US1843892A - Band filter - Google Patents

Description

M. VOS ET AL FebJ2, 1932.

BAND FILTER w11ea Feb. 14, 1929 3 SheetsSheet 1 Feb. 2, 1932. M. vos ET AL 1.843,892

BAND FILTER Filed Feb.*l4, 1929 5 Sheets S heet 2 Feb. 2, 1932. M. vos ET AL 3, 9

BAND FILTER Filed Feb. 14,. 1929 3 Sheets-Sheet 3 Patentecl Feb. 2, 1932 UNITED STATES PATENT-FOFFICE MAURIIZ vos ND VIIKAI\T Knnn AGUST STERKY, or STOKHOLM, SWEDEN, AS-

smnons T TELEFONAKTIEBLAGE L, M. nRmsso1v, on s rocxnorn, svvnnnn,- A

" BAND FILTER Applicatio filed February 14, 1929, Serial No. 339,946, and in Sweden '1"ebri1ary 27, 1928.

The present invention relates to s0 called band filters which are used in telephone and high frequencysysterhs to sejoarate oscillations of difierent frequencies. or difierent bai1ds 0f frequencies. Especially in connection 'With multiple telephon or telegraphy,

it often occurs that two or sveral band filters of difierent bands of frequenbies have to be connected in parallel oi in series in relation to eachbther either on the input sideS or on the output sides. In'hithrto known simple bahd filters i. e. those consisting ofa single section the characteristics of the two sides (i.-e. the input and the output characteristic) are similar so far as they are both falling or both rising on each side of the band of' frequen oies of the filter. In the former case the filter is not suitable forbeing connected in parallel because the charactefistic of each of the filters has 10W values for oscillations within the band of requenciesof the other filters the difit'erent filters connected in parallel thus forming short circuits for each other. Filters of the latter kind in which the characteriStic is rismg on both sides of the band of frequericies are, on the other hand, not suitable for being connected in series becausein this case each of the filters connected in series forms a very high impedance for oscillations within the band of frequencies of the other filters. The object 'of the present invention is to bring about a simple band filter, which may be connected advantageousl both in parallel and in series. The object of the invention is further to produce an efiicacious filter of a design so simple asypossible and to combine in such a simple filter all those qualities which are desirable in a bandfilter and which otherwise can be*obtained only by comparatively complicated devices. The invention Will be more closely de scribed with reference to the attache-d drawings in which Figure 1 shows an embodiment of a single section filter. Figures 2 and 3 are diagrams showing the variations of the input and output characteristics of the filter according to Figure 1, as a unction of the frequency. Figure 4 shows the connection of the filter accorcling to Figurl to the xnode.side of an electron tube and Figure 5 the connection of said filter to the grid Side of an electron tube. Figure 6 is a diagra n showing the damping as a function of the frequency in the filteraccording to Figure 1. Figures 7 and 8 show diflzerent connections in cascadeor in chain et sections according to Figurel. Figures9 and.l0 show modifications ofthe single section filter according to Figure 1. Figures 11 and 12 show connections in series or in parallel respectively of filtersaccording to the.inventioh[ The single section filter according to Figure lis composed of two induetively coupled inductances:Lf and L2 and a condenser C connectd in parallel with L and a condenser C connected in series with L The input terminals 1 of the filter are formed by the terminals of theinductance L Which are connectedto the condenser C The free end of the inductance L and the outer p.0le of the condenser C form the output termirials 2 of the filter. By suitably selecting the ratio et transformation and the coupling coefficient 76 and by a corresponding dimensioning of inductances and capacities the filter may.be adjusted to suit two impedancesZ and Z (limes or apparatus) connected to the input 7 and output terminals' respectively in;such a manner that oscillations within the desired band of frequencies pass the filter without reflectionlosses. The impedances Z and Z which may be of difi'erent values in relation to each other may be assumed to. have, at least approximately, purely realvalues o:t inupedance ithin the band of frequencies and especially for the geometric mean value of the twojlimiting. frequencieS f f of the filter. Provicled the filteris thus adjusted t0 predetermined values of:Z and Z,at the geometric -mean frequency betweenf and f and provided the losses in coils and condensers are negligible we obtain accorcling to the general theory of the ideal artificial line With two input and two output terminale or the so called transducer (see for instance Breisig, Theoretische Telegraphie, 2nd edition, page 401) the following formulas for a suitable dimensioning of the filter according to Figure 1.

An arrangement of the filter as described presents the following advantages:

A. As can be readily understood from the diagrams in Figures 2 and 3 which show the input and output characteristics of the filter drawn accordingto a symbolical method set forth by Zobel among others (The Bell System Technical Journal for J anuary 1925, page 62) the filter may suitably be connected in series with other similar filters (having, hoWever, difierent bands) on the input side 11 whereas the output side 22 may suitably be connected in parallel.

B. A filter section according to the invention has an impedance both on the input and the output side Which is practically constant over the greater portion of the band of frequencies.

C. On account of the transformation within the filter said filter may be connected without reflection losses to two impedances of different sizes. The band width is then as can be seen by the third formula not dependent upon the ratio of transformation of the transformer and determined solely by the coupling coefficient 7).

D. When connecting the filter section into the anode circuit of an amplifying tube according to Figure 4: the anode may be directly fed by continuons Current over the coil L without introducing a special choke coilin the supply lead and a blocking condenser or other coupling elements,

E. W'hen connecting the filter section into the grid circuit in an audion tube it is usually desirable to dimension the side of the filter facing the gridfor a high impedance i. e. to design the filter with a high ratio of transformation. The maximum attainable transformation ratio is then, however, limited by that the internal capacity of the inductance coil L apparently reduces the value of L In the present filter one may, however, as-

sume that both the internal coil capacity and the grid circuit capacity are inclucled in 0 the filter thus functioning independently of otherwise disturbing internal coil capacity.

F. The filter section according to the inimpedance diagrams represent purely imag- 'inary values of the impedances (pure inductances and capacitances) whereas the unbroken portion of the impedance diagrams extending between the limiting frequencies f f in each of the figures represents purely real values (ohmic resistances).

In order to render the filter symmetrical the condenser 0 connected in series with the inductance L (Figure 1) may be divided into two condensers each having the capacity 2C2 which are connected into circuit according to Figure 9. In this way another embodiment of the invention is realized.

It is also possible to unite a number of filter sections according to the invention by means of cascade coupling resulting in circuit diagrams according to Figures 7 and 8. In such Cases When two coi1densers are connected into circuit in parallel or in series by means of said cascade coupling they may, of course, be combined to one condenser having the double or the half capacity respectively.

A1so other embodiments of the filter in addition to the above mentioned ones are conceivabl such as for instance the substitution of the transformer L L by its equivalent circuit diagram according to Figure 10. Thus:

filters connected in series ofiers thus only' an insignificant impedance for oscillations within the band of frequencies of the other filters.

The cohnection in parallel of several fil- H to Figure 12 on those sides where the series capacities are connected into circuit. The different filters ofler thus' according to Figure 3 high impedance values for other oscillations than those lying within each of the appertaining band of frequ&encies.

We claim: 7

1. A bond filter section consisting of two inductively coupled inductances (L L a 10 capacity (0 connected in parallel with the one inductance (L and a capacity (C connected in series with the other inductance (L the inductances and the capacities and the coupling coefiicient (la) between the in-- ductances being so dimensioned that substantially A and f being the limiting frequencies and Z Z the terminal impedances, one of the two characteristics of the filter then falling on both sides of the band of freqnencies and the other rising onboth sides of said baud. 2. A band filter as claimed in claim 1, connected to anapparatus with inner capacity, characterized by that said inner capacity is includecl in the parallel condenser of the filter. 3. A bond filter as claimed in claim 1, characterized in thzit two or several sections having diflerent band of frequencies are connectecl in series in relation to each other on those sides where their characteristics are falling on both sides of the corresponding band of frequencies whereas the opposite sicles are connected to difi'erent impedances. 4:. A band filter as claimecl in claim 1, characterized in that two or several sections having diffrent band of requencies are connected in parallel in relation to each other on those sicles where their charactertistics are rising on both sides of the corresponding band of frequencies whereas the opposite sicles are connected to difl?erent impedances.

In testimony whereof we aifiX our signaturcs.

MAIRITZ VOS. m HKAN KARL AUGUST STERKY.

Images