with la pulse repetition rate discriminator arranged to eliminate pulses identical in shape with said given pulse shape but occurring outside of a given repetition rate.

9. The system dened in claim 6 wherein there are at least two ltering means, one of said filtering means being arranged to eliminate those pulses differing from the amplitude characteristic of said given pulse shape and the other of said ltering means being arranged to eliminate those pulses differing from the width characteristic of said given pulse shape.

10. The system dened in claim 6 wherein there are at least two filtering means, one of said filtering means being arranged to eliminate those pulses diiering from the amplitude characteristic of said given pulse shape and the other of said ltering means being arranged to eliminate those pulses diiering from the slope characteristic of said given pulse shape.

11. The system defined in claim 6 wherein there are at least two filtering means, one of said filtering means being arranged to eliminate those pulses differing from the slope characteristic of said given pulse shape and the other of said filtering means being arranged to eliminate those pulses differing from the width characteristic of said given pulse shape.

12. The system defined in claim 6 wherein the 10 iiltering means comprise an amplitude discriminator means, a slope discriminator means, a width discriminator means and a pulse repetition discriminator means.

13. A system for selectively filtering a train of pulses to eliminate those pulses differing from one or more of the given amplitude, given slope and given width characteristics of a given pulse shape, comprising filter means to pass energy of those pulses having said given amplitude characteristic, filter means to pass energy of those pulses having said given slope characteristic, lter means to pass energy of those pulses having said given width characteristic, and means to combine the operations of the several filter means to produce a nal unitary sequence of output pulses having a timing according to those pulses of said train having said various given shape characteristics.

14. The system dened in claim 13 wherein the means for combining the operations of the several filtering means includes means for connecting said lter means in tandem relation whereby certain of said lter means operate on the pulse energy output of other of said filter means.

15. The system defined in claim 13 wherein the means for combining the operation of the several lter means includes means for connecting said lter means in parallel circuit relation for separate operation on the pulses of said train and means for combining the output of the several filter means for producing therefrom said nal output pulses.

16. The system dened in claim 13 in combination with a pulse repetition rate discriminator for eliminating pulses of said given pulse shape occurring outside of a given repetition rate.

EMILE LABIN. DONALD D. GRIEG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,483,172 Gannett Feb. 12, 1924 2,132,655 Smith Oct. 11, 1938 2,151,149 Pach Mar. 21, 1939 2,211,942 White Aug. 20, 1940 2,231,792 Bingley Feb. 11, 1941 2,284,714 Bedford June 2, 1942

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