Patent GB-865052-A (GB)
Title: Improvements in or relating to bandpass electric filters
IPC: H03H9/62
Filed: 1958-10-31
Abstract: 865,052. Electromechanical filters; filter circuits. GENERAL ELECTRIC CO. Ltd., and SNOW, A. Oct. 29, 1959 [Oct. 31, 1958], No. 35104/58. Class 40(8) A bandpass electric filter comprises an elongated member 1 of magnetostrictive material, e.g. a tube of nickel-zinc ferrite, a sleeve 2, e.g. of polytetrafluorethylene, and input and output coils 8 and 9, the member 1 being a loose fit in the sleeve 2 and being supported at least throughout the major part of its length by the sleeve. The sleeve 2 is a loose fit in a rigid tube 3 of insulating material, e.g. paper impregnated with a phenolic resin. Each end of the tube 3 is partially closed by a member 4 of insulating material and an annular member 5 of insulating material embraces each end of the tube 3. The members 4 and 5 can be of a phenolic resin and secured to the tube 3 by glueing. A disc 6 of polytetrafluorethylene is provided at each end of the tube 1, which is held in position by pressure applied to the disc 6 by nylon screws 7. The ferrite tube 1 is permanently magnetized so that the lines of magnetic force surround the longitudinal axis of the tube 1, an oscillating input signal causing the tube 1 to vibrate torsionally. The ferrite tube 1 is magnetized by passing a direct current through a conductor positioned in the axial hole 14 of the tube. The length of the tube 1 is so chosen that it acts as a half-wave resonator at a frequency in the region of the filter passband. The width of the passband can be adjusted by the screws 7 which change the pressure applied to the ends of the tube 1. A further coil 10, forming a single coil assembly with the input coil 8, is connected in series with the output coil 9 in such manner that it serves to buck out part of the signal induced in the output coil 9 by direct magnetic coupling with the coil 8-this forms the subject of Specification 865,053. The filter can be used to select component signals in a narrow band of frequencies present in an electric signal covering a wider band supplied by a radio receiver. In this case the spacing of the output coil 9 and the assembly of the input and further coils 8 and 10 is adjusted so that there is minimum coupling between the input and output coils at frequencies outside the desired frequencies, e.g. by applying a signal having a random waveform to the terminal wires 11 and adjusting the positions of the coils until there is a minimum noise signal developed at the terminal wires 12. The filter has a plurality of passbands each corresponding to a resonance of the tube 1. In order to prevent the filter passing signals at frequencies outside the desired passband it is preceded by a bandpass filter having a wide passband which contains the desired passband but not the unwanted passbands, e.g. a coilcapacitor filter. If a plurality of filters of the present invention are arranged to select from an electric signal component signals lying in contiguous bands of frequencies, they can all be supplied in parallel by the wideband filter. If the device, e.g. an amplifier, supplying the electric signal to the plurality of filters has the appropriate filter characteristic the wideband filter is unnecessary.