US2187805A - High impedance band pass filter - Google Patents

Description

- Jail 1940v v. D. LANDON 2,187,805

HIGH IMPEDANCE BAND r1155 FILTER Filed Nov. so, 1936 3nventor practically low resistance and capacity.

Patented Jan. 23, 1940 ,,.2,1zrz,scs I HIGH IMPEDANCE BANDQPASS FILTER Vernon D. Landon, Haddonfield, N J., assignor to Radio Corporation of America, a corporation of Delaware Application November 30,1936, Serial.No.113,4 60

I I 5 Claims.

- "The present invention relates to band pass filters of the high frequency type, and has for its object to provide a .band pass filter of the socalled M-derived shunt type which may have high input and output impedance whereby-it is I adapted to provide coupling means between high A high gain amplifierrutilizing a crystal filter.

101 is shown in the patent to Mason 1,967,249v :of July 24, 1934. It is a further object of the present invention to provide a high frequency amplifier of the type shown in Mason wherein the crystal I may be. eliminated without sacrificing desirable operation characteristics.- 1 z The cost of providing-a satisfactory'crystal filter is relativelyhigh in comparison to thecost ofproviding tuned circuits and it is, therefore,

a still'further object of the present invention to provide as a substitute for a crystal in a high frequency filter, a tuned circuit which may opband and at a much lower cost.

' the theoretical performance of an M-derived band pass filter without requiring inductances of im- The invention will, however, be better under. 39 stood from the followingdescription when considered in connection with the accompanying I drawing, and itsscope will be pointed out in the ppended claims.

In the drawing, v 1 is a h matic diagram f work embodying the'invention,

Fig. 2 is a schematic diagram of an equivalent filter network used in analyzing the performance- 5 5; 1- is of theband pass, type adapted. to provide A further object of the invention is to provide a, filter net- Referring to Fig. 1, a signal transmission cir' cuit represented by input leads 5 are connected, through an inputcircuit impedanceB which may be the plate impedance of a preceding tube,- to a filter network I having output leads 8. i The filter sharp attenuationzoneach side of the pass band and may be of the M-derived type with midshunt termination, asshownin Fig. 2 to which attention is now directed along with Fig. 1.

Broadly considered, the circuit of Fig. 1 comprisesa high impedance input terminal 9,,a high impedance output terminal l0 and a common ground terminal H with a T structure of capacitors C1, Cl-and C2 :connected between the three terminals, the latter connection including aresistor R2, hereinafter referred to.

The inductance elements of the filter network comprising inductances L1, L1 and L2 likewiseprovide a T structureconnected tothe same three terminals and a resonant circuit 23 coupled to the two inductors forming the arms of the T of inductors and aresistance R connected in the stem of the T of the inductors. I i

The filter of Fig. 1 embodies a circuit which is the equivalent in performance to that shown in Fig."' 2. The same reference numerals and reference characters are used in both figures to designate like parts, and the circuit of Fig. 2 will be vrecognizedas a band pass filter in which the circuits comprising LzCz, L3C3 form the rejector elements of the filter. The circuit of Fig. 2 would be: ideal if reactors having the properconstants could be designed. However, this can not practicallybe done, particularly in connection with the inductance L3, as design formulas call. for the inductance L3 to be very large to beresonated by 'capacityCa which should be relatively low. 1

When an inductance of the proper value is. constructed for use at La, it is found to have a distributed capacity larger than the capacity called for at C3. Hence, it can not be resonated by C3. Also, thelosses in LsCs and L202 should be extremely small for best operation.

'Ihesedisadvantages in a circuit in accordance with. Fig.2 have heretofore been overcome by utilizing a crystal in place of the circuit CsLz to provide sharp attenuation at each side of the pass band. However, in the circuit shown in Fig. l, the difficulties have also been overcome in that the requirementfor a large inductance of unduly low capacity is eliminated by replacing L303 of Fig. 2 by a tuned circuit LsCsLs coupled to the input and output circuit of the filter. In this case the tuned circuit is coupled tothe input and output inductances L1 and L1 of the filter as shown.

'Referrin'g'now to Figs. 3 to 6 inclusive, in addition to Figs. 1 and 2. and in which the same reference characters are used throughout, the

circuit of Fig. 2 can best be resolved into the circuit of Fig. 1 by progressive steps illustrated by the figures referred to. For example, it will be seen that the circuit C3113 of Fig. 2 may be provided as shown in the circuit of Fig. 6, since the circuit of Fig. 6 may be shown to be equivalent to the circuit shown in Fig. 5, in which the series resonant circuit CsLx of Fig. 2 or of Fig. 5 is replaced by a circuit inductively coupled to the input and output inductances L1 and L1 of Fig. 6.

Similarly, L2C2 can be replaced by inductive and capacitive couplings as in Fig. 2. This is illustrated in Fig. 4 which can be shown to be equivalent to Fig. 3 by performing a T to 1r transformation in accordance with the conversion formulas, examples of which are found in Transmission Circuits for Telephone Communications by K. S. Johnson, page 282, Fig. 27. The transformation is taken first on the T of the inductances and secondly on the T of the capacity.

If, as shown, Fig. 4 is equivalent to Fig. 3, and Fig. 5 is equivalent to Fig. 6, it follows that Fig. 1 is equivalent to Fig. 2. However, Fig. 1 has the advantage that L3 and L2 may have more reasonable constants.

In Fig. 2 the reactances L2C2, L3--C3 are assumed to be dissipationless. If losses are present, the effect is chiefly to reduce the attenuation at the two frequencies adjacent to the pass band where the attenuation is desired to be infinite.

In the circuit of Fig. 1 the presence of losses in the circuit LsCa has an effect analogous to losses in L303 in Fig. 2. However, resistance in series with L2 appears mathematically as a negative resistance in series with the series arm of the filter when a T to 1r transformation is applied to the T of inductances. Thus, while dissipation in LsCs decreases the peak attenuation, dissipation in L2 or C2 increases the attenuation. A resistor may be inserted in series with L2 and adjusted to give infinite attenuation at the two attenuation peaks f1 and f2. This resistance may be inserted wholely in series with L2 in Fig. 1 as shown at R or may be included also in circuit with C2 as shown at R2.

The attenuation characteristic takes the form of the curve A of Fig. 8. The double attenuation peaks at frequencies f1 and f2 are provided by the combination of series resonant and parallel resonant circuits, L2C2, L3C3 of Fig. 2, which are tuned to the same frequency f. The pass band of the filter is effective between the frequencies is and f4.

It has been shown in the foregoing description that the circuit of Fig. l is equivalent to the circuit of Fig. 2. The latter is a well known M-derived filter, as may be seen with reference to page 255, Fig. 141, of Transmission Networks and Wave Filters, by T. E. Shea, published by D. Van Nostrand Company in 1929.

The circuit parameters of Fig. 1 are obtained by the method used in showing the equivalence, for example, of C1, C1, C2 of Fig. 1, which are obtained from the values of the same elements of Fig. 2 by well known formulas relating to T and 1r circuits given in the work by Johnson, hereinbefore referred to.

The value of R in Fig. 1 to use for peak attenuation is, in general, found by experiment. However, the value is adjusted for peak attenuation at f1, f2. This value may be solved for by the following:

where R2 is the total resistance of the link circuit.

From the above reference in Johnson, the series arm of the Ir network is, in Johnsons terminology:

(92L12 w L +R The non-reactive term is seen to be negative. If the band is narrow so that (in Fig. 1) L1 is much greater than L2, then the last term of the above may be neglected for a first approximation. Then (except for the negative sign) the power factor of the series arm of the 1r (L2 of Fig. 2) is the same as that of the stem of the T (L2 of Fig. 1). If this power factor is the same as that of L3 C; then the branch comprising L3 C3, L2 C2 (of Fig. 2) presents infinite impedance at frequencies f1 and f2.

Then

In regard to the relation of the L1C1, L202, L302, Fig. 2, combinations to the pass band and attenuation peaks, these may have various values, as discussed in Shea above referred to, where the circuit parameters are evaluated on Fig. 141 in terms of the equivalent constant K filter.

Referring to the circuit of Fig. '7, the use of the filter network as a coupling impedance between a high impedance high gain amplifier tube !2 and a second amplifier tube 13 is shown. The ground lead l4 forms the common ground lead for the amplifiers, and the output circuit I5 of theamplifler I2 is coupled to the impedance coupling means I8, I I with the high impedance input terminal I 8 of the filter network 19. The high potential output terminal of the filter network indicated at 2D is connected through a similar impedance coupling means 2|, 22 with the input electrode 23 of the amplifier I3. The signals-amplified by the device l2 are applied to the amplifier device l3 subject to the band pass characteristic of the filter and to the attenuation characteristic thereof. This filter network corresponds in form to that shown in Fig. 2 but indicates a practical coupling network for two amplifier tubes providing high impedance output and input circuits which are to be coupled.

In form, the filter network shown is a 1r network of resonant elements, each element being a parallel resonant circuit tuned to the mean pass band with a fourth circuit comprising the elements L3, C3, L3, also resonant to the mean pass band frequency and inductively coupled to the two shunt elements L1 and L1 of the 1r structure. The resistor R is common to the inductive branches L1 and L1 of the input and output circuits. Thus, as in Fig. 1, means are provided in the filter network for effecting the theoretical performance of a dissipationless M-derived filter.

In this circuit the 71' form of filter network is retained for the reactances L1L2L1 and the caatfthe'rejection frequencies.

pacity C1Ciz Ci, to show an operable variationof equivalent performance to that of Fig. '1. As in Fig. 1 the resistance R'in series with L202 is given such a value that infinite attenuation is obtained LZ'V'YIIII the clrcuit 'of 2, if LcCa=LzCz=LiCn each pair of elements being resonant at a midband, frequency, atsome lower frequency LcCs being a series circuit has a capacitive reactance while L2C2, being a parallel circuit, has an inductive reactance. Thus, at a certain frequency below the pass band of the filter network the combination acts as a parallelresonant circuit. A similar action takes place above the transformation band and the impedance developed by the combination of these two frequencies is inversely proportional to the resistance of the elements. This is likewise true of the circuits of Figs. 1 and 7. The attenuation peaks f1 and f2, one on each side of the desired transmission band, are provided by the combination L3--C3 and L2C2, acting like a parallel resonant circuit at each of the attenuation frequencies.

It will be seen that the difiiculty in the way of duplicating the performance of an ideal filter because of the presence of resistance in each element of. the circuit may be overcome by the corresponding filter networks, as shown in Figs.

1 and 7, and may be rendered substantially as Fig. 2 is raised to an infinitely high value, giving high attenuation at the proper points when the negative resistance is adjusted to balance out the other resistance of the combination.

The circuit of Fig. 1 has been tested and found to give results which are highly satisfactory as a substitute for a crystal filter. The advantages over the crystal filter are found in the low cost of the elements of the filter and flexibility in the design thereof.

An embodiment of the invention, as shown in Fig. 1, may have the following constants:

f :175 kc. R :51 ohms f1:165 kc. L1=4300 microhenries f2=185 kc. L3=4300 microhenries f3=170 kc. L2=940 microhenries f4=180 kc.

The capacitors are adjusted for resonance at 1'75 kc.

The terminal resonance for the above constants may be considered to be 120,000 ohms. At f2 the best value of R was 62 ohms, and at f1 40 ohms. Therefore, a compromise value of 51 ohms was considered best to use. The gain of the circuit when used between high impedance circuits, as in Fig. 7, was approximately 50.

The resulting response curve was flat from 1'71 to 1'79 kc. and the attenuation on f2 and f1 was more than to 1, as indicated by the curve in Fig. 8.

The use of a tertiary circuit as in Fig. 1 has the advantage that the cost of the filter circuit may be reduced materially and permits more nearly the ideal circuit of Fig. 2 to be provided by reactance elements having practical physical and electrical characteristics.

'I claim as my. invention: 1. A band pass filter comprising, in combination, means providing input and output circuits therefor each resonant to 'a mean pass band frequency'and including an inductance element,

means including additional inductance elements resistor in circuit-with the inductance elements of the input and output circuits, the resistance of the resistor being so proportioned with respect to the impedance of said circuits that substantially infinite impedance is effected at two frequencies adjacent to and on opposite sides of" resonance.

2. In a band pass filter,'the combination of a high signal potential input terminal, a high signal potential output terminal, a common ground terminal, a T structure of capacitors connected between said terminals, a T structure of inductances connected between said'terminals, the electrical values of the elements of the two T- structures being such that an arm and the stem of the inductive T is resonant with an arm and the stem of the capacitive T at the mean pass band frequency, means providing a resonant circuit inductively coupled to the arms of the inductance T-structure, and a resistance connected in the stem of said last named T structure having a value to provide substantially infinite impedance at rejection frequencies adjacent to and above and below the mean pass band frequency.

3. A bandpass filter comprising, in combination a 1r.St1llCtllI'8 of resonant elements, each element comprising a parallel resonant circuit having inductive and capacitive branches and being tuned to a mean pass band frequency, means providing a fourth circuit resonant to the mean pass band frequency and inductively coupled to the two shunt arms of the 11' structure, and a resistor common to the inductive branches of said shunt elements of the 1r structure, the resistance of the resistor being so proportioned with respect to the impedance of said circuits that substantially infinite impedance is effected at two frequencies adjacent to and on opposite sides of resonance.

4. In an electric discharge amplifier, the combination of means providing a high impedance output circuit and means providing a high impedance input circuit, and a band pass filter comprising parallel resonant circuits connected in parallel relation to said output and input .circuits, said parallel resonant circuits forming the arms of a 1r structure and having inductive branches, a resistor common to the inductive branches of said parallel resonant circuits, a third parallel resonant'circuit forming the series arm of said 1r structure, and a fourth circuit inductively coupled to the inductive branches of said first named parallel resonant circuits, all of said circuits being tuned to a mean pass band frequency withinwhich the amplifier -operates.

5. A band pass filter providing substantially the performance of a dissipationless M-derived filter comprising, in combination, a T network of inductance elements and a T network of capacity elements providing two parallel T structures between high potential input and output terminals for said filter and a common ground terminal, the electrical values of the elements of the two T networks being such that an arm and the stem of the inductive T is resonant with an arm and the stem of the capacitive T at the mean pass band frequency, means for increasing the resistance in the stem of at least one of said T structures to a value to provide substan-

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