Patent GB-531662-A (GB)
Title: Wave filter
IPC: H03H9/54
Filed: 1939-07-28
Abstract: 531,662. Impedance networks. STANDARD TELEPHONES & CABLES, Ltd. July 28, 1939, Nos. 21991, 21992 and 21993. Convention dates, July 28, 1938, Sept. 20, 1938, and Sept. 28, 1938. [Class 40 (iii)] In an unbalanced wave filter of the bridged-T type (which may degenerate into one of pi type), the series arms of the T comprise a single crystal having a split electrode on one or each side. Attenuation peaks may be located at any distance from the pass range, the number of such peaks may be increased by the addition of further crystals, and inherently high or low image impedances may be attained by the use of shunt or series terminal inductances. In the arrangement shown in Fig. 1, the series arms comprise the impedances between the part electrodes 5, 7 and 6, 8 respectively, together with shunt condensers C 1 . The bridging arm Z 1  and shunt arm Z 2  may have various forms, and the lower terminals 2, 4 may be earthed. The Specification gives the lattice network, Fig. 3 (not shown) equivalent to that shown in Fig. 1, and deduces the transmission characteristics of the filter. In a band-pass filter, Figs. 4 to 7 (not shown), the bridging arm Z 1  may be a capacitance while Z 2  is replaced by a direct connection ; the band-width can be increased by reducing the capacitance C 1  shunting the crystal, while the frequency of an attenuation peak below the pass band can be adjusted by varying the capacitance which forms the bridging arm Z 1 . The attenuation peak is located above the pass band if the poling of the part-electrodes 6, 8 is reversed, so that the electrodes shall be cross-connected, Figs. 8 to 11 (not shown). Attenuation peaks on both sides of the band can be obtained by connecting filters of the two types in cascade, Fig. 12 (not shown), their impedances being matched and their transmission bands identical. For a pass band of maximum width, the capacitances C 1  are omitted, so that the network consists of the crystal X with a bridging condenser at Z,. For a low-pass filter, Figs. 13 to 18 (not shown), the bridging arm Z 1  may be a parallel-tuned circuit while the shunt arm Z, is replaced by a direct connection. In a band-stop filter, with two attenuation peaks in the stop band, Figs. 19 to 22 (not shown), the bridging arm Z, is a parallel-resonant arm while the shunt arm Z 2  is an inductance. Fig. 23 shows a high-pass filter, in which the bridging arm Z, comprises a series resonant arm L 4 , C 9  which may be shunted by a capacitance C 10 , while the shunt arm Z 2  comprises an inductance ; and the former arm may be replaced by a crystal X, equivalent to it, Fig. 27. In another high-pass filter, Fig. 20, and Figs. 29 to 31 (not shown), the electrodes of the crystal X are reversely poled, the bridging arm Z, is a capacitance, and the shunt arm Z, is a series resonant circuit. In a band-pass filter with two attenuation peaks, Fig. 31 (and Figs. 32 to 35, not shown) the crystal X is crossconnected and is paralleled by a tee comprising a pair of inductances 4a which give the network a high-image impedance, in series with an adjustable resistance R 3 . The inductances La have a series-opposing mutual inductance. The bridging arm Z, is a capacitance which can be adjusted to vary the location of the peaks of attenuation ; and this adjustment can also be effected by adjusting the coupling of the inductances La. In the absence of such coupling one of the peaks is at zero frequency. The input and output terminals are shunted by capacitances C, which can be varied to vary the band-width. Instead of being cross-connected, the electrodes in Fig. 31 may be symmetrically connected as in Fig. 23 ; the circuit of Fig. 31 thus modified, Figs. 36, 37 (not shown) forms a band-pass filter with two attenuator peaks below the pass band ; the coupling between the inductances La may in this case be series-aiding or absent. The resistance R 5  may be replaced by a direct connection, while the capacity in the bridging arm Z, is shunted by an adjustable resistance, Figs. 30 to 41 (not shown) ; and such a shunt resistance may be used in the filters described above to compensate for dissipation in the inductances. A low image impedance is obtained by arranging inductances Lb, Fig. 42 (and Figs. 43 to 46, not shown), in series with the input and output terminals ; they may have a series-opposing mutual inductance, by decreasing which the frequency at which an attenuation peak occurs above the pass band may be raised; this peak is removed to infinite frequency when the mutual inductance falls to zero. If the crystal in Fig. 42 be cross-connected, Figs. 47, 48 (not shown), both attenuation peaks will lie above the pass band. Two crystals, X, X, Fig. 49 (and Figs. 50 to 52, not shown) may have their pairs of electrodes connected in parallel, one of the crystals having its electrodes cross connected as shown. Such a filter gives in general three attenuation peaks. To produce a high image impedance, the inductances La are connected in shunt instead of series with the input and output terminals, Figs. 53 to 56 (not shown).