US3316510A - Electrical ladder-type filter - Google Patents

Description

April 25, 1957 w. POSCHENRIEDER ET AL 3,316,510

ELECTRICAL LADDER-TYPE FILTER Original Filed April 1, 1963 2 she s- 1 Fig.1

PRIOR ART Fig.2

April 1967 w. POSCHENRIEDER ET AL 3,316,510

ELECTRICAL LADDER-TYPE FILTER Original Filed April 1, 1963 2 Sheets-Sheet 2 Fig.5

Fig 9 United States Patent Ofitice 3,315,5lfi Patented Apr. 25, 1967 7 Claims. (31. 333-42 This application is a continuation of Ser. No. 269,526, filed Apr. 1, 1963, now abandoned.

The invention disclosed herein is concerned with an electrical ladder-type filter, comprising at least one transverse branch containing an electromechanical oscillator at which appears, as seen in theladder at the terminals of the transverse branch, in addition to the series resonance of the electromechanical oscillator, which produces an attenuation pole, at least one parallel resonance frequency (repetition frequency) lying in the filter barrier range, the action of which is compensated by an attenuation produced in preceding and/or succeeding filter elements.

A ladder circuit of particular construction is frequently being used at the present time for making low pass filters and band pass filters with quartzes or other electromechanical oscillators. It is important in connection with these ladder circuits, on the one hand, that an electromechanical oscillator for the production of an attenuation pole is used in the transverse branch of the filter, and on the other hand, that so-called repetition poles appear directly adjacent to both sides of the branch containing the oscillator, that is, barrier points with substantially the same resonance frequency which produce, however, only one operating attenuation pole which coincides at least approximately with the parallel resonance appearing in the transverse branch and lying in the filter barrier range. The result obtained by this ladder circuit resides in that the structure of the electromechanical oscillator, especially the oscillator quartz, appears in the electrical equivalent or analog filter circuit in the desired manner.

It is in the design of low pass filters and broad band pass filters in the form of such ladder circuits desirable to obtain a value as great as possible for the ratio of static to dynamic capacity of the mechanical filter that is being used, for example, a quartz, and to provide the repetition pole in a suitable spacing from the attenuation pole produced by the oscillator, so that the losses and the barrier attenuation at the barrier border in the pass range of the filter are substantially determined by the oscillator and not by the elements which produce the repetition pole.

The problem and object of the invention reside in improving the properties of such ladder circuits, primarily so as to produce a particularly favorable ratio of static to dynamic capacity of the oscillator used, which may be a piezoelectric oscillator.

This object is realized, in connection with an electrical ladder-type filter having at least one transverse branch containing an electromechanical oscillator, in the branch circuit of which appears, as seen from the terminals of the transverse branch, the series resonance of the electromechanical oscillator, which produces an attenuation pole, and in addition thereto at least one parallel resonance frequency (repetition frequency) lying in the filter barrier range, the action of which is compensated by an attenuation produced in preceding and/or succeeding filter elements, by effecting the compensation, in accordance with the invention, in the form of a parallel resonance circuit in a longitudinal branch, or in the form of a series resonance circuit in a transverse branch, and providing this compensation in the branch circuit outside of the filter section which is determined by the transverse element containing the electromechanical oscillator and the longitudinal elements directly connected therewith.

'An advantageous embodiment for the realization of a low pass filter or of a band pass filter with steeply rising attenuation flanks, is obtained by using in the transverse branch, as an electromechanical oscillator, a piezoelectric element, preferably an oscillating quartz. Further advantageous embodiments of the invention are obtained by effecting the compensation by two parallel resonance circuits with identical intrinsic resonance fre quency, whereby one such resonance circuit is allocated to a preceding and the other to a succeeding longitudinal branch, or by effecting the compensation by two series resonance circuits with identical intrinsic resonance frequency, which resonance circuits are arranged in transverse branches, whereby one such series resonance circuit is disposed ahead of the transverse element containing the oscillating quartz while the other in disposed following this transverse element.

It is moreover advantageous to effect the compensation with the aid of a series resonance circuit in a transverse branch and a parallel resonance circuit with the same intrinsic resonance frequency in a longitudinal branch of the arrangement, whereby the series resonance circuit is disposed ahead of the transverse element containing the oscillating quartz while the parallel resonance circuit is disposed following this transverse element, or to effect the compensation with the aid of a parallel resonance circuit disposed in a longitudinal branch and a series resonance circuit with the same intrinsic resonance frequency disposed in a transverse branch, whereby the parallel resonance circuit is disposed ahead of the transverse element containing the oscillating quartz while the series resonance circuit is disposed following this transverse element.

According to another feature of the invention, an advantageous embodiment is obtained by using in the transverse branch, with the aid of a dual circuit, a magnetostrictive element in place of the piezoelectric element.

Further details of the invention will appear from the description of embodiments which is rendered below with reference to the accompanying drawings.

FIG. 1 shows a portion of a known ladder-type band pass filter;

FIG. 2 indicates equivalent circuits for transforming elements shown in FIG. 1;

FIG. 3 represents the result of the transformation according to FIG. 2;

FIG. 4 indicates another ladder-type circuit obtained by transformation of elements shown in FIG. 3;

FIG. 5 shows equivalent circuits for effecting a trans forrnation with respect to FIG. 4;

FIG. 6 illustrates a circuit obtained by transformation with the aid of the elements shown in FIG. 5; and

FIGS. 7 to 10 illustrate embodiments of the invention in which two-port nets are connected between the transverse branch containing the mechanical oscillator and respective resonance circuits which may be either parallel resonance circuits or series resonance circuits.

Referring now to FIG. 1 which shows, as noted above a portion of a known ladder-type band pass filter, wherein an attenuation pole is produced at the point jm of the attenuation diagram, by the series resonance of the piezoelectric oscillator indicated by a straight arrow. The

quartz is represented by its electrical equivalence or analog circuit, namely, by the element L C and G In the transverse branch of this circuit, in which the piezoelectric oscillator is disposed in parallel with an inductance L there appears a parallel resonance frequency f,-

which is indicated by a semicircular arrow. This resonance frequency f 'would produce an attenuation breakthrough since the corresponding transverse branch becomes high ohmic at a frequency 72;, practically causing a through-connection between the .circuit 3 and the circuit 4. g In order to avoid this undesired operation, there are provided, at both sides of the transverse branch which is here being considered, two parallel resonance circuits 1 and 2, in the longitudinal branch of the circuit, the intrinsic frequency of which is identical and indicated by fa These two parallel circuits produce at the frequency fang, the socalled repetition frequency, a common attenuation pole, the socalled repetition pole. The parallel circuits 1 and 2 which produce the repetition pole, are di- 7 mensioned so that their intrinsic resonance frequency f coincides approximately with the parallel resonance frequency f of the transverse branch referred to (f f whereby the apparent input resistance of the circuit assumes at the point 3:12. again approximately the value infinity, thereby avoiding an attenuation break-through at this point. a

As may be seen from FIG. 1, the arrangement also contains in the longitudinal branch, the parallel resonance circuits 3 and 4, such circuits producing further attenuation poles at the frequencies fw and f as Well as the parallel resonance circuits 5 and 6 in transverse branches, which aredimensioned according to further requirements posed for the filter. The dash lines indicate that there may be provided further circuit elements at the respec.

tive ends of the ladder, which is, however, unimportant in connection with the following considerations.

The bracketed two-port net VPI and VPII which embrace, respectively, the elements ofthe resonance circuits 5, 1 and 2,6, are now reformed with the aid of equivalent circuits, known per se, as shown in FIG. 2. (The circuit a of FIG. 2 merges with the aid of an ideal transformer with a transformation ratio lzu, into the circuit b, and vice versa.) The use of this transformation'in connection with the two-port nets VPI and VPII '(FIG. 1) results in the circuit shown in FIG; 3. As will be seen from FIG. 3, the branch containing the quartz as well as the parallel resonance circuits 3 and 4, are not affected by the transformation. An attenuation pole is. with a piezoelectrical oscillator also formed in this circuit, at the point fs which is again indicated by a straight arrow. 'The parallel resonance circuits 1 and 2.

which produce at the frequency fm the repetition pole are not any more. directly connected with the transverse branch which is provided with the oscillator, but are separated by the parallel circuits 7 and 8, the intrinsic frequency of which lies according to the transformation with the elements of FIG. 2, likewise at the frequency fm The repetition frequency fwQ of the parallel resonance circuits 1 and 2 coincide in the circuit according to FIG. 3 at least substantially with the parallel resonance frequency f lying in the filter barrier range of the transverse branch containing the oscillation quartz (fx mg). The circuit transformation entailed addition of the capacitor C and C and of the ideal transformers A and B.

The two-port nets VPIII and WW, bracketed in' FIG. 3,-which embrace respectively the elements from the parallel resonance circuits 7, 3 and 8, 4, are now again trans,-

formed with the aid of the equivalent circuits shown in FIG. 2, thus resulting in the ladder-type circuit repreresented'in- FIG. 4. The ideal transformers resulting in the transformation are thereby unimportant and for the sake of clarity have been omitted in FIG. 4, that is,

the transformers are placed at'the input or the output I of the circuit, which as is known merely signifies respectively a multiplication of all inductivities or a division of all capacitances of the respective circuit section, with the value u. As will beseen, the parallel resonance circuits land 2, which produce the repetition pole, are not affected by the transformation and the intrinsic resonance frequency lies now as before at the frequency fa The 7 addition of capacitors C7 to C capacitors C and C likewise appear in the circuit. The capacitors C and C are added by the transformation. The parallel circuits 3 and 4 with the intrinsic frequencies fm and fa; also appear again in the longitudinal branch of the circuit.

Upon measuring at the terminals K and K of FIG. 4 of the transverse branch containing the oscillation quartz, for example, with the aid of an impedance meter, the apparent resistance depending upon the frequencywhereby the input and the output of the filter can be terminated with real resistances, the value of which issuitably equal to the wave impedance of the filterthere will appear zero points and infinity points in the course of the apparent resistance. One of the infinity points corresponds to the repetition frequency fang, to which are tuned the parallel resonance circuits 1 and 2 which are disposed in the longitudinal branch and produce the repetitionpole.

As may also be seen from FIG. 4, the circuit transformation results in the addition of capacitors C and C as well as the coils L and L Since the capacitances C and C are due to the parallel connection to the original quartz capacitance C additive, the ratio of the static capacitance which now consists of C +C +C to the dynamic capacitance C is increased, which is extraordinarly advantageous for the realization of the oscillation quartz. The newly added elements C C and L which are disposed parallel with the original quartz, effect moreover a shifting of the original parallel resonance frequency f of the transverse branch containing the oscillation quartzconsidering such transverse branch by itselfto a new value f which is in FIG. 4 again indicated by a semicircular arrow. The parallel resonance frequency 3, lies as a rule adjacent to the repetition frequency or adjacent to one of the other pole frequencies 12. or fa so that the finite attenuation based upon these attenuation that the repetition pole can also be realized with the aid ofseries resonance circuits which are disposed in transverse branches of the arrangement. The bracketed twoport nets VPV and VPVI which consist of half-elements comprising respectively the, capacitor C and parallel circuit 1 and the capacitor C and parallel circuit 2, are for this purpose transformed with the aid of known equivalent circuits according to FIG. 5. The corresponding transformation results in the circuit shown in FIG. 6;

As will be seen from FIG. 6, the capacitors C and C; as well as the parallel resonance circuits 3 and 4, remain unchanged by the transformation, that is, they remain as they also appear in FIG. 4. The capacitances C C and C of FIG. 4 are combined to form the static quartz capacitance C and there applies the relation The inductivities L L and L of FIG. 4 are combined to form the inductivity L according to the relation The parallel resonance frequency f appearing in the transverse branch containing the oscillation-quartz, thus remains preserved. However, the transformation effects formation of the repetition pole at the point ja by the series resonance circuits 9' and 10 which lie in transverse branches of the arrangement and cause at the frequency n practically a shunt, such con-v dition being indicated by straight arrows. The circuit transformation results, as compared with FIG. 4, in the Upon measuring the apparent resistance at the terminals K and K of the, transverse branch containing the oscillating quartz, there will again appear an infinity point in the course of the apparent resistance, at the repetition frequency fs Various embodiments of the invention utilizing the principle of repetition poles are again represented in FIGS. 7 to 10,

In each of these circuits there is provided in a transverse branch an electromechanical oscillator Q, whereby an inductivity L and a balancing capacitor C, which may be connected in parallel therewith. Two-port nets D and E are respectively disposed at the sides of the transverse branch which contains the quartz. The repetition pole is in FIG. 7 produced by the parallel resonance circuits 1 and 2 (see also FIG. 4), which are disposed in the longitudinal branch of the circuit. The parallel oscillation circuit 1 is connected ahead of the two-port net D and the parallel oscillation circuit 2 is disposed serially following the two-port net E.

In FIG. 8, the repetition ole is produced by two series resonance circuits 9 and which are disposed in transverse branches (see also FIG. 6). The series oscillation circuit 9 is disposed ahead of two-port net D and the series oscillation circuit 10 is disposed serially following the two-port net E.

In FIG. 9, the repetition pole is produced by the series resonance circuit 11 lying in a transverse branch and by a arallel resonance circuit 12 lying in a longitudinal branch. The series circuit 11 is disposed ahead of the two-port net D while the parallel circuit 12 is disposed serially following the two-port net E. The two oscillation circuits 11 and 12 are tuned to the identical resonance frequency fraz- In FIG. 10, the repetition pole is produced by a parallel oscillator circuit 13 disposed ahead of the two-port net D and by a series oscillation circuit 14 disposed serially following the two-port net E.

The following particularly characteristic features result in connection with the circuits according to FIGS. 4 to 6 as well as in the circuits according to FIGS. 7 to 10:

(1) At least one attenuation pole is produced with an electromechanical oscillator in ladder circuit.

(2) At both sides of the branch containing the electromechanical oscillator, but not directly contiguous thereto, are disposed combinations of reactance elements which produce an attenuation pole at the identical resonance frequency (repetition pole).

(3) Upon measuring the apparent input resistance of the entire circuit, at the transverse branch which contains the electromechanical oscillator, there appear a number of pole points and zero points. One of these pole points coincides with the frequency of the repetition poles which lies in the barrier range.

(4) Upon cutting the circuits according to FIGS. 4 and 6 and the circuits according to FIGS. 7 to 10, at the points K /K and Kg/Kg, thus obtaining three partial two-port nets in which the reactance elements which pro duce the repetition pole do not any more belong to the partial two-port net which contains the electromechanical oscillator, and measuring the apparent input resistance at the terminal pairs of the respective partial two-port net which contains the electromechanical oscillator, the apparent input resistance becomes zero at the resonance frequency 72. of the repetition pole, when the repetition pole is produced by series resonance circuits or, respectively, infinitely high, when the repetition pole is produced by parallel oscillation circuits.

These four characteristic features will be fully preserved when the repetition pole is produced by bridge circuits or the like, within the ladder circuit.

Two band pass filter circuits (FIGS. 4 and 6) have been explained to bring out the invention more clearly, such circuits having, disposed in a transverse branch, an oscillating quartz, and being obtained by circuit transformation with the aid of equivalence circuits. The circuits according to FIGS. 4 and 6 as well as those accord- 6 ing to FIGS. 7 to 10 can 'be respectively calculated or designed according to the image parameter theory or according to the insertion loss theory, by a corresponding analysis of the circuit elements of a matrix according to the principle of the repetition poles.

There is, moreover, in connection with all circuits according to the invention, the possibility of using in the transverse branch, with the aid of dual circuits, magnetostrictive elements in place of the piezoelectric elements.

Changes may be made within the scope and spirit of the appended claims which define what is believed to be new and desired to have protected by Letters Patent.

The invention claimed is:

-1. An electrical ladder-type filter circuit having an electromechanical oscillator disposed in a transverse branch which generates an attenuation pole in the blocking range of the filter, a circuit element disposed in parallel with the electromechanical oscillator, as a result of which a parallel resonance frequency lying in the filter blocking range occurs in said transverse branch, a two-port net disposed in said circuit preceding said transverse branch and a twoport net disposed in said circuit following said transverse branch, said two-port nets each containing at least one longitudinal branch and one transverse branch, with a longitudinal branch of each disposed adjacent the transverse branch containing the electromechanical oscillator, a resonance circuit connected ahead of the first mentioned two-port net which is disposed in a branch thereat of said filter circuit, and a resonance circuit connected behind said second mentioned two-port net which is disposed in a branch thereat of said filter circuit, the resonance frequency of said last mentioned resonance circuits disposed ahead of and behind said two-port nets corresponding, at least approximately, to the parallel resonance frequency occurring in the filter blocking range of the transverse branch containing the electromechanical oscillator, such resonance circuits being so constructed, with respect to the nature of the branch in which they are disposed, that they effect a compensation of the parallel resonance frequency occurring in said transverse branch containing the electromechanical oscillator.

2. A filter according to claim 1, wherein said resonance circuits disposed ahead of and following said two-port nets are like resonance circuits with identical intrinsic resonance frequency, disposed in like branches ahead of and following the transverse branch containing said electromechanical oscillator.

3. A filter according to claim 2, wherein said resonance circuits are parallel resonance circuits disposed in respective longitudinal branches.

4. A filter according to claim 2, wherein said resonance circuits are series resonance circuits disposed in respective transverse branches.

5. A filter according to claim 1, wherein, of said resonance circuits disposed ahead of and following said twoport nets, one of said circuits is a series resonance circuit disposed in a transverse branch, and the other circuit is a parallel resonance circuit disposed in a longitudinal branch.

6. A filter according to claim 5, wherein the resonance circuit disposed ahead of the preceding two-port branch.

7. A filter according to claim 5, wherein the resonance circuit disposed ahead of the preceding two-port net is a parallel resonance circuit disposed in a longitudinal branch.

No references cited.

ELI LIEBERMAN, Primary Examiner. C. BARAFF, Assistant Examiner.

Claims (1)

1. AN ELECTRICAL LADDER-TYPE FILTER CIRCUIT HAVING AN ELECTROMECHANICAL OSCILLATOR DISPOSED IN A TRANSVERSE BRANCH WHICH GENERATES AN ATTENUATION POLE IN THE BLOCKING RANGE OF THE FILTER, A CIRCUIT ELEMENT DISPOSED IN PARALLEL WITH THE ELECTROMECHANICAL OSCILLATOR, AS A RESULT OF WHICH A PARALLEL RESONANCE FREQUENCY LYING IN THE FILTER BLOCKING RANGE OCCURS IN SAID TRANSVERSE BRANCH, A TWO-PORT NET DISPOSED IN SAID CIRCUIT PRECEDING SAID TRANSVERSE BRANCH AND A TWOPORT NET DISPOSED IN SAID CIRCUIT FOLLOWING SAID TRANSVERSE BRANCH, SAID TWO-PORT NETS EACH CONTAINING AT LEAST ONE LONGITUDINAL BRANCH AND ONE TRANSVERSE BRANCH, WITH A LONGITUDINAL BRANCH OF EACH DISPOSED ADJACENT THE TRANSVERSE BRANCH CONTAINING THE ELECTROMECHANICAL OSCILLATOR, A RESONANCE CIRCUIT CONNECTED AHEAD OF THE FIRST MENTIONED TWO-PORT NET WHICH IS DISPOSED IN A BRANCH THEREAT OF

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