Patent GB-268030-A (GB)
Title: Improvements relating to electrical wave filters
IPC: H03H7/01
Filed: 1925-12-23
Abstract: 268,030. Standard Telephones & Cables, Ltd., (Western Electric Co., Inc.). Dec. 23, 1925. Filter circuits; wired wireless systems.-The impedance of a filter circuit is matched to the load impedance at one end for a frequency near to one of the cut-off frequencies only; at the other end a similar match may be made for a frequency near the other cut-off. The device may be applied to connect an ordinary telephone line, terminating in a two-way - translating system, with a power line conveying carrier signalling currents. The Specification gives an outline of Zobel's theory of image parameters, and applies it to elucidate the invention. In the system shown in Fig. 1, which is of the kind described in Specification 241,368, an ordinary telephone line 1 terminates in a translating system 2 comprising bridge-coils 5 and a line balance 6, with a modulator 7 and band pass filter 8 for working from left to right, and a band filter 10 and detector 11 for working from right to left. The translator has a high impedance as measured at the terminals 9, while the power line 4 has a low impedance, both being approximately independent of frequency over the range considered. The two are connected by a filter 3, of the type described in Specification 142,115, the series impedances being resonant and the shunt impedances anti-resonant to the same frequency. The filter comprises a complete # section on the left and a mid-series half section on the right, the latter including high-voltage condensers 12 ; the invention makes it practicable for these condensers to be substantially smaller than those otherwise required. Fig. 3 shows the image impedance of the filter 3, as measured at the terminals 16, 16, plotted as ordinate against the frequency as abscissa, the unbroken line A showing the resistive impedance within the transmission band. The " image impedance " at 16, 16, is the geometric mean of the impedances which obtain respectively when the terminals 9, 9, are open circuited or short-circuited, and is equal to the external or load impedance which fails to cause reflection. The maximum impedance or " nominal impedance " PR occurs at the abscissa OP which is the geometric mean of the cut-off frequencies t1, t2 ; but instead of arranging, as is usual, that the load impedance (viz. that of the line 4) shall be equal to this nominal impedance, the invention provides that this load impedance, DD<1>, shall be equal to the image impedance of the filter at two frequencies near the cut-off frequencies, as shown at e, e. The image impedance of the filter 3 as measured at the points 9, 9, is different from that measured at 16, 16, owing to the asymmetry of the filter; it is shown in Fig. 4, where GG<1> indicates the impedance of the translator 2 as measured at 9, 9, and the intersections h, h, indicate that the impedances are matched to avoid reflection at two frequencies near the cut-offs. The effect is illustrated in Fig. 5, in which frequency as abscissa is plotted against the " transmission characteristic " as ordinate, the latter quantity being a function which indicates the choking effect of the filter. The dotted curve 14 relates to a fil. ter whose impedance matches the end impedances at the mean frequency of the band, while the full line 15 relates to a filter which is constructed according to the invention and has a minimum choking effect in two regions near the cut-off frequencies t1, t2. Curve 13 represents the " transfer constant " of the filter, a quantity depending on the ratio of input to output of current. In modifications, the load impedances may be equal and the filter may be terminated mid-series at both ends, in which case Fig. 3 applies to both ends; or, the end impedances being unequal, the filter may be matched to the smaller impedance near the cut-off frequencies and to the larger impedance near the mean frequency : this arrangement gives a fairly uniform value to the reflection losses throughout the frequency band. In all cases the "nominal impedance " of the filter, viz. its impedance at the geometric mean of the cut-off frequencies, differs greatly from one or both of the load impedances.