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MONOLITHIC CRYSTAL BANDPASS FILTER

IP.com Disclosure Number: IPCOM000005412D
Original Publication Date: 1980-Aug-01
Included in the Prior Art Database: 2001-Oct-10
Document File: 2 page(s) / 89K

Publishing Venue

Motorola

Related People

Bob Kinsman: AUTHOR [+2]

Abstract

Crystal filters with tighty controlled parameters are needed for SSB radios and telephone carrier modems. Theoretically, the desired response characteristics can be otained most efficiently with elliptic function type filters. However, in the past it has been difficult to achieve a practical realization of this type design using conventional two-pole monolithic crystals. By utilizing the design technique described hereinbelow, a filter with the desired response characteristics can be obtained by cascading conven- tional two-pole monolithic crystals.

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MOTOROLA Technical Disclosure Bulletin Vol. 1 No. 1 August 1980

MONOLITHIC CRYSTAL BANDPASS FILTER By Bob Kinsman and Frank D'Alexander

   Crystal filters with tighty controlled parameters are needed for SSB radios and telephone carrier modems. Theoretically, the desired response characteristics can be otained most efficiently with elliptic function type filters. However, in the past it has been difficult to achieve a practical realization of this type design using conventional two-pole monolithic crystals. By utilizing the design technique described hereinbelow, a filter with the desired response characteristics can be obtained by cascading conven- tional two-pole monolithic crystals.

   The realization of the filter can be best illustrated by the following step-by-step design process. First, a conventional ladder filter network is designed with the desired bandwidth and passband ripple. Such a network utilizing two-pole resonators is illustrated in Figure 1A for a multiple pole filter. All series resonators have equal impedance, and each pair of coupled resonators is connected to the adjacent pair with a"T" inverter network.

Figure la.

  Attenuation peaks can be generated by shunting each coupled two-pole resonator with a capacitor. However, this will cause severe assymmetry in the passband characteristic. To maintain symmetry, the two-pole resonator must be shunted by an L-C pi-network as shown in Figure 1B. The addition of these networks will change the passband characteristics slightly? but this can be corrected by a small change in the impedance level of the "T" networks and the terminatmg impedance.

Figure lb.

   Finally, to convert the circuit to a more practical from the '7" networks are replaced with equivalent pi-networks as shown in Figure 1C. The incorporation of the two-pole monolithic crystal element into the netw...