GB516422A - Improvements in or relating to electrical filters - Google Patents

Abstract

516,422. Impedance networks; valve amplifying circuits. MARCONI'S WIRELESS TELEGRAPH CO.; Ltd., and RUST, N. M. April 28, 1938, Nos. 12771/38 and 30614/39. Addition to 474,387. [Classes 40 (iii) and 40 (v)] In a filter with adjustable band-width for superheterodyne intermediate frequencies and like purposes, tuning-condensers IT, OT, are shunted across inductances which are coupled inductively at M and also through a crystal X. The coils coupled at M, Fig. 1, may be replaced by a single coil, Fig. 2 (not shown), which may be in series with a resistance, Fig. 3 (not shown), or a resistance may be inserted in series with the coupled-coils M, Fig. 4 (not shown). The crystal X is shunted by an adjustable condenser VC and may be connected between the upper ends of the inductances or between tapping- points TI, TO. The capacities IT, OT may be ganged so that one increases while the other decreases, and the tappings TI, TO also may be ganged. The transmission characteristic may be varied as follows : (i) the slope of the skirts sk1, sk2 near the cut-off frequencies f1,f2 may be varied by simultaneous adjustment of the parts M, VC, Fig. 5 ; (ii) the cut-off frequencies f1, f2 may be shifted in one direction to produce asymmetrical skirts sk1, sk2, the flat top t remaining stationary, by altering either the inductive coupling M or the capacity VC, Fig. 6 (not shown) ; (iii) the top t may be narrowed, the cut-off frequencies f<1>, f2 remaining constant while the skirts sk1, sk2 are deformed, by adjustment of the tappings TI, TO or of the condensers IT, OT, Fig. 7 (not shown) ; (iv) the band-width may be changed, the shape of the skirts remaining unaltered, by simultaneous adjustment of the parts M, VC, and TI, TO or IT, OT. The specification gives calculations, taking account of the resistance inherent in the inductances, relating the arms of a pi-section, Figs. 9, 10 (not shown) equivalent to the network under the invention. In a modification, Fig. 11, the need for a condenser VC shunting the crystal is eliminated, and difficulties arising out of the setting of the coupling M to neutralize the capacity of the crystal-electrodes are avoided, by the use of a coupling condenser C instead of the mutual coupling M. In further modifications, the coupling M may be shunted by a condenser Fig. 13, (not shown), or replaced by a single anti-resonant or resonant circuit, Figs. 14, 15 (not shown). The effective impedance may be raised by shunting negative impedance upon the input and output terminals. Valve circuits. An intervalve coupling in the intermediate-frequency amplifier of a superheterodyne receiver, constructed as described above, may be provided with magnetic, resistance-capacity or other reaction coupling to sustain the attenuation outside the pass band and the impedance inside it.