US1969571A

US1969571A - Transmission network

Info

Publication number: US1969571A
Application number: US661268A
Authority: US
Inventors: Warren P Mason
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1933-03-17
Filing date: 1933-03-17
Publication date: 1934-08-07
Anticipated expiration: 1951-08-07

Description

Aug. 7, 1934. w. P. MASON TRANSMISSION NETWORK Filed March 17, 1933 FILTER SECTION F /1. TE R SECTION FILTER SECTION F/L TE R SECTION FREQUENCY /N VENTUR W P. MAS 0N FREQUENCY FREQUENCY A TTORNEY Patented Aug. 7, 1934 1,969,571 Ta NsMIssroN NETWORK VJarren Mason, West Orange, N. .L, assignorito Bell Telephone Laboratories, Incorporated, New York, N. Y, a corporation of New York p Application March 17, 1933, semi No. 661,268 7 f 16 Claims. (01. 172F443 This invention relates to wave transmission networks and more particularly to wave filters employing in combination piezoelectric crystals and inductance coils as impedance elements.

An object of the invention is to improve the transmission and impedance characteristics of wave filters.

Another object is to reduce the attenuation distortion in the transmission band of. wave 10 filters due to the inherent resistance associated with the component inductance coils.

Another object is to decrease the impedance irregularities at the input terminals of a multisection Wave filter caused by reflection effects at the junction points of the filtersections.

A further object is the elimination of longitudi-' nal currents from the output of a balanced wave filter.

A feature of the invention is a multi-section "2.0 wave filter in which an ohmic resistance has been inserted between adjacent sections, and the load impedances between which the filter is connected have been modified in order to reduce'the reflection eifects otherwise caused by the effective resistance inherent in the reactance ele ments comprising the filter.

The wellknown property of resonance in piezoelectric crystals and their extremely small energy dissipation make them highly suitable for use as impedance elements in wave transmission networks. When they are thus used in wave filters it has been found necessary to associate inductance coils with the crystals in order to broaden the transmission band obtainable.- The introduction of'the more highly dissipative in ductances tends to increase the transmission loss in; the transmission range and also to distort the attenuation characteristic in this range.

The attenuation distortion is the more objectionable of these two eifects, since the increased transmission loss may easily be made up by the use 'of a properly designed thermionic tube amplifier. Therefore. if the dissipation in the coils,

needed to widen the band of the filter, has onlybe considered as effective at the ends of thesection, either in seriesor in parallel with the filter.;

In accordance with the present invention these resistances are incorporated with the load im-;

coil resistances are effectively] 'inlseries or in shuntwith the filter. Also,.between filter sec-' tions these effective resistances are incorporated with added ohmic resistances in such a way as to form a constant resistance attenuator of essentially the same impedance as the filter. If the resistances are effectively in series with the filter rss Mae 1 ems-E then a shunt resistanceis added between the sections; if the eiiective resistances are in shunt with the filten'the' added-resistance 'is placed in series between the sections. In this'way the'filter is terminated in itscharacteristic impedance both at the ends and at the junctions of the sectionsj As a result a constant loss independent of fre'quencyjis added to'the attenuation characteristic of the filter which would be" obtainable if the elementswere dislsipatioriless; Also, the terminal impedance of the filter in the-transmission bandis made much more uniform, since the reflection efiects are eliminatedat the ends of the filter and at the points where the sections are joinedp 7 In the case where the added resistance is connected in shunt, this resistance maybe tapped and grounded atits center point, thus providing a low impedance'path to groundforundesired longitudinal currents which may be fiowing in the same direction along the two: sides 'of a balanced filter. g Q I 1 The invention will'be more fully understood from the following detaileddescription and by reference "to the accompanying drawing, *of which: I

Figs. land 2 are 'schematicsof typical'wave filter sections to which the invention is ap; plicable; i- Figs. 3 and 4 show diagrammatically'two em-- bodiments'o'f the'i nvention'; and i Figs. 5, 6 and '7 are diagrams to which reference is made in explaining the advantages of the invention. 1 V

A typical wave filter section to which the invention is applicable is shown in Fig.1. 'The section consists of a lattice of piezoelectric crystals, shunted at each end by condens'ersand with four'equal inductance coils L1 irrseries with the combination. Condensers are 'alsoshunted 55. smaller or larger, depending upon whether the acrossthe ftwo crystalsin series withthe' line. For a more detailed descriptionof this'ty'pe; of filtersectionjreference is made to my copending application, Serial No. 489,268, filed October 17, 1.936. The energy dissipation in the crystals and the condensers will "ordinarily be very small in amount-and; therefore may safely be neglected The resistance associated with eachinductance L1 is effectively in series with each coil, as represented by the resistances R1 of Fig. 3. The value of the resistance R1 may be found by direct measurement or by well known methods of computation. In accordance with the invention each load impedance R5 is reduced in value to allow for the two series coil resistances R1. In equation form,

R5=Ro2R1 (1) where R0 is the characteristic impedance of the filter in the transmission band. In this way, the

' filter is terminated at each end in its characteristic impedance, and therefore, reflection effects at these points are eliminated.

Between filter sections "there will be two coil resistances in series in each side of the line. If left uncorrected, there will thus be an impedance mismatch between the sections. In accordance with the invention, however, an ohmic resistance Ra'is shunted across the line between the two junction points of the series coil resistances R1. The value of R3 is so chosen that, in combination with the four resistances R1, it forms a constant resistance attenuator of impedance R0. In equation form When the resistance R3, evaluated in accordance with Equation (2), is added each filter section faces its characteristic impedance at the junction of the sections, and reflection effects at this point are thereby eliminated. The multi-section filter shownin Fig. 3 will, therefore, have the attenuation characteristic which would be obtained if elements having no dissipation were used, with the addition of a constant loss over the entire extent of the frequency range. This additional loss may readily be compensated, when necessary, by the use of a distortionless amplifier, as mentioned above.

As shown in Fig. 3 the added shunt resistance R3 may be tapped at its electrical mid-point and connected to the ground G in order to eliminate undesired longitudinal currents which may be flowing in phase along the two sides of a balanced filter. Grounding the center point of this resistance has no effect upon the loop current flowing in the filter, since the voltage producing this current is balanced with respect to ground. However, a low impedance path to ground is thereby provided for the longitudinal currents which fiow along both sides of the filter in the same direction and utilize ground as the return path. These undesired longitudinal currents are in this way effectively eliminated and do not appear in the output of the filter.

Another type of filter section to which the invention is applicable is the one shown in Fig. 2,

which differs from the section shown in Fig. 1

only in that the inductance coils L2 are shunted across the ends of the filter instead of being in series therewith. At any one frequency the resistance associated with the inductances may be represented as eiiectively in shunt across the input and output filter terminals, as shown by the resistances R2 of Fig. 4. Inorder to match the impedance of theload to that of the filter, the load impedance R6 is increased in value to compensate for the-shunt resistance R2. The required value for R6 is givenby the equation I Q 2 o R0 R2 R0 (3) where Rois the characteristic impedance of the filter section.

Where two of the sections join, a pair of equal resistances R4 are inserted in series between the sections. The value of the resistance R4 is given by the equation o z R4 R22 R02 When these modifications have been made the filter sections shown in Fig. 4 are terminated at each end in their characteristic impedance and, as a result, reflection effects at the ends of the filter and between sections are eliminated. The filter will then have the attenuation characteristic of a filter composed of dissipationless elements, with a small added loss which is constant with frequency. At the same time, the terminal impedance of the filter will be more uniform in the transmission band.

Some of the advantages of the application of the invention to a band pass filter composed of two sections of the type shown in Fig. '1 are illustrated by the diagrams of Figs. 5, 6 and '7. Fig. 5 shows the attenuation-frequency characteristic of the filter in the transmission band. The dotted line curve,gives the attenuation of the filter, when shunt resistance R3, determined according to Equation (2), is added. It is apparent from a comparison of the two. curves that the attenuation distortion in the transmission band of the filter has been eliminated. The small amount of added-loss may easily be made up in an associated amplifier. The dotted line curve and the solid line curve of Fig. 6 give a comparison of the terminal resistance of the two-section filter of Fig. 3, respectively, before and after the application of the invention. The two curvesof Fig. 7 show the improvement in the terminal reactance of the filter under similar conditions. It will be noted that the impedance of the filter in the transmission band is made considerably more uniform by the-application of the invention. As a result, the reflection efiects which occur between the filter and the load are greatly reduced.

The inductance coils L1 and L2 of Figs. 1 and 2 are shown as located external to the latticeportion of the filter sections but these coils may, of course, be incorporated within the lattices and the principles of the invention may be applied equally the values of the various resistances in accord-.

ance with Equations (1), (2), (3) and (4).

What is claimed is: r

1. In combination, a multi-sectioh wave filter and an attenuator located'between two sections of said filter, the characteristic impedance'of said attenuator being substantially equal to the characteristic impedance of said filter, and at least one of the component resistances of said attenuator being formed by the effective resistance inherent in the elements comprising said filter.

2. In combination, a wave filter having a plurality of-sections and a constant resistance attenuator located between two adjacent sections of said filter, the characteristic impedance of said attenuator being substantially equal to the characteristic impedance of the sections of said filter, and at least one of the component resistances which comprise said attenuator being formed by the effective resistance inherent in the elements which comprise the sections of said filter.

3. In combination with a multi-section wave filter, a resistance inserted between two adjacent sections of said filter, the magnitude of said re sistance depending upon the characteristic impedance of said filter and also upon the effective resistance inherent in the reactance elements which comprise the sections of said filter, whereby each of said two adjacent sections at their junction point faces an impedance which is substantially equal to the characteristic impedance of the section.

4. In combination, two wave filter sections con- '1 nected in tandem and a resistance shunted across the line at the junction of said two sections, the magnitude of said resistance depending upon the characteristic impedance of said filter sections and also upon the effective resistance in- 'I herent in the reactance elements which form said sections, whereby reflection effects at the junction of said two sections are reduced.

5. In combination, two wave filter sections connected in cascade and a resistance connected in II series between said two sections, the magnitude of said resistance depending upon the characteristic impedance of said filter sections and also upon the inherent resistance of the reactance elements comprising said sections, whereby the at- ..tenuation distortion in the transmission band of said filter is reduced.

6. In combination, a wave filter and a load impedance connected to the terminals of said filter, the magnitude of said load impedance differing TI from the characteristic impedance of said filter by an amount which depends upon the inherent effective resistance of the reactance elements comprising said filter, whereby said filter is terminated in its characteristic impedance, thereby reducing the attenuation distortion and impedance irregularities in the transmission band of said filter.

7. In combination, a wave filter and a load impedance associated with said filter, the magnitude "of said load impedance being smaller than the characteristic impedance of said filter by an amount equal to the inherent resistance of the reactance elements comprising said filter and effectively in series with said filter, whereby said filter is terminated in its characteristic impedance.

8. In combination, a wave filter and a load impedance connected to the terminals of said filter, the magnitude of said load impedance being larger than the characteristic impedance of said filter by an amount which depends upon the inherent resistance of the reactance elements comprising said filter and effectively in shunt with the terminals of said filter, whereby said filter is terminated in its characteristic impedance.

9. In combination, a multi-section wave filter comprising piezoelectric crystals and inductance coils as reactance elements, and an attenuator located between two sections of said filter,

at least one of the component resistances of said attenuator being formed by the-effective resistance inherent in the inductance coils comprising said filter. A a

10. In combination, a multi-section wave filter comprising piezoelectric crystals and inductance coils as reactance elements, and a resistance -'inserted' between two adjacent sections of saidfilter, the magnitude of said resistance depend ing upon the characteristic impedance of said filter and the effective resistance inherent in said inductance coils, whereby each of said two adjacent sections at their junction point faces an impedance which is substantially equal to the characteristic impedance of said filter. I

11. In combination, two wave filter sections connected in series relation, and a resistance connected in shunt with the line at the junction of said two sections, said filter sections comprising piezoelectric crystals and inductance coils as reactance elements, and the magnitude of said resistance depending upon the characteristic impedance of said filter sections and also upon the magnitude of the effective resistance inherent in said inductance coils, whereby the attenuation distortion otherwise present in the transmission band of said filter is reduced.

12. In combination, two wave filter sections connected in tandem, and a resistance connected in series between said two sections, said filter sections comprising piezoelectric crystals and inductance coils as reactance elements, and the magnitude of said resistance depending upon the characteristic impedance of said filter sections and also upon the magnitude of the effective resistance inherent in said inductance coils, whereby refiection loss is eliminated in the transmission band of said filter.

13. A frequency selective wave transmission system comprising a wave filter having two pairs of terminals, and transmission devices representing terminal impedances'connected to said pairs of terminals, said filter comprising piezoelectric crystals and inductance coils as reactance elements, and the magnitude of at least one of said terminal impedances differing from the charac teristic impedance of said filter by an amount which depends upon the inherent effective resistance of said inductance coils, whereby said filter is effectively terminated in its characteristic impedance.

14. In a frequency selective transmission system, a multi-section broad band wave filter having piezoelectric crystals and inductance coils as reactance elements, a resistance inserted between two adjacent sections of said filter, and transmission devices representing terminal impedances connected to the ends of said filter, the magnitude of at least one of said terminal impedances differing from the characteristic impedance of said filter by an amount which depends upon the inherent efiective'resistance of said inductance coils, and the magnitude of said inserted resistance depending upon the characteristic imped-- 15. In combination, two lattice type wave filter sections connected in cascade, and a resistance inserted between said two sections, said filter sections comprising piezoelectric crystals and in-- ductance coils as reactance elements, and the magnitude of said inserted resistance depending upon the characteristic impedance of said filter sections and also upon the magnitude of the effective resistance inherent in said inductance coils, whereby each of said two sections at their junction faces an impedance which is substantially equal to the characteristic impedance of said filter, thereby reducing the attenuation dis tortion and impedance irregularities otherwise present in the transmission band of said filter due to theinherent effective resistance of said inductance coils.

16. In combination, two balanced wave filter sections connected in tandem and a resistance shunted across the line at the junction of said two sections, said resistance being grounded at its electrical mid-point, and the magnitude of said resistance being determined with respect to the characteristic impedance of said filter sections and also with respect to the effective resistance inherent in the reactance elements which comprise said sections, whereby the attenuation and impedance characteristics of the filter are improved and longitudinal currents are effectively drained off and thereby eliminated from the output of the filter.

WARREN P. MASON.