US2247898A - Band-pass filter, including trap circuit - Google Patents

Description

July 1, 1941- H. A. WHEELER BAND-PASS FILTER INCLUDING TRAP CIRCUIT Filed Sept. 29, 1959 2 Sheets-Sheet 1 g E0 3, o n f: 1E: Frequency.

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INVENTOR HAROLD A.WHEELER BYf ATTORN EY y 1941- H. A. WHEELER 2,247,898v

BAND-PASS FILTER INCLUDING TRAP CIRCUIT Filed Sept. 29, 1939 2 Sheets-Sheet 2 INVENTOR HAROLD A. WH EELE R vmm/ ATTORNEY Patented July 1, 1941 2,247,898 BAND-PASS FILTER, INCLUDING TRAP CIRCUIT Harold A. Wheeler, Great Neck, N. Y., assignor to Hazcltine Corporation, a corporation of Dela ware Application September 29, 1939, Serial No. 297,081

19 Claims.

This invention relates to band-pass filters including filter sections having trap circuits therein and having an image impedance at a pair of terminals which has opposite extreme values at the upper and lower cutoff frequencies of the pass band. While the filters of the invention are of general application, they are of particular utility in effecting an impedance transformation between two pairs of terminals while providing traps for frequencies just outside of a desired continuous pass band.

A double-tuned transformer is a commonly used form of band-pass filter section. Such a transformer circuit may be regarded as two halfsections joined together, each comprising a shuntand a series-reactance arm. Each of these half-sections comprises at their junction a pair of terminals across which the image impedance has opposite extreme values at the upper and lower cutoff frequencies of the pass band, which form of image impedance is not matched with the more familiar constant-k type of bandpass image impedance. However, such filter half-sections have many uses and are of particular utility because oftheir extreme simplicity and their image-impedance transforming properties between their tWo pairs of terminals. Band-pass filters utilizing only half-sections of the type under discussion, or such half-sections in combination with other known filter sections, have the disadvantage that, for some purposes, sufiicient attenuation at frequencies just above and below the pass band is not secured with a minimum number of circuit elements. This is true, even if the other known filter sections are of types including trap circuits to provide maximum attenuation at particular frequencies outside the pass band. Some attempts to design simple filters with traps have yielded filters with spurious pass bands in addition to the desired pass band, and such spurious bands are usually undesirable.

It is an object of the present invention, therefore, to provide an improved impedance-transforming band-pass filter half-section which includes a trap for frequencies outside the pass band.

It is another object of the invention to provide an improved band-pass filter half-section which passes only a single continuous band and which includes a trap circuit for frequencies outside the pass band.

It isanother object of the invention to provide an improved band-pass filter section for passing a single band of frequencies and which includes a trap circuit for frequencies above the pass band and a trap circuit for frequencies below the pass band.

It is still another object of the'invention to provide a band-pass filter network for passing a single continuous band of frequencies which includes one or more trap circuits for frequencies outside of the pass band and which tolerates maximum shuntcapacitance across one or both pairs of terminals.

In accordance with one feature of the invention, a band-pass filter half-section having in put and output pairs of terminals effectively comprises seriesand shunt-reactance arms of which at least each of two includes a portion individually of the same reactance-frequency characteristic and of a given kind of reactance at frequencies within the passband; that is, all such portions individually hav the same variation of reactance with frequency over the pass band and effectively provide either only inductive or only capacitivereactance at frequencies within the pass band. A trap circuit is included in one of the reactancearms which is adjacent to one of the pairs of terminals and comprises the only other reactance of the half-section. The trap circuit is resonant at a. frequency outside of the pass band andis so connected to present reactance of the kind opposite to that of the said portions at frequencies within the pass band, whereby the image impedance at the other pair of terminals is caused to present opposite extreme values atthe upper and lower cutoff frequencies of the pass band.

In accordance with another feature of the invention, a band-pass filter section has input and output pairs of terminals and effectively comprises, seriesand shunt-reactance arms at least each of three of which includes a portion of the same reactance-frequency characteristic and of a given kind at frequencies in the pass band; the filter section also includes a trap circuit in on of its reactance arms which is connected between two of the said three reactance arms and which comprises the only other reactance of the filter section. The trap circuit is resonant at a frequency outside of the pass band and presents reactance of the kind opposite to that of the above-mentioned portions .at frequencies in the pass band, whereby the image impedance at both pairs of terminals is caused to have opposite extreme values at the upper and lower frequencies of the pass band.

In accordance with another feature of the invention, aconfiuent band-pass filter effectively comprises a series of filter half-sections with image-impedance matching at their junctions, each of two adjacent ones of said half-sections having both seriesand shunt-reactance arms of which each of two of the reactance arms adjacent to their junction includes a portion individually of the samereactance-frequency characteristic and of a given. kind of reactance at frequencies in the pass band. The filter of this embodiment also includes a trap circuit in each of the said two adjacent half-sections which is removed from their junction by at least one of the said adjacent reactance arms and which comprises the only other reactance of th half-sec tion. One of the trap circuits comprises a shunt arm of series-connected capacitance and induct ance resonant at a frequency on one side of the pass band, and the other of the trap circuits comprises a series arm having parallel-connected capacitance and inductanc resonant at a frequency on the other side of the pass band. The matched image impedance at the above-mentioned junction therefore has opposite extreme yalues at the upper and lower cutoff frequencies of the pass band and unlike trap circuits are provided in the two adjacent half-sections.

In accordance with still another feature of the invention, a band-pass filter comprises two resonant circuits each having an inductor coupled therewith by mutual inductance, a series circuit including the said inductors and a trap circuit with parallel-connected capacitance and inductance resonant at a frequency below the pass band, and a second trap circuit connected in parallel with one of the above-mentioned inductors and including series-connected capacitance and inductance resonant at a frequency above the pass band. The filter is preferably proportioned to pass a continuous band of frequencies and to secure maximum attenuation at frequencies above and below the pass band.

It is seen, therefore, that, while some arrangements of the prior art utilize both a band-pass filter circuit and. one or more trap circuits for frequencies outside the pass band of the filter, such prior art arrangements are subject to the disadvantage that they require more than the minimum number of circuit elements or the filter properties of the filter are disturbed by the addition of the trap circuit. In the latter case, the filter may be caused to have one or more spurious pass bands other than the desired pass band. The invention, however, provides a most simple bandpass filter which effectively includes the required number of trap circuits with filtering properties undisturbed by the inclusion of the traps. Substantially only one single continuous band of frequencies is passed by the filter circuits of the invention.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Each of Figs. 1a. and 1b of the drawings represents a filter half-section which is equivalent to one-half of a double-tuned transformer circuit and which is utilized to explain the principles of operation of the present invention; Fig. 1c is a graph of the image-impedance characteristic at one pair of terminals of each of the circuits of Figs. la and 1b, which characteristic is similar to the image-impedance characteristic at one pair of terminals of each of the filter half-sections of the invention; each of Figs. 2a and 2b is a circuit diagram of a band-pass filter half-section of the invention comprising a trap circuit for frequencies below the pass band; each of Figs. 3a and 32: represents a band-pass filter half-section of the invention comprising a trap circuit for frequencies above the pass band; Fig. 4a is a circuit diagram of a band-pass filter in accordance with the invention which is utilized to couple two vacuum tubes in cascade and which comprises a trap circuit for frequencies above the pass band of the system; Figs. ib-4e, inclusive, represent equivalent modifications of the circuit of Fig. 4a; Fig. 5a is a circuit diagram of a band-pass filter in accordance with the invention which comprises a trap circuit for frequencies above the pass band and a trap circuit for frequencies below the pass band; each of Figs. 5b to 5e, inc., represents an equivalent modification of the circuit of Fig. 5a.

Referring now more particularly to Fig. la of the drawings, there is shown a filter half-section comprising a shunt reactance arm including parallel-connected inductance L and capacitance C1 and a series-reactance arm including inductance L2. The filter half-section of Fig. 1a is a known electrical equivalent of one-half a filter whole-section comprising a conventional doubletuned transformer. The filter comprises one pair of input terminals I, I across which appears the image impedance of nominal value R1 and another pair of input terminals 2, 2 across which appears the image impedance R2. The conventional double-tuned transformer circuit may also be divided in a known manner into two filter halfsections of the form shown in Fig. 1b, each of Which includes a shunt-reactance arm comprising capacitance C1 across terminals I, I, a seriesreactance arm comprising inductance L3, and a shunt-reactance arm comprising inductance L4 connected across terminals 2, 2. Each of the following equations is applicable to one or both of the circuits of Figs. 1a and 1b:

Zz' image impedance at terminals 2, 2 of Fig. 1b;

fm the mean frequency of the pass band;

wz=the angular frequency at the lower cutoff frequency;

wa=the angular frequency at the upper cutoff frequency;

Ri=noinilnal image impedance across terminals Rz nominal image impedance across terminals 2, 2 of Fig. la;

Rc=nominal image impedance across terminals 2, 2 of Fig. lb.

In Fig. 10 there is illustrated the form of the image-impedance characteristic at the terminals 2, 2 of each of the filter half-sections of Figs. 1a and lb. Each half-section includes one tuned pass band.

circuit" and has at the terminals l, I .a constant-k mid-shunt impedance of mid-band valueRi associate'dwith the maximum tolerable shunt capacitance C1, where From Fig. lc it is seen that the image impedance across terminals 2, 2 has opposite extreme values at the cutoff frequencies of the pass band; that is, it is infinite at the lower cutofl' frequency hand zero at the upper cutofi frequency f2, and this type of image impedance serves as a basis for joining the half-sections of further embodiments of the invention in filter networks comprising additional filter-sections of different types.

In Fig. 2a there is illustrated a filter half-sectionin accordance-with the present invention which includes a trap for frequencies below the pass band. This filter half-section includes a shunt-reactance arm across terminals 3, 3 comprising inductance L5 and a series-reactance arm including inductance L6 in series with parallelconnected trap-circuit inductance L7 and capacitance C7. The following equations are applicable to the circuit of Fig.2a:

where, I h h Z3=image impedance at terminals 3, 3;

' f1=resonant frequency of trap circuit L7, C1; 7

w1=angu1ar frequency corresponding to is; and Ra=mid-band image resistance at terminals 3, 3.

The image-impedance characteristic of thejfilter of Fig. 2a at the'ter- minals 3, 3 has the same ing inductance L5 and the series-reactance arm includes a portion com-prising inductance Ls, the portions both having inductive reactance at frequencies in the pass band and the same react- ;ance at frequencies in the pass band and the same reactance-frequency characteristic. A trap circuit is 'included in the series-reactance arm which is adjacent to the pair of terminals 4, 4

"and comprises the only other reactance of the half-section. 'Thetrap circuit is resonant at a frequency below the passband and presents capacitive reactance, which is of the kind. opposite to that of the said portions at frequencies inthe half-section of Fig. 31) comprises pairs of ter-,

Another filter half-section in accordance with the present invention, including a trap circuit for frequencies below the pass band and having the same cutoff frequencies, is illustrated in Fig. 2b. This filter half-section comprises pairs of terminals -5, -5 and 6, 6 and includes a series-reactance arm comprising inductance La, a shunt-reactance arm comprising inductance L9, and a series arm including parallel connected inductance Lie-and capacitance C10. The circuit of Fig. 2b is readily derived from that of Fig. 2a by a simple impedance transformation. 1

The image impedance Z5 at terminals 5, 5 is identical with that at terminals 3, 3 of Fig. 2a. That at terminals 6, 6 has the same form as that at terminals 4, 4 but a different magnitude.

In Fig. 3a of the drawings there is illustrated a band-pass filter half-section in accordance with the invention, including a trap circuit forffrequencies above the pass band. This filter halfsection includes a series-reactance arm compris- 'ing inductance L11 and a shunt-reactance arm including inductance L12 connected in parallel with series-connected inductance L1: and capacitance C13, the pairs of terminalsof the filter'halfsection z-being identified by numerals I, I and 8, 8.

The following equations are applicable to the cir- 7 Zi=iinage impedance at terminals I, I;

R7=mid-band image resistance at terminals 1, I;

w and F 'w3 ='angular frequency of trap circuit L13, C13.

minals. 9, 9 and III, III, a shunt-reactance arm across terminals 9, 9 including inductance L14, a series-reactance arm including inductance L15, and a shunt reactance arm across terminals i0, i0 including series-connected inductance llm and capacitance Cl6.- The circuit of Fig. 3b is vacuum tubes l5 and IS in cascade. The bandpass filter circuit cf Fig. 40. includes four filter half-sections; they are connected with'imageimpedance matching at their respective junctions sections to which the opposite terminals of each of the intermediate half-sections are connected provide. a constant-k band-pass mid-shunt image impedance across the input and output terminals. The end half-sections and, therefore, the composite filter circuit is of a type which tolerates maximum shunt capacitance across the output circuit of tube I5 and across the input circuit of tube Hi. It will be understood that the capacitances C20, C20 may be comprised in whole or in part of the interelectrode capacitance-s of the tubes to .which they are coupled. It is also seen that the band-pass filter section, including the series-inductance arms L15, L15 of Fig. 4a effectively com-prises seriesand shunt-reactance arms, of which each of at least three includes portions individually of the same impedancefrequency characteristic and of a given kind of reactance at frequencies within the pass band. Specific-ally, four of the arms individually include inductances L14, L15, L15, and L14, each of which portions has an inductive reactance at frequencies within the pass band. Furthermore, a trap circuit, including both trap circuits L16, C16, is included in the one of the reactance arms of the filter section which is between two of the three inductance arms and comprises the only other reactance of the filter section. This resultant trap circuit is resonant at a frequency above the pass band and presents reactance of the opposite kind, that is, capacitance, at frequencies within the pass band.

The circuit of Fig. 4b is the full electrical equivalent of that of Fig. 4a. and differs only in that adjacent shunt-inductance arms L22 and L14 of Fig. 4a have been combined into the single inductance element L25 and the adjacent seriesinductance arms L21 and L15 have been combined single shunt-inductance element L28.

The circuit of Fig. 4:2 is identical with that of Fig. 40 except that a transformer including inductively-coupled primary and secondary inductances Lao, L31 has been substituted for the adjacent inductance elements L23, L28 of Fig. 4c in a manner which is wellunderstood in the art.

The circuit of Fig, 4c is identical with that of Fig. 4d except that a further similar transformation has been made to provide an equivalent circuit including three inductors L31, L32, and L33 each of which is coupled to each of the others. They are respectively tuned by condensers C31, C32, and C33.

In.-Fig..5a there is illustrated another modification of the invention comprising trap circuits resonant at frequencies both below and above the pass band which may be utilized for effecting an impedance transformation between two pairs of terminals. The filter of Fig. 5a comprises all of the elements of the filter -of Fig. 4a. and includes in addition a filter section comprising two filter half-sections, each identical with the half-section of the filter of Fig. 2a. Circuit elements which are similar to those. of Fig. 2a. and Fig. 4a have identical reference numerals. The filter section com-prisingg the two half-sections similar to that of Fig. 2a is included between one of the end filter half-sections including. elements L21, L22, C20 and the adjacent half-section including a trap circuit which is resonant at a frequency above the pass band. The operation of the circuit of Fig. 50. will be understood from the description which has been given with reference to the individual filter half-sections which form component parts thereof, rendering a further description unnecessary herein. As stated above, the image impedance at the terminals 3, 3 of Fig, 2a is of the same form as that at terminals 2, 2 of Fig. 1b and the terminals 9, 9 of Fig. 3b, so that these filter sections canbe connected in cascade with image-impedance matching at their junctions.

It is, therefore, seen that the filter circuit of Fig. 5a is a confluent band-pass filter effectively comprising a series of filter half-sections with image-impedance'matching at their junctions, each of two adjacent .ones of the half-sections comprising both seriesand shunt-reactance arms of which each of two of the four arms adjacent their junction includes a portion individually of the same reactance-frequency characteristic and of a given kind of reactance at frequencies in the pass band; specifically, the said two halfsections include inductances L15, L14 and L5, and L6, each having an inductive reactance at frequencies in the pass band. A trap circuit is included in each of the two adjacent half-sections under discussion which is removed from their junction by at least one of the above-mentioned adjacent arms-and comprises the only other reactance of such half-section. One of the trap circuits comprises a shunt 'arm of seriesconnected capacitance C16 and inductance L16 resonant at a frequencyion one side of the pass band and the other trap-circuit comprises a series arm having parallel-connected capacitance C7 and inductance L2 resonant at a frequency on the other side of the-pass band. 1

In Fig. 5b is showna' modification of the filter of Fig.-5a-in which'single-trap circuits are substituted for-the two adjacent-trap circuits of each type in the filter 'of Fig. 5a, adjacent inductance elements of similar kind being combined in a single inductanceeleme'nt and" impedance transformation being effected between like elements in adjacent arms of opposite type. The circuit of Fig. 5b-thus comprises a single trap circuit L36, C36 resonant at a frequency above the pass band a single trap circuitLa's; C39 resonant at a frequency below the pass band, shunt-inductance elements Lu and L40, and series-inductance elements Laaand L38,

The circuit of Fig. 5c is identical to that of Fig. 5b except that'an equivalent transformer L42, L114 has been substitutedin a manner well understood in the art for the adjacent inductance elements L31, L 8 of Fig. 5b.

The filter circuit of Fig. 5d is also the .eleccircuits are provided in trical equivalentlof thefilter circuit of Fig. a and is derived from the circuit of Fig. 5a by combining adjacent inductance elements, combining each of the two pairs of trap circuits into a single pairof trap circuits, and effecting an impedance transformation. The filter of Fig.'5d thus comprises a single trap circuit L51, C51 resonant at a frequency above the pass band, a single trap circuit L53,- C53 resonant at a frequency below the pass band, series-inductance elements L56 and L57, and shunt-inductance elements L50 and Len The filter circuit of Fig. 5e is similar to that of Fig. 5d except that an equivalent transformer L60, L61 has been. substituted for the adjacent inductance elements 1150,1156 of Fig. 5d and a second transformer L64, Lesha's been substituted for the adjacent inductance elements L54, L51 of Fig. 5d in a manner well understood in the art.

It is apparent that the band-pass filter of Fig. 5e comprises two resonant circuits L60, C20 and L65, 020 having inductors L61 and L64 individually coupled therewith by mutual inductance. The filter comprises a series circuit including the inductors'Lsi, L64 and a trap 'circuit with parallelconnected capacitance C53 and inductance L53 resonant at a frequency below the pass band. The filter also'comprises a second trap circuit connectedin parallel with inductor L61 including series-connected capacitance'csi and inductance Lsiresonant at a frequency above the pass band,

whereby a single continuous band of frequencies is caused to be'passed by the filter and maximum attenuation is'secured at the resonant frequencies of the trap circuits above and below the pass band.

While therei'have been described what are at presentrconsidered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modificationssmay be made therein without departing from-the invention, and it is, therefore, aimed in the appended claims to cover all such changes and imodificationsas fall within the true spirit and scope of the invention.

' What is claimed is:

1. A confluent band-pass filter effectively comprising, a series of filter half-sections with imageimpedance matching at their junctions, each of two 'adjacent'ones of said half-'sectionshaving both'seriesand shunt-reactance arms of which 'each'of two of said arms adjacent their junction includes aportion individually of the same reactance-frequency characteristic and of a given kindof reactance at frequencies within the pass band,"a' trap circuit included in each of said two adjacent'half-sections and removed from said junction by at least one'of 'said adjacent reactance arms and comprising the only other reactance of the half-section, one of said trap circuits comprising a shunt arm of series-connected capacitance and inductance reson'ant'at a frequencyonone side of the pass band, 'and the *otherof said trap circuits comprising a series arm having parallel-connected capacitance and inductance resonant at a frequency on the other side of the pass band, whereby the matched image impedance at said junction is caused to have opposite extreme values at the upper and lower cutoff frequencies of the pass band and unlike trap the said two adjacent half-sections.

2.A confluent band-pass filter effectively com- "prising, a series of filter half-sections with imageimpedancematching at their junctions, each of ,two adjacent ones of said half-sections havin both seriesand shunt-reactance arms of which each of two of said arms adjacent their junction includes a portion individually of the same reactance-frequency characteristic'and of inductive reactance at frequencies in the pass band, a trap circuit included in each of said two adjacent halfsections and removed from said junction by at least one of said adjacent reactance arms and comprising the only other reactance of the halfsection, one of said trap circuits comprising a shunt arm of series-connected capacitance and inductance resonant at a frequency above the pass band, and the other of said trap circuits comprising a series arm having parallel-connected capacitance and inductance resonant at a frequency below the pass band, whereby the matched image impedance at said junction is caused to have a maximum value at the lower cutoff frequency and a minimum value at the upper cutoff frequency of the pass band and unlike trap circuits are provided in the said two adjacent half-sections.

3. A band-pass filter half-section having input and output pairs of terminals and effectively comprising, seriesand shunt-reactance arms of which each of at least two includes a portion individually of the same reactance-frequency characteristic. and of a given kind of reactance at frequencies within the'pass band, and a trap circuit included in' one of said reactance arms which is adjacent to one of said'pairs ofterminals and which comprises the only other reactanceof said half-section, said trap circuit being resonant at a frequency outside of the pass band and having "in one of said reactance arms which is adjacent to one of said pairs of terminals and comprising the only other reactance of said half-section, said trap circuit being resonant at a frequency outside of the pass band and having capacitive reactance at frequencies within the'pass band,whereby the .image'impedance at the other of said pairs of terminals is causedto'have a maximum value at the lower -cut-off frequency and aminimum value at the upper cutoff frequency of the pass band.

5.A- band-pass filter-'half-section having input and output pairs of terminals and effectively comprising, seriesand shunt-reactance arms of whicheach of at least two includes a. portion individually having-aninductive reactance at frequencies' within the pass band, and a trap circuit comprising aseries-connected inductance and 'capacitance' resonant "at a frequency above the band and having capacitive'reactance at frequencies within the pass band. said trap circuit being included in one of said shunt arms which is ada max mum value "at'the lower cutoff frequency and a minimum value at'the upper cutoff fre quency of th'ep'ass band.

6. A band-pass filter half-section having input and output pairs of terminals and effectively comprising, seriesand shunt-inductance arms of which one is adjacent to one of said pairs of terminals, and a shunt-reactance arm across the other of said pairs of terminals consisting of a trap circuit comprising a series-connected inductance and capacitance resonant at a frequency above the band, whereby the image impedance at said one of said pairs of terminals is caused to have a maximum value at the lower cutoff frequency and a minimum value at the upper cutoff frequency of the pass band.

'7. A band-pass filter section having input and output pairs of terminals and effectively comprising, two shunt arms each comprising a single inductance connected across a different one of said pairs of terminals, two series arms each including a single inductance, and a shunt arm between said series arms consisting of a trap circuit comprising series-connected inductance and capacitance resonant at a frequency above the pass band, said trap circuit presenting capacitive reactance at frequencies within the ass band.

8. A band-pass filter half-section having input and output pairs of terminals and effectively comprising, seriesand shunt-reactance arms of which each of at least two includes a portion individually having an inductive reactance at frequencies in the pass 'band, and a trap circuit comprising parallel-connected inductance and capacitance resonant at a frequency below said pass band and having capacitive reactance at frequencies within the pass band, said trap circuit being included in one of said series-reactance arms which is adjacent to one of said pairs of terminals and comprising the only other reactance of said'half-section, whereby the image impedance at the other of said pairs of terminals is caused to have a maximum value at the lower cutoff frequency and a minimum value at the upper cutoff frequency of the pass band.

9. A band-pass filter half-section having input and output pairs of terminals and effectively comprising, seriesand shunt-reactance arms of which each of at least two includes an inductance, and a trap circuit included in one of said quencies in the pass band, whereby the image impedance at the other of said pairs of terminals is caused to have a maximum value at the lower cutoff frequency and a minimum value at the upper cutoff frequency of the pass band.

10. A band-pass filter section having input and output pairs of terminals and effectively comprising, two shunt-reactance arms each including a single inductance and connected across a different one of said pairs of terminals, a series-reactance arm between said shunt arms including in series an inductance and a trap circuit having parallel-connected inductance and capacitance which is resonant at a frequency belowthe pass band and which has capacitive reactance at frequencies within the pass band.

11. A band-pass filter half-section having inputand output pairs'of terminals and effectively comprising, a series arm including inductance, a shunt arm including inductance, and a trap circuit included in one of said arms and comprising the only other reactance of said half-section, said trap circuit being resonant at a frequency outside of the pass band and having capacitive reactance at frequencies within the pass band, whereby the image impedance at the pair of said terminals furthest removed from the 'arm including said trap circuit is caused to have a maximum value at the lower cutoff frequency and a minimum value at the upper cutoff frequency of the pass band.

12'. A band-pass filter section having input and output pairs of terminals and effectively comprising, seriesand shunt-reactance arms of which each of at leastthree includes a portion individually of the same reactance-frequency characteristic and of a :given kind of reactance at frequencies'within the pass band, and a trap circuit included in'one of said reactance arms which is between two of said three'reactance arms and comprising the only other reactance of said filter section, said trap circuit being resonant at a frequency outside of the pass bandand having reactance of the opposite kind at frequencies in the pass band, whereby the image impedance at both said pairs of terminals is caused to have opposite extreme values at the respective cutoff frequencies of th'e'pass band.

13. A band-pass filter section having input and output pairs of terminals and comprising, seriesand shunt-reactance arms of .which'each of at least three includes a portioniindividually having inductive reactance at frequencies within the pass band, anda trap 'circuit'included'in one of said reactance arms which is between two of said three reactance arms and comprising the only other reactance of said 'filter section, said trap circuit being resonant at a frequency outside of the pass band and having'capacitive reactance at frequencies within the pass band, whereby the image impedance at both said pairs of terminals is caused to have a maximum value at the lower cutoff frequency and 'a minimum value at the upper cutoff frequency of the pass band.

14. A band-passfilter. section having input and output pairs of terminals and effectively comprising, seriesand shunt reac'tance arms of which each of at least three'in'cludes a portion individually-having inductive reactance at frequencies within the pass band, and a trap circuit comprising a series-connected inductance and capacitance included in one 'of said shuntreactance arms which is between two of said three reactance arms and comprising the only other reactance of said filtersection, said trap circuit being resonant at 'a'frequency above the pass band and having capacitive reactance at frequencies within the'pass band, whereby the image impedance at both said pairs of terminals is caused to have a maximum value at the lower cutoff frequency and a minimurn value at the upper cutoff frequency of the pass band."

15. A band-pass'filter section having input and output pairs of terminals and effectively comprising, seriesand shunt-reactance arms of which each of at least three includes a portion individually having inductive reactance at frequencies within the pass band, and a trap circuit including a parallel-connected inductance and capacitance included in one of said series arms whichis between two of said three reactance arms and comprising the only other reactance of said filter section, said trap circuit being resonant at a frequency below the pass band and having capacitive reactance at frequencies within the pass band, whereby the image impedance at both said pairs of terminals is caused to have a maximum value at the lower cutoff frequency and a minimum value at the upper cuto-fi frequency of the pass band.

16. A band-pass filter comprising, two resonant circuits each having an inductor coupled therewith by mutual inductance, a series circuit including said inductors and a trap circuit with parallel-connected capacitance and inductance resonant at a frequency below the pass band, and a second trap circuit connected in parallel with one of said inductors including series-connected capacitance and inductance resonant at a frequency above the pass band, whereby a continuous band of frequencies is passed by said filter and maximum attenuation is secured at frequencies above and below the pass band.

1'7. A confluent band-pass filter comprising, two resonant circuits each having an inductor coupled therewith by mutual inductance, a series circuit including said inductors and a trap circuit with parallel-connected capacitance and inductance resonant at a frequency below the pass band, and a second trap circuit connected in parallel with one of said inductors including series-connected capacitance and inductance resonant at a frequency above the pass band, the reactive constants of the filter being proportioned to pass a single continuous band and to provide a band-pass constant-k form of image impedance at each of said two resonant circuits.

18. A confluent band-pass filter comprising, two parallel-resonant circuits each having an inductor coupled therewith by mutual inductance, a series circuit including said inductors and a trap circuit with parallel-connected capacitance and inductance resonant at a frequency below the pass band, and a second trap circuit connected in parallel with one of said inductors including series-connected capacitance and inductance resonant at a frequency above the pass band, the reactive constants of the filter being proportioned to pass a single continuous band and to provide a band-pass constant-k mid-shunt form of image impedance across each of said two resonant circuits.

19. A band-pass filter comprising, a plurality of seriesand shunt-reactance arms, one said shunt arm including a trap circuit with seriesconnected inductance and capacitance resonant at a frequency on one side of the pass band, and an adjacent series arm including a trap circuit with parallel-connected inductance and capacitance resonant at a frequency on the other side of the pass band, whereby said trap circuits are caused to present reactance of opposite kind at frequencies within the pass band and to give maximum attenuation at frequencies above and below the pass band.

HAROLD A. WHEELER.

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