POLYLITHIC CRYSTAL BANDPASS FILTER HAVING ATTENUATION POLE FREQUENCIES IN THE LOWER STOPBAND

• US 3,676,806 A
• Filed: 11/06/1969
• Issued: 07/11/1972
• Est. Priority Date: 11/06/1969
• Status: Expired due to Term

• Current Assignee: AG Communication Systems Corp. (Nokia Corporation)
• Original Assignee: Desmond F. Sheahan, Henry J. Orchard
• Inventors: Orchard, Henry J., Sheahan, Desmond F.
• Primary Examiner(s): Saalbach, Herman Karl
• Application Number:
  US04/874,522
• Time in Patent Office: 978 Days
• Field of Search: 333/70,76,72 310/8.2,8.4,9.4
• US Class Current: 333/187
• CPC Class Codes: H03H 9/60 Electric coupling means therefor H03H9/0095 takes precedence

Abstract

A bandpass crystal filter of the type which uses coupled resonators and has a shunt capacitor at the junction between pairs of coupled resonators exhibits a stopband loss-frequency characteristic which is approximately symmetrical and monotonic. In some applications, it is necessary to sharpen up at least one side of the filter and this extra loss could be obtained if one or more frequencies of infinite loss could be added in the adjacent stopband. By connecting the parallel combination of a capacitor and a two-terminal quartz resonator across the junction between pairs of coupled resonators, the loss-frequency characteristic may be modified to include frequencies of infinite loss in the lower stopband.

Claims

1. A bandpass filter employing piezoelectric crystals as the primary frequency determining elements comprising:

a plurality of coupled resonators, having separate piezoelectric crystal bases, connected in tandem, each said coupled resonator comprising a pair of individual resonators sharing the corresponding crystal base, and each said individual resonator having an electrode area on each of the two sides of the base, one electrode being immediately above the other and having physical electrical connections made to each electrode to provide input and output connections, said resonator pair being in spaced relation so that an energy coupling path exists therebetween via the coupled crystal base which path is represented electrically by a mechanical coupling capacitor;

circuit connections for connecting the output of one of the plurality of coupled resonators to the input of the succeeding coupled resonator thereby forming a junction between adjacent pairs of coupled resonators; and

resonance means connected across each said junction said resonance means comprising a capacitor and a piezoelectric crystal resonator connected across each junction, said capacitor being selected to provide an equivalent shunt capacitance at said junction that is of the same order of magnitude as the mechanical coupling capacitor of the associated pair of coupled resonators, said resonance means providing an attenuation pole in the lower stopband.

2. The filter as defined in claim 1 wherein the ratio of the electroded area of the coupled resonators to the electroded area of said single crystal resonator is kept to near unity.

3. The filter as defined in claim 1 wherein the ratio of the electroded area of the coupled resonators to the electroded area of said single crystal resonator is not more than 10:

1.

4. The filter defined in claim 1 wherein said resonators are formed on AT-cut quartz crystals.

5. The filter as defined in claim 1 wherein the equivalent circuit of the crystal resonator is represented by a capacitor in parallel with a series inductor-capacitor circuit and the inductor of each single crystal resonator at said junction has a value in the range of 3 to 5 times the inductance value exhibited by each resonator of the associated pair of coupled resonators.