Patent GB-527496-A (GB)
Title: Improvements in or relating to automatically variable frequency-selective electric wave filters
IPC: H03G5/26
Filed: 1939-03-17
Abstract: 527,496. Impedance networks; wireless receiving apparatus. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., RUST, N. M., BRAILSFORD, J. D., OLIVER, A. L., and RAMSAY, J. F. March 17, 1939, No. 8609. Divided out of 522,476. [Classes 40 (iii) and 40 (v)] An automatically variable frequency selective filter comprises at least one element constituted by a potential variable condenser which may be of the kind described in Specifications 522,476 and 522,564, [Group XXXVI]. The filter may be used in a wireless receiver to produce automatic frequency control or so as to move the acceptance band away from an interfering signal or to narrow the band in the presence of interfering signals on either side of the band. Fig. 2 shows a filter coupling intermediate frequency amplifying valves 1, 2, which includes series shunt arms, including potential variable condensers VC<SP>1</SP>, VC<SP>11</SP>, resonant to the limiting frequencies of the band passed. Control of the band passed is effected by voltages applied to condensers VC<SP>1</SP>, VC<SP>11</SP> derived through filters FS<SP>1</SP>, FS<SP>11</SP> responsive to the limiting frequencies of the band and rectifier combinations D<SP>1</SP>, RE<SP>1</SP> and D<SP>11</SP>, RE<SP>11</SP>, opposite biases being superposed by resistance R<SP>11</SP> on the condensers VC<SP>1</SP>, VC<SP>11</SP>. In a modification, Fig. 4 (not shown), the shunt arms each comprise an inductance and a condenser in series with the common condenser terminal connected through a further series resonant circuit which includes a potential variable condenser, controlled from a discriminator so that the filter tuning accords with the intermediate frequency band fed to the amplifier In a further arrangement Fig. 5 (not shown) AVC voltage is applied to voltage variable condensers associated with an intermediate frequency transformer so as to oppositely detune the two windings with a strong signal. Alternatively a pair of tuned circuits Fig. 6, may have top and bottom capacity couplings C<SP>1</SP>, C" respectively associated with potential variable capacities VCI, VC<SP>11</SP> so that increase of the potential of one of the condensers will widen the band pass in one direction and of the other condenser in the opposite direction. This effect is enhanced if shunt inductance is associated with capacity C<SP>1</SP> and series inductance with capacity C<SP>11</SP>. Fig. 8 shows an intermediate frequency filter in which positive mutual coupling is provided between the coils L<SP>1</SP>1, L<SP>1</SP>2 and L<SP>11</SP>1, L<SP>11</SP>2 and rectifiers D<SP>1</SP>, D<SP>11</SP> are connected across condensers to provide the controlling potentials for the potential variable condensers VC<SP>1</SP>, VC<SP>11</SP>. Whistle in a wireless receiver may be eliminated by the provision of an acceptor circuit in a low frequency amplifier intervalve coupling Fig. 9 (not shown) with diodes in parallel with the inductance and capacity respectively and arranged to control the potential applied to a potential variable condenser included in the acceptor circuit.