GB837275A - Improvements in or relating to electric wave filters - Google Patents

Abstract

837,275. Filter circuits. SIEMENS & HALSKE A.G. May 8, 1956 [May 9, 1955], No. 14285/56. Class 40 (8). One arm of a ladder-type filter having highpass, low-pass, band-stop or band-pass characteristics comprises a tuned element in the form of an electro-mechanical resonator (piezo-electric or magnetostrictive) and at least one further reactive element which gives the arm a secondary resonance, an adjacent arm having a tuned element which repeats this secondary resonance. In the band-pass filter shown the shunt arm contains a piezo-electric crystal element 1 in parallel with a parallel-resonant circuit 4 and the series arms contain parallel-resonant circuits 2 identical with the circuit 4. The attenuation peak produced by the crystal 1 may be arranged to determine the lower limit of the pass band whilst an adjacent attenuation peak at a frequency below that of the first peak is determined by the circuits 2, 4. The upper attenuation peak can be determined (or eliminated) by further parallel-resonant circuits 3 which may in some cases be simply inductors. Alternatively the attenuation peak produced by the crystal may determine the upper limit of the pass band in which case the tuned circuits 3 may be simply capacitors. In a further arrangement both the upper and lower limits are produced by crystal resonance in which case the circuits 2, 4 consist of crystal resonators. The filter may be designed so that the mid-shunt impedance of a half-section is given by the expression where Á is a constant, #2 and #1 are the limit frequencies, #α is the resonant frequency of the crystal filter and ## the resonant frequency of the tuned element in parallel with it. Where the circuit is in the form of a # network the shunt arms may include crystals resonant at frequencies which determine the limits of the filter band. Where the filter contains a number of sections it is not necessary for the resonant frequency of the third tuned elements to be exactly the same as the resonant frequency of the second tuned elements ; small deviations are permissible. The network may incorporate impedance transformation sections consisting of capacitors. Multi-electrode crystals may be used in some embodiments.