GB2100950A

GB2100950A - Crystal filter

Info

Publication number: GB2100950A
Application number: GB08119363A
Authority: GB
Inventors: Antony Harry Truelove
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-06-23
Filing date: 1981-06-23
Publication date: 1983-01-06
Also published as: DE3220698A1

Abstract

A monolithic band-pass crystal filter, for use, for example, as a radio channel filter, uses a single plate of quartz (1) operating in the thickness shear mode. This gives over 90 dB attenuation in the stop band. To prevent unwanted acoustic couplings between the resonators of the various filter sections due to mechanical acoustic vibrations in the quartz, one or more acoustic barriers are used. Such a barrier consists of a metal-loaded epoxy paste, which heavily attenuates the interfering modes of vibration, and can have grooves and/or holes under the paste to reinforce the attenuation. Some of the resonators on the same side of the barrier, i.e. in the same region of the quartz, are coupled acoustically by an evanescent wave. The or each acoustic barrier may be so oriented as to absorb acoustic waves propagating in the X direction of the quartz while the resonators of the filter sections are oriented in the rotated Z direction of the quartz. To further improve the characteristics, the geometries of the resonator section and/or of the regions on the quartz plate can be varied. <IMAGE>

Description

SPECIFICATION Crystal filter The present invention relates to monolithic crystal filters.

A monoplate quartz filter is a high-frequency bandpass filter using the thickness shear resonance of quartz and made on a single piece of quartz.

To produce a narrow bandpassfilter, it is usual to lightly couple a string of resonators, which can be done with purely mechanical resonances and couplings at low frequencies, or electrically with tuned circuits, and at microwave frequencies with resonant cavities. Between 5 MHz and 30 MHz 'AT shear quartz resonators are normally used, the techniques of energy trapping by the loading effect of the metal electrodes confining the resonance largely to the region ofthe electrodes. Coupling to the mechanical resonance of the quartz is by the piezo-electric effect.

Coupling between two separate resonators can be electric for resonators on separate pieces of quartz or for resonators on the same piece of quartz. If resonators on a single piece of quartz are close enough to each other, the coupling may be acoustic by the evanescent wave: this is the basis of the monolithic quartz filter. Thus one known example of such a filter, which defines a narrow band telephone channel at about 8 MHz consists of two plates each having four resonators which are acoustically coupled, the two plates being coupled by a single capacitor.

It is common practice when making a channel filter for radio purposes to use four individual units each consisting of a pair of acoustically coupled resonators, with three capacitors used to couple the units. Typical frequencies are 10.7MHz and 21.4 MHz. To produce a desired filter characteristic it is necessary to adjust the frequencies and coupling coefficents to precise values, and for well-known characteristics the values to be used can be read from published tables. Also important are the Q factors: the higherthe Qfactorthe more selective the filter can be made.

An object of the invention is to produce a monolithic quartz crystal filter which is simpler and more economical both in size and in manufacturing cost.

According to the invention there is provided a monolithic quartz crystal filter, which includes a single plate of quartz as the resonator material, a plurality of sets of electrodes deposited or otherwise applied to the surface of the quartz, each of which sets of electrodes defines a crystal filter section, terminals associated with the electrodes ofthe filter section to which electrical circuit means may be connected to couple the filter sections to each other when the filter is in use, and one or more acoustic barriers formed on the surface of the quartz, the or each said barrier being located between two adjacent filter sections so as to prevent acoustic coupling between filter sections via the quartz, when the or each said acoustic barrier is so oriented as to absorb acoustic waves propagating in a direction of the quartz other than the direction in which the resonators of the filter sections are oriented.

An embodiment of the invention will now be described with reference to the accompanying drawing.

The filter includes a single plate 1 of quartz which is operated in the thickness shear mode: in one example this plate is 9 mm wide by about 32 mm long, and 0.15 mm thick. Electrodes shown at 2 are produced on the surface of the quartz, e.g. by deposition of a metallic film, and four such electrode sets are shown. Each such electrode set, in the arrangement shown, consists of one large electrode on one face of the quartz plate and two smaller electrodes (shown in broken lines) on the reverse face of the quartz plate. Thus each such electrode set provides two resonators with a common electrode. These electrodes are connected to the leads such as 3 via conductive tracks such as 4, the tracks on the one face being shown in solid lines and those on the reverse face being shown in broken lines.

The electrical couplings needed between the various filter sections are made via capacitors (not shown) connected to appropriate ones of the leads.

In addition some ofthe leads may be commoned together if this is required.

Where it is necessary to prevent or reduce unwanted couplings between sections due to mechanical plate resonances, this is effected by providing one or more suitably located acoustic barriers on the plate 1. As shown, one such barrier is shown at 5, this being a stripe of an acoustic damping material.

Monoplate filters made as described above provide the tight specification needed for radio channel filters, i.e. a low loss, relatively flat passband, a sharp cut-off and an extended stop band to approximately 100 dB down from the passband. The fact that the filter is made on a single plate simplifies manufacture, making it cheaper, more reliable and more compact.

As already indicated, the mechanical plate resonances which might otherwise interfere with the overall performance are reduced by one or more acoustic barriers across the plate to separate it into two or more regions. Such a barrier absorbs and scatters mechanical waves passing along the plate, and can be a stripe or stripes of an acoustic damping material such as a metal-loaded epoxy paste. In addition, the barrier can have grooves or holes in the quartz under the paste. In the arrangement shown one barrier absorbs waves which propagate mainly in the X direction ofthe quartz: and the acoustically coupled sets of resonators are oriented in the rotated Z direction of the quartz.

To make a compact filter it is necessary to separate in frequency the inharmonic resonances on either side of the acoustic barrier: this is achieved by altering the geometry of the individual pairs of resonators and of the regions on either side of the barrier.

The arrangement shown in the drawing is encapsulated in a suitable casing.

1. A monolithic quartz crystal filter, which includes a single plate of quartz as the resonator material, a plurality of sets of electrodes deposited

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Crystal filter The present invention relates to monolithic crystal filters. A monoplate quartz filter is a high-frequency bandpass filter using the thickness shear resonance of quartz and made on a single piece of quartz. To produce a narrow bandpassfilter, it is usual to lightly couple a string of resonators, which can be done with purely mechanical resonances and couplings at low frequencies, or electrically with tuned circuits, and at microwave frequencies with resonant cavities. Between 5 MHz and 30 MHz 'AT shear quartz resonators are normally used, the techniques of energy trapping by the loading effect of the metal electrodes confining the resonance largely to the region ofthe electrodes. Coupling to the mechanical resonance of the quartz is by the piezo-electric effect. Coupling between two separate resonators can be electric for resonators on separate pieces of quartz or for resonators on the same piece of quartz. If resonators on a single piece of quartz are close enough to each other, the coupling may be acoustic by the evanescent wave: this is the basis of the monolithic quartz filter. Thus one known example of such a filter, which defines a narrow band telephone channel at about 8 MHz consists of two plates each having four resonators which are acoustically coupled, the two plates being coupled by a single capacitor. It is common practice when making a channel filter for radio purposes to use four individual units each consisting of a pair of acoustically coupled resonators, with three capacitors used to couple the units. Typical frequencies are 10.7MHz and 21.4 MHz. To produce a desired filter characteristic it is necessary to adjust the frequencies and coupling coefficents to precise values, and for well-known characteristics the values to be used can be read from published tables. Also important are the Q factors: the higherthe Qfactorthe more selective the filter can be made. An object of the invention is to produce a monolithic quartz crystal filter which is simpler and more economical both in size and in manufacturing cost. According to the invention there is provided a monolithic quartz crystal filter, which includes a single plate of quartz as the resonator material, a plurality of sets of electrodes deposited or otherwise applied to the surface of the quartz, each of which sets of electrodes defines a crystal filter section, terminals associated with the electrodes ofthe filter section to which electrical circuit means may be connected to couple the filter sections to each other when the filter is in use, and one or more acoustic barriers formed on the surface of the quartz, the or each said barrier being located between two adjacent filter sections so as to prevent acoustic coupling between filter sections via the quartz, when the or each said acoustic barrier is so oriented as to absorb acoustic waves propagating in a direction of the quartz other than the direction in which the resonators of the filter sections are oriented. An embodiment of the invention will now be described with reference to the accompanying drawing. The filter includes a single plate 1 of quartz which is operated in the thickness shear mode: in one example this plate is 9 mm wide by about 32 mm long, and 0.15 mm thick. Electrodes shown at 2 are produced on the surface of the quartz, e.g. by deposition of a metallic film, and four such electrode sets are shown. Each such electrode set, in the arrangement shown, consists of one large electrode on one face of the quartz plate and two smaller electrodes (shown in broken lines) on the reverse face of the quartz plate. Thus each such electrode set provides two resonators with a common electrode. These electrodes are connected to the leads such as 3 via conductive tracks such as 4, the tracks on the one face being shown in solid lines and those on the reverse face being shown in broken lines. The electrical couplings needed between the various filter sections are made via capacitors (not shown) connected to appropriate ones of the leads. In addition some ofthe leads may be commoned together if this is required. Where it is necessary to prevent or reduce unwanted couplings between sections due to mechanical plate resonances, this is effected by providing one or more suitably located acoustic barriers on the plate 1. As shown, one such barrier is shown at 5, this being a stripe of an acoustic damping material. Monoplate filters made as described above provide the tight specification needed for radio channel filters, i.e. a low loss, relatively flat passband, a sharp cut-off and an extended stop band to approximately 100 dB down from the passband. The fact that the filter is made on a single plate simplifies manufacture, making it cheaper, more reliable and more compact. As already indicated, the mechanical plate resonances which might otherwise interfere with the overall performance are reduced by one or more acoustic barriers across the plate to separate it into two or more regions. Such a barrier absorbs and scatters mechanical waves passing along the plate, and can be a stripe or stripes of an acoustic damping material such as a metal-loaded epoxy paste. In addition, the barrier can have grooves or holes in the quartz under the paste. In the arrangement shown one barrier absorbs waves which propagate mainly in the X direction ofthe quartz: and the acoustically coupled sets of resonators are oriented in the rotated Z direction of the quartz. To make a compact filter it is necessary to separate in frequency the inharmonic resonances on either side of the acoustic barrier: this is achieved by altering the geometry of the individual pairs of resonators and of the regions on either side of the barrier. The arrangement shown in the drawing is encapsulated in a suitable casing. CLAIMS

1. A monolithic quartz crystal filter, which includes a single plate of quartz as the resonator material, a plurality of sets of electrodes deposited or otherwise applied to the surface of the quartz, each of which sets of electrodes defines a crystal filter section, terminals associated with the electrodes of the filter sections to which electrical circuit means may be connected to couple the filter sections to each other when the filter is in use, and one or more acoustic barriers formed on the surface of the quartz, the or each said barrier being located between two adjacent filter sections so as to prevent acoustic coupling between filter sections via the quartz, when the or each said acoustic barrier is so oriented as to absorb acoustic waves propagating in a direction of the quartz other then the direction in which the resonators ofthefilter sections are oriented.

2. A monolithic quartz crystal filter, which includes a single plate of quartz as the resonator material, a plurality of sets of electrodes deposited or otherwise applied to the surface of the quartz, each of which sets of electrodes defines a crystal filter section, the resonators of said filter sections being oriented in the rotated Z direction of the quartz, terminals associated with the electrodes of the filter section to which electrical circuit means may be connected to couple the filter sections to each other when the filter is in use, and one or more acoustic barriers formed on the surface of the quartz, the or each acoustic barrier being so oriented as to absorb waves in the X direction of the quartz, so that said acoustic barriers prevent acoustic coupling between said filter section via the quartz.

3. A crystal filter as claimed in claim 1 or 2, and in which the or each said barrier consists of one or more stripes of a metal loaded epoxy paste on the surface of the quartz plate.

4. A crystal filter as claimed in claim 4, and in which the or each said barrier has grooves and/or holes in the quartz under the paste.

5. A crystal filter as claimed in claim 1, 2, 3, or 4, and in which the geometrics of the resonator sections on either side of the barrier, and/or of the regions on either side of the barrier differ one from the other.

6. A crystal filter as claimed in claim 1, 2, 3, or 4, and in which coupling between resonators separated from each other by a said barrier is capacitive.

7. A monolithic quartz crystal filter, substantially as described with reference to the accompanying drawings.