US2792552A - Attenuation equalizer - Google Patents

Description

Y M Q 2 m 2 T 7 L N. T 2 E A n a i 15 0 W c m M V ATTENUATION EQUALIZER Fild Nov. 27, 1953 May 14, 1957 ATTENUATION EQUALIZER Stephen Bobis, Summit, N. J Laboratories, incorporated, ration of New York Application November 27, 1953, Serial No. 394,663 7 Claims. (Cl. 333-28) assignor to Bell Telephone New York, N. Y., a corpo- This invention relates to wave transmission networks and more particularly to adjustable attenuation equalizers.

An object of the invention is to provide an adjustable attenuation equalizer whose deviation characteristics is a parabola. A more specific object is to provide an adjustable equalizer which, over the operating frequency range, has a fiat insertion loss characteristic for the normal setting of the control element and, for any other setting, has a deviation characteristic of such a shape that the area between the characteristics and above the normal characteristic is equal to the area between the characteristics and below the normal characteristic.

in long signal circuits the transmission loss varies with changes in temperature or humidity, or for other reasons. it is customary to compensate for these transmission variations by means of an automatic attenuation regulator which comprises a number of independently adjustable equalizer shapes. Interaction or hunting between the equalizers can be avoided if the shapes are orthogonal to each other. One such orthogonal shape found very useful is a parabola.

The adjustable attenuation equalizer in accordance with the present invention has such a parabolic deviation characteristic. The equalizer comprises an adjustable impedance adapted for insertion between a wave source impedance and a load impedance. The adjustable impedance includes a shaping network terminated in an adjustable resistor. The shaping network is an all-pass structure having an image impedance which is a constant resistance. The network includes a series impedance branch which is resonant at approximately the midband frequency of the operating range and antiresonant at two frequencies on opposite sides of f This branch includes a resistive element which affects the impedance thereof primarily in the neighborhood of 5,. The shaping network may be a lattice, bridged-T, or other equivalent structure.

The component elements of the equalizer are so proportioned that one setting of the adjustable resistor produces a normal insertion loss-frequency characteristic which is substantially fiat over the operating range. Any other setting produces a deviation characteristic which over this range, is essentially a parabola having its apex at approximately i The parabola crosses the normal characteristic at two points approximately equally spaced from i These are pivot points for the parabola and remain substantially fixed for all adjustments. The apex of the parabola may be either above or below the normal characteristic and its distance therefrom depends upon the setting. An important feature is that within the operating range, for every setting the area between the parabola and the normal characteristic and above the normal characteristic is approximately equal to the area between these characteristics and below the normal characteristic.

The nature of the invention and its various objects, features, and advantages will appear more fully in the following detailed description of preferred embodiments illustrated in the accompanying drawing, of which:

Fig. l is a schematic circuit of an adjustable attenuation equalizer in accordance with the invention;

Fig. 2 shows the normal and one extreme insertion loss-frequency characteristic obtainable with the equalizer of Fig. 1, and also a reference characteristic;

Fig. 3 is a schematic circuit of a lattice structure suitable for the shaping network in Fig. l; and

Figs. 4, 5, 6, and 7 are schematic circuits of equiva lent, unbalanced, bridged-T structures suitable for the shaping network.

The adjustable attenuation equalizer in accordance with the present invention is of the general type disclosed in Patent 2,096,627, issued October 19, 1937, to H. W. Bode. The embodiment shown in Fig. 1 is a modification of the circuits shown in Figs. 12, 15, and 27 of the Bode patent. The equalizer comprises a pair of input terminals 1, 2, a pair of output terminals 3, 4, and an interposed adjustable impedance 5. The impedance 5 comprises a four-terminal shaping network 7 having input terminals 8, 9 and output terminals 10, 11, a symmetry resistance Rs, and an adjustable terminating resistance of value Rx. The resistance R5 is connected between the input network terminal 8 and the equalizer terminals 1 and 3. The other input network terminal 9 is connected to the equalizer terminals 2 and 4. The resistor Rx is connected between the output network terminals 1i) and 11. A wave source 14 of impedance Zs is connected between the input terminals 1, 2 of the equalizer and a load of impedance Zr. is connected between the output terminals 3, 4. Thus, the impedance 5 is connected in parallel with the source and the load.

Pig. 2 shows two insertion loss-frequency characteristics obtainable with the equalizer of Fig. 1. The straight line 15, of zero slope, represents the substantially fiat loss A0 obtainable for the normal setting of the terminating resistance Rx. The curve 16, over the operating frequency range 1, to is, is an acceptably close approximation to the desired parabolic characteristic. The solidline curve 16 represents one extreme adjustment limit of the loss and is obtained when Rx has the minimum value. For other values of Rx, the curve will pivot about the crossing points at the frequencies f, and it. When RX has its maximum value, the characteristic will be substantially a mirror image of the curve 16, with respect to the line 15. Any desired loss characteristic between these extremes may be obtained by properly setting Rx. For the curve to be orthogonal, the cross-hatched area 17 between the curves 15 and 16 and above the curve 15 must be equal to the sum of the oppositely cross-hatched areas 18 and 19 between the curves 15 and 16 andbelow the curve 15. Also, for symmetry, the apex 21 should be located at the frequency f midway between 1, and f6 and the areas 18 and 19 should be equal.

Fig. 3 is a schematic circuit of a constant-resistance, lattice-type network suitable for the shaping network 7 of Fig. 1. The input terminals 8, 9 and the output terminals 1o, 11 have the same designations as are used in Fig. 1. The network comprises a pair of equal series impedance branches Z1 and a pair of equal diagonal impedance branches Z2. For simplicity, only one series branch and one diagonal branch are shown in detail. The other two branches are indicated in the usual manner by broken lines connecting the terminal 9 with the terminals 10 and 11. Each of the branches Z1 consists of an inductance L2 and a capacitance C2 connected in parallel with an arm comprising the series combination of a resistance R1, an inductance L1, and a capacitance C1. Each of the branches Z2 is made up of an inductance L3 and a capacitance C3 connected in series with the parallel co'moinationof an inductance Li, a capacitance C4, and a resistance R2. In order to make the a frequency f,

greases A suggested procedure for designing the shaping network of Fig. 3 is as follows: First, assume that each series branch Z1 comprises only C2 and L2 and each diagonal branch Z2 comprises only C3 and L3. This lattice is then designed as an all-pass structure having an image impedance R chosento provide the desired extreme deviation swings. Therefore, C2 and L2 will be antiresonant, and C3 and L3 resonant, at approximately f Thestiffness of each branch, which is dependent upon the ratio Cz/Lz or the ratio Ls/Cs, determines the frequencies at which thevextreme deviation characteristic crosses the normal characteristic 15. This stiffness is so chosen that, when thenetwork is used as the shaping network 7 in the equalizer of Fig. 1, an extreme deviation characteristic shown by the broken-line curve 22 in Fig. 2 is obtained. This is essentially a sinusoidal shape, with a maximum at f and crossings of the normal curve 15 at the frequencies f2 and f which are the pivot points. The frequencies f,., f and 1, approximately correspond, respectively, to the frequencies at which the phase shift in the shaping network 7 is 135 degrees, 180 degrees and 225 degrees. It will be noted that the pivot point 1, is somewhat iower than 1, and the pivot point 1, is somewhat higher than 1}. The value of the symmetry resistance Rs is so chosen that the inverse extreme deviation characteristic is the mirror image of the curve 22 with respect to the normal characteristic 15.

As compared to the desired parabolic characteristic 16, the sinusoidal curve 22 is too low at the limiting frequencies f, and f, of the operating range and too high at the midband frequency f In accordance with the presentinvention, the shape 16 is obtained by adding the arm comprising L1, C1, and the damping resistance R1 to each series impedance branch Z1 and the corresponding inverse combination of L4, C4, and a damping resistance R2 to each diagonal branch Z2. The elements L1 and C1 are resonant, and L4 and C4 are antiresonant, at approximately f Therefore, the modified series branch Z1 is resonant at f and antiresonant at two frequencies substantially equally spaced above and below f -The inverse diagonal branch Z2 is antiresonant at f and resonant at the two antiresonant frequencies of-Z1. main effect of adding the elements L1,,C1, R1, L4, C4, and R2 is to lower the curve 22 at f and raise it at f, and f Incidentally, the addition of these elements also raises the pivot point 1, to f, and lowers the pivot point to f,. The amount by which the curve 22 is lowered at f is determined by the magnitudes of the added resistances R1 and R2. Thewidth of the frequency range atfected is determined by the stiffness of the added arms, that is, by the ratios Ci/Li and L4/C4. After a few trials, a value of R1 and a ratio Cl/Ll can ordinarily be found which will give a sufiiciently close approximation to the desired parabolic characteristic 16. The elements in the series branch are thus determined. The values of the corresponding elements in the inverse diagonal branch Z2 may be found from the relationship given in Equation 1.

As an illustrative example only, the element values will begiven for an adjustable equalizer of the type shown in Fig. l in which the shaping network 7 is a lattice structure of the type shown in Fig. 3; It is assumed that the source impedance Zs and the load impedance Z1. are each 2830 ohms, the operating range extends from of 176 kilocycles to a frequency f, of 264 kilocycles, the pivot frequencies f, and f, are approxi- V mately 191 and 247 kilocycles, respectively, the midband frequency f is 217 kiiocycles, and the permissible fiat loss A is 6.82 decibels. 'Ihe symmetry resistance Rs has a value of 373 ohms,the imageimpedance Re The.

4 of the shaping network 7 is 818 ohms, and terminating resistance Rx is adjustable-between 134 and 5000 ohms; The component elements of the network '7 have approximately the following values:

C1=63 L2=300 C2=17,9l5 V L3=1,200 C3=448 L4=42.22 C4=12,742 R1=4,910 L1=8,532 R2=136.4 V

The capacitances are given in micromicrofarads, the inductances in microhenries, and the resistances in ohms.

The chracteristic 16 shown in Fig. 2 is obtained when the terminating resistance Rx is set at its minimum value of 134 ohms. The maximum swing of the equalization characteristic, which is the difference between the insertion loss at i and at f,, is five decibels. The equalizer is thus adaptedto equalize a maximum parabolic bump or dip of five decibels. The swing could be increased some what by extending the adjustment range of Rx.

In order to save'elements and avoid providing pairs of equal'impedance branches such as Z1, Z1 and Z2, Z2, it is desirable to transform the symmetrical lattice network of Fig. 3 into an equivalent unbalanced structure. For example, Fig. 4- shows an equivalent unbalanced bridged-T network into which the lattice of Fig. 3 may be transformed when C2 is larger than C3; In Fig. 4, the values of the component elements are given in terms of the elements appearing in Fig. 3. Fig. 5 shows another equivalent bridged-T structure, obtained by transforming the T of capacitances C2, C2, C into the equivalent 7r arrangement C3, C3, C6.

Fig. 6 shows a third equivalent bridged-T network. it is obtained by adding the redundant capacitance C to the bridging branch of Fig. '4 and changing the values of the capacitances C2, C2, and C5 to C2, C2, and C5, determined by the choice of C7. The capacitance C7 is addedto permit further transformations which will result in more convenient element sizes. Fig. 7 shows the final circuit. The component elements haveapproximately the following values:

The capacitances are given in micromicrofarads, the inductances in microhenries, and the resistances in ohms. it will be noted that the inductance L5 has a value of only 764, whereas the corresponding inductance 2L1 has a value of 17,064. Also, the inductance Ls has a value of 1,000, which is much more easily obtained within close limits than is the value 21.11 of the corresponding inductance L4/2. The other component elements also have values which are readily obtainable in standard designs. a

It is to be understood that the above-described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. An adjustable attenuation equalizer for operation over a range of frequencies comprising an adjustable impedance adapted for insertion'between' a wave source and a load, said adjustable'impedance comprising an all-pass, four-terminal, shaping network having an' image irnpedance which is a'constant resistance R1: and a phase shift which continuously increases over said range and is equal to at least degrees at the 'midband frequency of said range, a symmetry resistrconnected to the input thereof, and an adjustable resistor of value Rxconnected .tothe output terminals ereoff, said network comprising a series impedance connected between an input terminal and the corresponding output terminal, said series impedance comprising a damping resistance and being resonant at approximately and antiresonant at two frequencies substantially equally spaced above and below the component elements of said adjustable impedance being so proportioned that, over said range, when Rx is equal to Rn the insertion loss-frequency characteristic of the equalizer is a first curve substantially flat, when Rx has a value other than Rn said characteristic is essentially a parabolic curve having its apex at f and crossing said first curve at two frequencies within said range approximately equally spaced from f and the area between said curves and above said one curve is approximately equal to the area between said curves and below said one curve.

2. An equalizer in accordance with claim 1 in which said shaping network is of the lattice type.

3. An equalizer in accordance with claim 1 in which said shaping network is of the bridged-T type.

4. An equalizer in accordance with claim 1 in which said shaping network is an unbalanced structure.

5. An equalizer in accordance with claim 1 in which said shaping network is an unbalanced, bridged-T struc ture.

6. An equalizer in accordance with claim 1 in which said adjustable impedance is adapted to be connected in parallel with said source and said load.

7. An equalizer in accordance with claim 6 in which said symmetry resistor is connected at one end to an input terminal of said shaping network and adapted for connection at the other end to said source and said load.

References Cited in the file of this patent UNITED STATES PATENTS 2,096,027 Bode Oct. 19, 1937 2,153,743 Darlington Apr. 11, 1939 2,374,872 Lundry May 1, 1945

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