GB859535A - Intermediate band width crystal filter - Google Patents

Abstract

859,535. Crystal filter networks. HYCON EASTERN Inc. March 9, 1959, No. 8066/59. Class 40(8). A band-pass filter comprises two crystal resonators of different resonant frequency and effectively in parallel with each crystal a parallel combination of a capacitor and an inductor, the combination producing with the crystal an anti-resonant condition displaced from the crystal resonant frequency by an amount greater than the displacement therefrom of the crystal antiresonant frequency. In the lattice network of Fig. 3, the resonant frequencies (marked o on Fig. 4) of crystals A, B are arranged to define the lower half of the pass-band B o . The networks Lp, Cp resonate at frequencies slightly above the crystal resonant frequencies so that at these frequencies they appear as very high inductances. Thus the combinations of crystals and reactances produce antiresonant conditions at frequencies x which together define the upper half of the pass band B o . Each frequency x is greater than the anti-resonant frequency of the corresponding crystal. The networks Lp, Cp also produce antiresonances x defining a spurious pass-band B s , this being caused by the fact that the inductive reactance of each network L p , C p resonates with the lattice which, in the region of B s , has an equal and opposite reactance (see the equivalent circuit, Fig. 5). This condition may be eliminated by arranging across the output a network C a , L a . The values of these reactances are arranged to be equal and opposite at the centre of the pass-band B o so that they have no effect in this region. In the region B s , the network C a , L a has a relatively low inductance and the attenuation of the filter remains high throughout this region. In practice, the network C a , L a may be combined with the adjacent Lp, Cp network. By assigning slightly lower values to Lp, C p a network may be produced having the attenuation characteristic of Fig. 8 in which the positions of the poles and zeros of the pass-band are effectively reversed. Fig. 9 shows a hybridtransformer equivalent network. The values of the impedances are modified as indicated, Lp<SP>1</SP> and Cp<SP>1</SP> denoting values of reactances including both L p , L a and C p , C a .