US2444063A - Electric circuit equalization - Google Patents

Description

June 29, 1948.

Filed Dec. 7, 1944 K. W. PFLEG ER 3 SheetsSheet 1 FIG.

I -10 3 /4 TRANSDUCER 9 a?) DELAY MEMBER s E F 2 \D [7 //a DELAY MEMBER I /9 l v I 22 ATTENUATOR DE), ("4'' BE IN MEMBER EOUALIZER) 3 F l G. 2

( TRANSDUCER 2Q DELAY MEMBER 2/ 2 DELAY MEMBER ATTE/VUATOR DELAY,

(MAY BE IN MEMBER EQUAL/25R) a TRAMmqcER TRANSDUCER TRANSDUCER -f-- CORRESPONDING DELI) MEMBERS IN TRANSDUCER-f MAY HAVE DIFFERENT DELAYS lNl/E N TOR K M. PFLEGER ATTORNEY June 29, 1948. K. w. PFLEGER 2,444,063

' ELECTRIC IQIRCUIIEQUALIZATION Filed Dec. 7, 1944 3 Sheets-Sheet '2 FIG. 4

/N VENTOR K w P LEGER ATTORNEY June 29, 1948. I K, w, PFLEGER i 2,444,063 v ELECTRIC cmcum EQUALIZATION Filed De. 7, 1944 s Sheets-Sheet a FIG. 6

v RESULTAIVT FIG-8 CHARlGZ'ER/STIC 0F LINE .j y W w TRANSDUC R CHARACTER/SWO FREQUENCY RESULTA/VT OF" TWO LOWER CURVES 9 A AVAQA i Aim/WA \AfiV v v u u v u rnsouguc? uvvavron K W PFLEGER ATTORNEY Patented June 29, 1948 2,444,063 ELECTRIC omourr EQUALIZATION Kenneth W. Pfleger, Arlington, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. I, a corporation of New York Application December '7, 1944, Serial No. 567,065

9 Claims.

This invention relates to electric circuit arrangements and more particularly to the attenuation and envelope delay equalization of waves transmitted over transmission lines.

It is a primary object of this invention to provide novel means for equalizing the attenuation and envelope delay versus frequency characteristics of broad band transmission lines 01' systems.

Due to the fact that in long wire line signal transmitting systems the line is usually not electrically smooth, that is, electrical impedance irregularities appear at many points along the line, small wiggles in the attenuation and envelope delay (dc/do, where p is the phase retardation and w is 21r times the frequency in cycles per second) versus frequency characteristics of the system occur. In one of its more important aspects, the invention relates to the removal or reduction of these so-called wiggles.

On a coaxial circuit transmitting a band of from 0.3 to 2.8 megacycles, for example, it is possible that the periodicity of the wiggles will be as low as 0.1 of a megacycle which gives around 25 humps or wiggles in a transmission band 2.5 megacycles Wide. The spacing between humps is not necessarily constant over the frequency range, nor are their amplitudes. On long circuits ordinary equalization produces a more nearly. flat characteristic but may give rise to an increased number of wiggles and it is possible to design rather complicated mop-up equalizers to equalize these wiggles. In accordance with the present invention, however, compensatin wiggles are produced in the attenuation and envelope delay frequency characteristics by means of delayed signals and the apparatus required is very much less than that of a mop-up equalizer which may require many sharply tuned resonant circuits. When the humps or wiggles are spaced at regular intervals along the frequency band and when they are of fairly uniform amplitudes or of amplitudes which vary in a known manner, the saving in apparatus resulting when the present invention is utilized may be very great. The compensating humps or wiggles are produced by combining a pair of properly delayed signals (one delay may be substantially zero if desired) produced in branch circuits with the delayed signal in the main transmission path and, if necessary, by repeating this process any desired number of times.

In a specific form of .the invention, chosen by way of example for illustrative purposes, means are provided for diverting at a selected point in the transmission system a portion of the signal and this portion is passed through an attenuator and then through two parallel-connected delay networks to produce two delayed signals which are combined with the .original signal after it has passed through a third delay network. The phase shifts for any particular frequency of the signal band in the two parallel-connected delay net works are made equal to each other but their phases are respectively of opposite sign with respect to the phase of the signal which is passed through the third delay network, that is, their phases are respectively +0 and -0 with respect to that of the wave at the output of the third delay network. The same is true for all frequencies in the band. By means of this arrangement compensating attenuation humps with practically no delay efie'cts are produced, but if the connections from one of the two parallel-connected delay networks are reversed, compensating envelope delay humps can be produced with practically no attenuation effects. If it is desired to produce wiggles of uniform periodicity and varying amplitude, the loss can be varied over the frequency range either by manual or automatic adjustment. If it is desired to produce wiggles of non-uniform periodicity, the delay networks, which in the former cases were of the :type to produce uniform envelope delay over the frequency band, can be replaced by delay networks or members which do not produce a flat delay. Where the desired correction is not a simple sinusoidal characteristic with respect to frequency more than one set of delay members can. be used in tandem, each with a different amount of delay. By means of a suflicient number of such devices in tandem superimposed sinusoidal components can be obtained to simulate any loss or delay characteristic which i resolvable into a Fourier series.

The invention will be more readily understood by referring to the following description taken in connection, with the accompanying drawings forming a part thereof in which:

Fig. 1 is a block diagram of a transmission system employing a transducer in accordance with the invention to reduce regularly recurring humps in the envelope delay versus frequency characteristic of the system;

Fig. 2 is a block diagram of a transmission system employing a transducer to reduce regularly recurring humps in an attenuation versus frequency characteristic of the system;

Fig. 3 is a block diagram of a transmission system employing a plurality of the transducers shown in the systems of Fig. 1 or Fig. 2;

, Figs. 4 and 5 are block diagrams of systems employing transducers which are modifications of those shown in Fig. 1; and

Figs. 6 to 9, inclusive, are diagrammatic and and a transducer ll! con-- The purpose of the transducer transducer is as shown in line Aof Fig.8 and.

comprises a number of wiggles of substantially sinusoidal form and of substantially equal amplitude. The purpose, of the transducer IQ is to introduce a characteristic such as that shown in line B of Fig. 8, which will compensate for these wiggles and produce a substantially flat characteristic over the transmitted band. This is accomplished by diverting a portion of the energy from lines [2 and it through an attenuating pad I 3 and through an attenuator- !9 which has a constant loss over the entire frequency range. The output energy of the attenuator I9 is divided-between a delay member 2| (called delay member No. l or D1) and, delay member 22 (called delay member No. 3 or D3) the output of the delay members I and 3 being combined and applied to the lines 12 and E3 in whichv is inserted a delay member (called delay member No. 2 or D2). The delay members can be electrical non-dissipative delay networks or cable pairs or-radio channels or'they can be any other means for producing constant envelope delay regardless of changein frequency. As one example, each--member electric waves may be converted into sound orsupersonic waves by means or a crystal or other form of loudspeaker for slower propagation in gases or milduids and may be reconverted thereafter into electric waves by meansof acrystal-or-other form of microphone; The transmitted frequency band may be shifted to higher frequenciesbeforeconversionfromelectric into supersonic'waves. and after transmission overthe medium maybe shiftedbacls again in order tomakeuseof-particular speakers -or microphones or desirable. flat characteristics. The resultant current at the output of the del ay memberDi D2 and Da-iscomputedbelow. Assume' unit current emergingfrom- Dz; a network or other member haying constant loss and em velope delay. To this must also lie-added currents from D1 and-D3. Let these currents be of small amplitude r and" let their phases be I respectively +0-and 0 with respect to -the phase ofunittcurrent from D2. This conditionis obtained by making the losses of-DrandDx equal-and bymaking the total delay ofthe-pad' I8; attenuator I9 and thenetworkDrlessthan the-delay ofDzby the same amount, 'I, asthe total delay ofthepad, attenuator and network D3 is greater than-the delay of D2. The combined output is:

l-|-re+ "re (1) where the minus sign ofthe. latter term is due to a reversal of the wires atthe outputof Dc. Equation 1 may be reduced .to:

l+i2r-sin 0 (2) 4 The phase retardation of the resultant, as com-' pared to unit undistorted current is fii== tan 2?" sin 0=2r sin 0 when 21 is small. I

The envelope delay distortion, that is, the departure from flatness, is

d0 2r Tcosl? (4) where r is the ratio of the current amplitude at the output of D1 or D3 to the amplitude at the output of D2 at a frequency of i 21r and 0 is the phase. difi'erence between currents at the outputs of D1 and D2 or D2 and D3 at such frequency.

Since the desired Wiggles have maxima occurring at uniform frequency of F cycles over the frequency range of interest asshown in. Fig 8, line A, it is obvious from Equatione that this condition is realized when is constant and 0 varies as a straight line function of frequency increasing by 2 radians every time the frequency varies an amount F so that in 1 dc: F

Accordingly the wiggles expressed by Equation 4 have the amplitude ru 'r27'dw F (5) and therefore T=%TF If the three delaying transducers D1, D2 and D3 have equal losses, then-r denotes the current ratio corresponding to the loss in the pad Hi; For example, if the delay humps are twomicro-seconds in-amplitude and-26,-460 cycles apart,

which corresponds to a 31.6-decibel pad.

Since envelope delay is defined as'the derivative of the phase shift versus 1 characteristic, the phase shift may be determined from the delay by integration. Thus the phase shifts corresponding to D2 andDsare respectively.

where M and'm are-constantsofintegration. In order to conserve-apparatus, let the delay member Diand'its phaseshift be zero.- In order to fulfill the requirement given abovethat' the phases of current D1 andDa" shall be +0and 0'with respect to the ph-ase ofcurrent in the member D2,

+0=wD2+k2=wD3+M-(wD2+)\2)- (8) therefore i 26=wD3+A3=2wD2I-2k2 (9) and Let Equation 8 be differentiated withrespect to w. Then d0 1 1 d =D =37.75 10* 'seconds (11) Similarly from Equation: 9':

D :-%=-7 5:5 X10? secondse (l2) "an iaoce Delay members-are known which produce envelope delays of these magnitudes.

It is stated above that the required. phase shiftsare oDz-l-Az and 2wD2+2Aa A2 controls the location of the humps as may be seen when Equation 8 is substituted in Equation 4 giving:

"Accordingly, A2 may be any constant between 0 and Zr depending upon the location of F1. When the envelope delay has a minimum at F1 it is evident from Equation 13 thatAat this frequency Suppose, for example, that Dz=37.75 10- seconds and F1=300,000 cycles, then:

A2=21r(N300,000 X 37.75 X 10- A 21r(N 11.325) (16) In order that A2 shall be small it may be desirable to select N:1l so that:

'When A2 is other than 0 or 180 it becomes necessary to add a constant phase shift at all frequencies to that of the flat delay medium. Perhaps the simplest way to obtain the desired over-all phase characteristic is to make the path in the delay medium slightl shorter than would be required and then add an electrical network or just a few sections of lattice or bridged-T type phaseequalizers in order to make up the difference plus A2. Methods for designing the latter networks are already well known to those skilled in the art of equalizer design and need not be covered here. Thus it is seen that the apparatus required to produce the delay wiggles of line B l of Fig. 8 consists of a pad la plus two constant delay paths D2 and D3 and a few lattice type networks or their equivalent to obtain the desired values for A2 and As.

The amplitude of the resultant current is /l+lr sin @=1+2 sin 0 when r is small.

Since loge(1+21 :21' when 21 1:; small, it follows that the resultant current has amplitude wiggles corresponding to 2r =2(.02646) =.00l4 neper or .012 db. (19) A shown in line B of Fig. 8 can be made to vary over the frequency range rather than all of them being constant as shown in line B of this figure. Thus only four wide band transducers D1, D2, D3 and the attenuator I9 can be made to introduce many delay wiggles with amplitude and periodicity varying by suitable amountsover the frequency range with negligible attenuation eifects.

Fig. 6 shows vectorially the relation of the voltages V20 (the output ofthe delay member 20), the voltage V22 (the voltage of the output of the delay member 22) and the voltage V21 (the volt- A 15 'tia-lly no phase change.

6 age ofthe output of the delay member-2i This diagram represents vectorially Equation l and it can easily be seen that the resultant voltage Va "for any particular frequency substantially 5 equal to the'voltage V20.

"Thesystem shown in Fig. 2 is like that in Fig. l except that the connections from the output ofdelay member No. 1 are reversedwith respect to these output connections in the arrangement loshown in Fig. '1. The relations of the voltages V20, Vzr-and V22 in the transducer l I of Fig, 2 are as= shown in Fig.7. This diagram shows clearly that the resultant voltage Va for any particular frequency has an amplitude change but substan- The output current at .the terminal l6, I1 is: A

Theredsino delay. effect inthis case. and only ..attenuation-humps:result. Therefore this arrangement can pe used aswanover-all attenuation equalizer withnodelay effects. If the atten- Auation equalizer i9contained within one or'the A other of the arrangements of Figs 1 and 2 does 1 not have constant delay, the over-all output has 5 cy, more than one device may be used in tandem each with a different value for T. Fig. 3 shows three such devices in tandem each transducer being like that shown in Fig. l orAFig. 2. :Bymeans of a suificient number of devices in .w'tandem, superimposed A sinusoidal components, such as are shown graphically in Fig. 9 (where the upper curve is the resultant of the two lower curves which are sine waves of difierentfrequem cies), may be obtained to simulate any loss or delay characteristic-which is resolvable into a Fourier series; It is obvious that where the char- A acteristic is quite irregular the method of utilizing delayed signals may be more expensive than I otherzmeans .on account orthe number of delay members in tandem,

"When a large number of transducers It or II are connected in tandem, the amountof apparatus-in the delay networks or other delay members may become excessive. Figs. 4 and5 disclose A arrangements wherein some of the delay members do multiple duty, thus saving aconsiderable quantity of delay networks.

' Im-ithearrangement of Fig. 4 the transducer A tlldncludes three paths D1, D2 and D3 .as before 69 but each of these paths includes three delay members. The upper path Dz includes the three delay members 50 (having a delay d21),- 5| (having a delaydzz), and 52 (having a delay (22:). "The middle path D1 includes the delay members '65 53 (delay (211x54 (delay c112), and55 (delay (in) while the lower path D3 includes the delay membersEE (delay'-d31);5l (delay (Z32), and 58 (delay dss). Signals from the lines l2, I3 are applied 'to the delay memberfill and the output thereof 70 is applied to the delay member 5! and also, by means'of lines 59 and to an attenuating pad Bl the output of which isapplied through an attenuator $2 to conductors B3 and--64 which are connected to theginput terminals of the delay 76" member-54in the path Brand to theinputtermin-als of the delay member 51 in the path D3. The output terminals of the delay member 5| are connected to the input terminals of the member 52 and by means of lines 65 and 66 to an attenuating pad 61 the output of which is applied through attenuator 68 to the lines 59 and i which are connected to the input terminals of the delay member 55 in the path D1 and to the input terminals of the member 58 in the path D3. The signals from the lines 12 and Hi are also applied through attenuating pad 18 and attenuator I9 to the input terminals of both the members 53 and 56, the output terminals of which are connected respectively to the input terminals of the delay members 54 and 51. In turn, the output terminals of the members 54 and 51 respectively are connected to the input terminals of the members 55 and 58. The output terminals of the delay members 55 and 53 are connected to the output terminals I6, I! of the transducer 30' to'which output terminals are also applied the output signals from the delay member 52 in the path D2. By way of example, each of the members 50, and 52 may have a delay (d21, dzz or 1123) of 30 microseconds, each member 53, 54 and 55 may have a delay (du, dlz or (ha) of 15 microseconds while each of the members 56, 51 and 58 may have a delay (0131, (132 or 133) equal to .45 microseconds. (In other examples, each delay member in a path need not equal the other delay members in the path.) Then Delay D2=d21+d22+d23=9O microseconds, Delay D1=d11+d12+d1a=45 microseconds, and Delay D3=d31+d32+d33=135 microseconds.

With this arrangement delayed signals from 45 microseconds to 135 microseconds in 15-microsecond steps can be obtained and these are all applied to the output terminals l6, II. To obtain a delay of 45 microseconds the signal travels through the delay members (111, 112 and 0313. To obtain a delay of 60 microseconds the signal travels through the delay members 0121, 1112 and 213.

' The 75-microsecond delay is obtained by the signal passing through the delay members (in, dzz and 1113 in tandem. A 90-microsecond delay is obtained by the signal passing through the delay members 1221, 122 and dzs. A delayed signal I05 microseconds behind the input signal is obtained by the signal passing through delay members r121, (Z22 and (133. A 120-microsecond delay is produced by the signal traversing in turn the delay members 0121, (Z32 and (Z33. The l35-microsecond delay is produced by the signal passing in turn through the delay members (Z31, (132 and dis. All of these figures neglect the delays of the pads and attenuation equalizers which are small compared with the delays produced by the delay memhere, It will be seen that if 90 microseconds is considered to be the mean delay, these delay times can be arranged in three groups, viz., '75, 90 and 105; 45, 90 and 135; and 120, 90 and 60. In each group the difference between the smaller delay and the mean delay is equal to the difference between the mean delay and the larger delay. Thus three sets of wiggles are produced to compensate corresponding wiggles in the input signal characteristic, The delay members may be nondissipative networks or cable pairs or radio channels or acoustic members or any other suitable means. The delay in the path D1 can be zero, if desired, It should be understood that power flows in Fig. 4 only in the directions shown by the arrows and that one-way devices (not shown) such as hybrid coils or amplifiers are provided to prevent power flow in directions contrary to the arrows.

An alternative arrangement to that of Fig. 4 is shown in Fig. 5. In the arrangement of Fig. 5 the path D2 of the transducer 40 comprises a single large delay member I!) and the path D1 comprises the three smaller delay members H, i2 and "it (they may be equal or unequal) in tandem, these members giving the delays du, 112 and d13, respectively. The path D3 contains the delay members M, 15 and 16 (which may be equal or unequal) having delays of 131, (132 and (133,168- spectively. Signals from the input terminals l4, l5 are applied through the attenuating pads 11, it and "i9, respectively, to the attenuators 89, BI and 82, respectively. The attenuator is connected to the delay member ll by means of the conductors 53 and 84 and is connected to the delay member 16 by means of the conductors 85 and 88. The attenuator BI is connected by means of the conductors 8'! and 88 to the input terminals of the members 12 and I5 while the attenuatcr 52 is connected by means of the conductors S5 and 533 to the input terminals of the member 53 and by means of the conductors 9| and 92 to the input terminals of the delay member 14. The output terminals of the members D2, 13 and. '55 are connected to the output terminals 15, ll of the transducer 50. These output terminals in Fig. 5 are connected to produce regularly recurring humps in the attenuation versus frequency characteristic of the system while the arrangement of Fig. 4 is connected to produce regularly recurring humps in the envelope delay versus frequency characteristic of the system but it is to be understood that either of the transducers 35 and 45 can be readily adapted by proper connections of these output terminals to produce humps in either of the envelope delay or attenuation characteristics. In the arrangement of Fig. 5, one large delay member 15 is used instead of the three delay members 55, 5! and 52 in the transducer 39 in Fig. 4 but in the arrangement shown in Fig. 5 a finite delay is required in the path D1 which in the arrangement of Figs. 1, 2 and i can be zero. In the arrangement of Fig. 5, wiggles are produced having crests far apart using delays so that the delay in the path D2 minus the delay in the path D1 equals the delay in the member 15 minus the delay in the path D2. Similarly, there are produced wiggles having crests close together by proportioning the delays so that the delay in the path D2 minus the delay in the members l2 and I3 equals the delay in the members '65 and 35 minus the delay in the path D2. This positive delay is greater than the positive delay produced in the path D2 minus that produced in the path D1. There are also produced wiggles having crests still closer together by proportioning the delays so that that produced in the path D2 minus that produced in the member 13 equals the delay produced in the path D3 minus that produced in the path D2. This is a positive delay much greater than that produced in the paths D2 minus that produced in the path D1. Since the delay in the path D1 equals d11+d12+d13 and the delay in the path D3 equals d33+d3z+ds1, it follows that 1211 equals (Z32 and (112 equals (231.

Although the present invention has been described in terms of certain illustrative embodi 2,444,oes

59 forms as may fairly-come within the spirit and letter-of -the appended claims.

What is claimed is:

1. Ina signal transmission system, the method of reducing substantially regularly recurring humps in a, transmission characteristic of the system which varies with tfrequencywhich comprises diverting a portion of the main signal through two branch paths and passing the main signal through a third path, the transmission characteristics of the three paths being such that the phases of any sinusoidal component of the resultant signal in one of the two branch paths and of the same sinusoidal component of the resultant signal in the other of the two branch paths are respectively +6 and with respect to the phase of the same sinusoidal component of thesignal in the thirdpath, where a'is any convenient. phase difference, and combining the output signals of the three paths to form a composite signal.

2. In a signal transmission system, the method of reducing substantially regularly recurring humps in an envelope delay versus frequency characteristic of the system which comprises diverting portion of the main signal through two branch paths and passing the main signals through a third path, the transmission characteristics of the three paths being such that the phases of any sinusoidal component of the resultant signal in one of the two branch paths and of the same sinusoidal component of the resultant signal in the other of the two branch paths are respectively +0 and 0 with respect to the phase of the same sinusoidal component of the signal in the third path, where 0 is any convenient phase difference, and combining the output signals of the three paths to form a composite signal.

3. In a signal transmission system, the method of reducing substantially regularly recurring humps in attenuation versus frequency characteristic of the system which comprises diverting a portion of the main signal through two branch paths and passing the main signals through a third path, the transmission characteristics of the three paths being such that the phases of any sinusoidal component of the resultant signal in one of the two branch paths and of the same sinusoidal component of the resultant signal in the other of the two branch paths are respectively +0 and +0 with respect to the phase of the same sinusoidal component of the signal in the third path, where 0 is any convenient phase difference, and combining the output signals of the three paths to form a composite signal.

4. The method of claim 1 in further combination with the step of repeating this method with the output signal produced thereby but utilizing paths having transmission characteristics so that 0 has a different value than before.

5. In combination, a signal path having impedance irregularities which produce a multiplicity of small humps in a transmission characteristic of the path which varies with frequency and a transducer connected in said signal path for reducing certain ones, at least, of said humps, said transducer comprising means for diverting a portion of the transducer input signal through two branch paths, means for passing another portion of this input signal through a third path including a delay member, the transmission characteristics of said three paths being such that output signals are produced having for any particular sinusoidal component phases which are respectively +0 and +0 with respect to the same 10 sinusoidal component of'the signal produced in said third path, where 0' is any convenient phase difference, and means for combining the output signals of said three paths.

6. In combination, a signal path having impedance irregularities which produce a multiplicity of smallhumps in the envelope delay versus frequency characteristic of said path and a transducer connected in said signal path for reducing certain ones, at least, of said humps, said transducer comprising means for diverting a portion of the transducer input signal throug'htwo branch paths, means for passing another portion of this input signal through a third path including a delay member, the transmission characteristics of said thr'eepaths being such that output signals areproduced having for anyparticular sinusoidal component phases which are respectively +9 and +0 with respect to the same sinusoidal component of the signal produced in said third path, where 9 is any convenient phase difference, and means for combining the output signals of said three paths according to the formula l+re+ +re where r is the magnitude of the current in each of the two branch paths referred to the magnitude of the output signal in said third path as unity.

7. In combination, a signal path having impedance irregularities which produce a multiplicity of small humps in the attenuation versus frequency characteristic of said path and a transducer connected in said signal path for reducing certain ones, at least, of said humps, said transducer comprising means for diverting a portion of the transducer input signal through two branch paths, means for passing another portion of this input signal through a third path including a delay member, the transmission characteristics of said three paths being such that output signals are produced having for any particular sinusoidal component phases which are respectively +0 and +0 with respect to the same sinusoidal component of the signal produced in said third path, where 0 is any convenient phase difference, and means for combining the output signals of said three paths according to the formula where r is the magnitude of the current in each of the branch paths referred to the magnitude of the output signal in said third path as unity.

8. In combination, a signal path having impedance irregularities which produce a multiplicity of small humps in a transmission characteristic of the path which varies with frequency and a transducer connected in said signal path for reducing certain ones, at least, of said humps, said transducer comprising means fordiverting a portion of the transducer input signal through two branch paths, means for passing another portion of this input signal through a third path including a delay member, the transmission characteristics of said three paths being such that output signals are produced having for any particular sinusoidal component phases which are respectively 6+ and +0 with respect to the same sinusoidal component of the signal produced in said third path, where 9 is any convenient phase difference, means for combining the output signals of said three paths, each of said three paths comprising a plurality of delay members in tandem, and means for passing portions of the signal in said third path, delayed to extents which are less than that of the total delay of said path,

tion of this input signal through 'a third path ineluding a delay member, the transmission characteristics of said three paths being such that output signals are produced having for any particular sinusoidal component phases which are respectively +0 and 9 with respect to the same sinusoidal component of the signal produced in said third path, where a is any convenient phase difierence, means for combining the output signals of said three paths, each of said two branch paths comprising a plurality of delay members in tandem, and means for diverting portions of said input signal in each case through one or more of the members in one of said branch paths and, in addition, through one or more of the members in another of said paths.

KENNETH \V. PFLEGER.

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

UNITED STATES PATENTS Number Name Date 1,644,395 Pohlman' Oct. 4, 1927 1,681,252 Nyquist Aug. 21, 1928

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