US2265042A - Attenuation equalizer - Google Patents

Description

Dec' 2, W. R. ATTENUATION EQUALIZER v Filed oct. 24, 1940 mm-/ncl PER sico/va Jg ATTORNE V Patented Dec. 2, 1941 ATTENUATION EQUaLIzEn Walter R. Lundry, Maplewood, N. J., assigner to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application october 24, 1940, serial No. 362,545

(ci. 17a-,44)

Claims.

This invention relates to wave transmission networks and more particularly to networks used as attenuation equalizers.

An object of the invention is to vary continuously the transmission loss of an attenuation equalizer to provide a family of similar characteristic curves.

' Another object is to provide such a family of curves `all of which have the same loss at some fixed reference frequency.

Another object is to extend the curves to the region below the axis of zero loss.

Another object is to provide a variable attenuation equalizer having a linear loss characteristic which may be continuouslyvaried from a positive slope to a negative slope and which will have the same loss, at some reference frequency, for all settings.

A further object of the invention is to provide an attenuation equalizer which will give a voltage gain when operating between a constant nonreactive input load and a high impedance output load. K

Transmission circuits often require the use of variable attenuation equalizers which may be regulated to compensate for attenuation changes caused by changes in the temperature or other weather conditions. Under some circumstances it is desirable that the equalizer be continuously adjustable to provide a family of similar loss characteristics all of which pass through a comlmon point. To make the equalizer more useful it is also sometimes required that the family of curves be extended into the region of transmission gain.

The attenuation equalizer of the present invention meets all of these requirements in a simple and eicient manner. The network comprises a number of sections connected in parallel at their input ends and coupled attheir output ends by means of a potentiometer to a high impedance load. The potentiometer may, for example, be of the condenser type, and the output load may be the input circuit of a thermionic tube. 'I'he equalizer sections are preferably ofthe constant resistance type and one or more of the sections may be ldesigned to provide a voltage gain. The individual sections may, for example, be so designedthat their loss characteristics over the useful frequency range are substantially straight lines having different slopes, some positive and some negative, but all having the same loss at some reference frequency. Under these circumstances by an adjustment ofthe potentiometer there may be selected any one of an infinite family of loss characteristics all of which are linear and in effect pivot about a fixed point.

The nature of the invention will be more fully understood from the following detailed description and by reference to the accompanying drawing, in which like reference characters represent p like ior similar parts and in which:

Fig. 1 is a diagrammatic representation of a variable attenuation equalizer in accordance with the invention; Y

Fig. 2 shows the circuit of an equalizer section the loss characteristic of which will have a positive slope;

Fig. 3 shows an equalizer section the loss characteristic of which will have a negative slope;

Fig. 4 shows the circuit of an equalizer section which will provide a voltage gain; and

Fig. 5 gives typical transmission loss characteristics obtainable with the variable equalizer of Fig. 1.

A typical circuit of a variable attenuation equalizer in acordance with .the present invention-is shown schematically in Fig. 1, wherein 2| and 22 are the input terminals and 23 and 24 are the output terminals. The circuit is shown in the unbalanced form and the path between terminals 22 and 24 may be grounded or otherwise fixed in potential. It is preferable that the equalizer work into a high impedance load, as, for example, the input circuit of a thermionic tube I9, which may be the first tube of an amplier.

The component equalizer sections 2, 3, 5, 6 and l and the resistance pad 4 are all connected in parallel at their input ends. Section 2 is connected in tandem with section I, and the latter is terminated at its output` end in a resistance 25 equalin value to the image impedance of the section. Sections I and 2 should have matching image impedances at their junction. Sections 3, 5 and 6 are likewise terminated in their image impedances 26, 2l and 28, while section l, as explained hereinafter, is designed to work into a high impedance load.

'Ihe output voltages of the component equalizer sections are selectively combined by means of the condenser potentiometer 20. The high potential sides of the output ends of the sections I to 'I are connected, respectively, to the fixed condenser plates 3| to 3l. The movable plate 30 is connected to the output terminal 23 of the equalizer as a whole.

The equalizer sections are preferably of the `constant resistance type and may, for example,

be bridged-.T structures such as are shown in Figs. 2 and 3 and more fully described in U. S. Patent 1,606,817 to G. H. Stevenson issued November 16, 1926. Section 2 may, for example, be of the type shown in Fig. 2 and may have its component impedance elements so proportioned that its loss characteristic over the useful frequency range extending from f1 to f2 is substantially a straight line with a positive slope, such as curve I2 of Fig. 5. The resistance pad 8 may be connected in tandem with the section 2 to increase the minimum loss at the lower end of the frequency range to some desired value. This pad may be designed to provide an impedance transformation, if desired. Equalizer section l may also be of the type shown in Fig. 2 Aand may be designed so that, when connected in tandem with section 2 and pad 3, the entire combination will have the transmission loss characteristic shown by curve II ci Fig. 5. Likewise, section 3 may have the configuration of Fig. 2 and maybe proportioned so that it, together with its associated pad 9, will have the loss characteristic shown by curve I3 of Fig. 5.

The equalizer sections 5 and 6 may have the configuration shown in Fig. 3 and may be designed to have linear loss characteristics with negative slopes. Section 5 and the associated series resistance IJ may, for example, provide the loss of curve I5 of Fig. 5 and section 6 alone may have the loss shown by curve I6.

Section 'I may take the form shown in Fig. 4, described in more detail hereinafter. This section may be so designed that its loss characteristic, as shown by curve I 'I of Fig. 5, has a negative slope and, at its lower end, crosses the zero 1 axis and extends into the region of negative loss, that is, transmission gain.

In the preferred form of the equalizer the pad 4 has the constant loss given by curve I4 of Fig.

5 and each of the sections 2, 3, 5, 6 and 'I 'in conjunction with its associated pad, if any, has this same loss at Ysome frequency f1 at the lower end of the frequency range covered. Section I is designed to have a minimum loss at f1 and at any other frequency has a loss equal to the difference between curves II and I2.

Now if the movable condenser plate is placed directly opposite the fixed plate 32, for example, the loss of the equalizer as a Whole will be that shown by curve l2. Also, if the plate 3i) is moved to a position opposite to any of the other fixed plates 33, 34, 35, 3S or 3l the loss characteristic will be that given, respectively, by curves I3, I4, I5, I6 or II. Furthermore, lif the plate 30 is stopped at some intermediate position,

so that part of it is opposite one fixed plate and another part is opposite an adjacent plate, the loss characteristic Will be a curve falling between the curves associated with the two plates. For

example, if the plate 30 overlaps both xed" plates S5 and 35 the loss will he that given by the dotted curve I8. For any of these settings the loss curve will be substantially linear and at the frequency f1 will pass through the point 39.

There is thus provided an infinite family of lini' ear curves which may have a positive, a zero or a negative slope and which, in effect, pivot about the point 39. The particular characteristic obtained depends upon the setting of the movable condenser plate 30.

Ii the plate 39 is placed opposite the fixed plate 3l the loss of the equalizer will be the sum of the losses of the sections I and 2 and the pad 8, given by curve II'. This curve does not pass through the point 3%, because of the inherent minimum loss of section I, but it is substantially linear and passes close to the pivot point. By making the plate 30 overlap portions of both of the plates 3I and 32 there may be provided a series of curves which fall between curves II and I2.

The movable plate 30 may be adjusted manually or, if desired, it may be placed under the automatic control of a pilot wire or a pilot channel to provide attenuation equalization for an associated transmission circuit.

As already mentioned, equalizer section I has the property that, over at least a portion of the frequency range it provides a transmission gain, as shown by curve I'l of Fig. 5. The design of this section will now be considered in greater detail. I

As shown in Fig. 4 the circuit consists of a serios impedance branch Ze, a shunt impedance branch Zb at the output end and a second shunt impedance branch at the input end. The ratio of the input voltage Eo across terminals 4I, 42 to the output voltage E across terminals 43, 44 is given by the expression be made a constant pure resistance by adding a.

shunt branch at the input end provided either Za or Zt includes a seriesresistance, such as R in Fig. 4, which is equal to or greater than Ro. This shunt branch consists of a resistance Ro in series with a general impedance Ze which is the inverse, with respect to Ro, of the sum of the impedances Za and Zh reduced by the value of the resistance Ro. In equation form,

So long as Za and Zb are physical and Ris greater than Ro than Ze will be physical.

In the above discussion it has been assumed that the equalizer section of Fig. 4 is to work into a load impedance which is high compared to the shunt impedance Zt. This condition is satisfied in the equalizer system of Fig. 1 where the load is the input circuit of the thermionic tube I9.

What is claimed is:

1. An equalizer having an input impedance Ro whichis a constant pure resistance for providing a voltage gain when operating between an input load of impedance Ro and an output load of high impedance comprising a series branch of impedance Za, an output shunt branch of impedance Zt and an input shunt branch including a resistance of value Ro and an impedance Ze connected in series, the impedances Za and Zb having over some portion of the useful frequency range a ratio not exceeding two, reactive components of opposite sign and resistive components the sum of which is at least equal to Ru, and the impedance Ze being the inverse, with respect to Ro, of the sum of the impedances Za and Zh reduced by the value of the resistance Re.

2. An equalizer in accordance with claim 1 in which said output shunt branch includes a se- Which the impedance Za is of the same order of ries-connected resistor. magnitude as the impedance Zh. 5. An equalizer in accordance with claim 1 3. An equalizer in accordance with claim 1 in having a transmission loss characteristic which which the impedance Za has substantially no re- 5 is substantially a straight line over said useful sistive component. frequency range.

4. An equalizer in accordance with claim 1 in WALTER R. LUNDRY.

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