Patent GB-746236-A (GB)
Title: Improvements in or relating to electrical attenuation equalizers
IPC: H04B3/14
Filed: 1954-02-24
Abstract: 746,236. Attenuation equalizers. WESTERN ELECTRIC CO., Inc. Feb. 24, 1954 [March 16, .1953], No. 5442/54. Class 40 (8). An adjustable attenuation equalizer is made up of a series of equalizer units 1, 2 ... N connected in tandem each provided with two seriesconnected resistors of value Ro, where Ro is the image impedance of the equalizer, a shunt impedance branch and a bridging impedance branch. The bridging impedance branch comprises a @ resistor RA1 connected in parallel with the series resistors Ro and a shaping network SA1 which is in shunt with the resistor RA1 and is terminated by a fourth resistor of value k 1 R, where k 1  is a variable coefficient. The shunt impedance branch comprises a fifth resistor RB1 connected at one end to the common terminal of the series resistances Ro and a shaping network SB1 which is in series with the resistor RB1 and is terminated by a resistor R/K 1 . Each of the shaping networks SA1, SB1 has an image impedance approximately equal to R, which is made equal to Ro. The whole equalizer is similar in principle to known types of equalizers which have a number of tandem connected units having adjustable deviation characteristics which are harmonically related cosine curves and which correspond to the lower terms of a Fourier Series. The equalizer of the invention is, however, modified in that the shaping networks have non-linear phase shifts so that the matching points may be concentrated in the portion of the band where the best equalization is desired. The bridging impedance also includes a resistive pad PA1 and a dissipation corrector DA1. The shunt impedance includes a resistive pad PB1 and a dissipation corrector DB1. The networks SA1, DA1 are identical respectively with the networks SB1, DB1 and each has, at each end, an image impedance which is a substantially constant resistance equal to R throughout the band to be equalized, say from zero to f 1 . The network SA1 is an all-pass structure providing an equalizing characteristic in the form of a warped cosine curve between 0 degrees and 90 degrees, the phase-shift being non-linear. The Specification gives values of components of a suitable bridged T shaping network, Fig. 2, which provides a phase-shift characteristic having a decreasing positive slope which means that the lower frequencies are weighted. A similar network providing an increasing positive phase-shift slope is also discussed (Fig. 3, not shown). The function of the dissipation correctors DA1, DB1 is to compensate for the dissipation in the elements of networks SA1, SB1. They comprise unbalanced, bridged-T, constantresistance networks having series resistors of value R, Fig. 5, and shunt and bridging branches designed for the desired equalization. The terminating resistors K 1 R and R/K 1  have a unitary control so that as one is increased the other is decreased. Each covers the range zero to infinity. It is preferable to restrict the loss range in each of the units by the provision of the impedance transforming resistive pads PA1, PB1 which are in the form shown in Fig. 4. They are connected so that, for the bridging branch, the resistor RY shunts the resistor RA1 and, for the shunt branch, RX is in series with RB1. The inclusion of the pads reduces the flat loss of the section. They may, however, be omitted from the first unit or first few units where it is desirable to have the full loss range. The 2nd, 3rd, Nth, units provide 2nd, 3rd, Nth warped harmonic cosine characteristics. The network SA2 may comprise two tandem-connected networks identical with the network SA1; the shaping networks in the Nth unit comprising N networks identical with SA1