GB2352101A

GB2352101A - Balanced monolithic piezoelectric resonator

Info

Publication number: GB2352101A
Application number: GB9916265A
Authority: GB
Inventors: Peter Kenneth Wall
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-07-13
Filing date: 1999-07-13
Publication date: 2001-01-17
Anticipated expiration: 2019-07-13
Also published as: GB2352101B; GB9916265D0

Abstract

The resonator comprises a plate-shaped piezoelectric substrate 9 on one side of which is a ground electrode 10. The other side of the substrate carries a pair of input electrodes 1,2 and a pair of output electrodes 3,4 such that the resonator may be used in a balanced electrical circuit. The bandwidth of the resonator is set by the separation of the input and output electrodes. This form of construction allows all connections except the ground connection to be made from one side of the substrate 9, and improves the stopband performance of the resonator if there are imbalances in the construction or terminating impedances.

Description

2352101 BALANCED MONOLITHIC RESONATORS - The invention relates to a

balanced monolithic, resonators for use in many types of electronic communication equipment. Monolithic resonators on piezoelectric plates have been known since early work in the USA ISykes et all and Japan [Makazawa et all in the 1960's decade which were based on the principle of energy trapping first pointed out by Mortley [19461 and explained in detail by Shockley, Curran & Koneval [1963].

The simplest version of such resonators uses an electrode pattern in which a first pair of electrodes of equal shape and dimensions are deposited opposite one another on either side of the plate and a second similar pair is deposited in like manner but spaced by a uniform gap width from the first pair. The energy trapping principle and the width of the gap then determine the electrical coupling between these two electrode pairs.

During manufacture, the thicknesses of the electrodes are selectively controlled in order to achieve the desired resonant frequencies of the two electrode pairs and the desired coupling between them in order to produce the correct centre frequency and filter bandwidth from the device.

In such an arrangement, these two electrode pairs can be connected into either a balanced or unbalanced electrical circuit.

Monolithic resonators capable of operating in a balanced electronic circuit potentially can provide specific performance advantages for the equipment in which they are used. The practical stopband performance of such balanced resonators described above is limited by the effect of small unbalances in the impedances on the two sides because no impedances are connected to the earth of the external balanced circuit.

According to the present invention, the balanced monolithic structure is provided by splitting input and output electrode areas on one side of the plate into two parts to permit connection to the external balanced circuits and using a single electrode on the second side to provide the required connection to the earth of the external balanced circuit.

2 This structure allows the two monolithic st - ructures on either side of the balanced device to be close together, which minimizes the size of the oomplete device. This is important for use in products requiring critically small dimensions such as pagers and mobile handsets. It also allows the manufacturer to carry out the major processing of the balanced monolithic resonator in a similar way to unbalanced resonators by connection of the electrode pairs forming the balanced input and output during processing.

They are then separated for subsequent use either by removal of external connections used during adjustment or if these are not used, by creating a narrow gap which divides the electrodes into two balanced parts by laser division or other means. The major part of the processing according to the invention can be carried out from one side of the plate. A further benefit is that the only connection to the external circuitry from the underside of the plate is to earth.

A 'specific embodiment of the invention -will now be described with reference to the accompanying drawings in which:

FIG.1 (a) and (b) are plan drawings of the two sides of a structure embodying features of the invention; FIG.2 is a sectional view of the device including the electrodes on the two sides of the resonator plate; FIGA is a special case of FIG.1 in which the structure is balanced on one side and unbalanced on the other side; FICA illustrates a frequency response obtainable from the devices in FIGS. 1, 2 and 3;

Claims

FIG.5 is an equivalent electrical circuit for the device in Claim 1;

In FIG.1, 9 is the device plate of piezoelectric material, 1, 2, 3, 4 are electrodes on one side of the plate and 10 is the single electrode on the reverse side of the plate. Electrodes 1,2 form the balanced input electrodes of the device and 3,4 form the balanced output electrodes of the device. Metallization strips 51,6',71,81,111,12' provide the connections to points 5'', 6'', 71' 811 11'' and 121 'which form the external connections of the device. The gap between electrodes 1, 2 on the one hand and 3, 4 on the other hand needs to be carefully dimensioned to-achieve the desired coupling between the input electrodes 1, 2 and the output electrodes 3, 4.. Coupling decreases as the gap increases in width.

3 The division of plating between electrodes 2 and between 3, 4 needs to be sufficiently wide' to reduce coupling between them to an adequately low level for the required performance which means its width needs to be approximately the same as the width of the electfodes. This is achieved by the choice of crystallographic orientation of the crystal plate as shown in FIG. 1. in which coupling along the Z-axis is several times smaller than along the X axis for the same separation of the electrodes. The reason for this arrangement is to minimize the total surface area occupied by the electrodes.

Electrodes 1,2- do not need to be the same length- as electrodes 3A.

FIG.2 shows that electrode 10 is one continuous metallization.

FIG.3 shows that a resonator constructed for operating in a circuit which is balanced on one side of -the monolithic resonator and unbalanced on the other side.

FIG.4 illustrates a typical attenuation characteristic of a device made according to the invention.

FIG.5 shows an equivalent circuit for the device, which ignores the residual coupling between electrode pair 1,2 and pair 3,4. When the device is balanced, the series resonance formed by elements 13 and 14 is identical to the resonance of elements 15 and 16. A similar situation exists with elements 17,18 and 19,20. To a first order therefore, the effective series resonance formed by elements 13,14,15 and 16 is the average value from the individual electrode pairs. Provided the imbalance is not too high, this average value can be detected by examining the minimum impedance between electrodes 1, 2 and the electrode 10 in FIG.1.

A similar situation exists for elements 17,18 and 19.20.

Although the embodiments have been described in connection with the case in which the plate is basically rectangular and the monolithic resonator uses an AT cut fundamental qd,artz crystal plate, the invention does not need to be limited specifically thereto and for example could use a round plate or other piezoelectric material such as lithium tetraborate or langasite and the electrodes can be basically fabricated from any symmetrical metallization with dividing gaps along two axes of symmetry at an angular alignment of 90 degrees..- What is claimed is 1. A balanced monolithic resonator comprising in combination, a piezoelectric crystal plate, a first continuous' electrode on the first face of the plate for connection to the earth of the external balanced circuit, four electrodes on the second face separated by one gap parallel with the Z-axis, the width of which controls the bandwidth of the device and separates the electrodes connected to the input from those connected to the output of the device and a second gap parallel with the Xaxis of sufficient width to minimize the coupling between the electrodes on either side which are connected to the balanced input or balanced load of the external circuit to which they are connected by metallization strips to the edge of the plate.
2. A resonator as in claim 1 wherein the-gap parallel with the X-axis is achieved by removal of the metallization after the device has.been adjusted.
3. A resonator as in claim I in which the piezoelectric body consists of a quartz plate of uniform thickness operating in AT fundamental mode.
4. A resonator as in claim I in which the gap on the second face of the plate parallel with the X-axis only exists between 2 electrode areas.
5. A filter using one or more devices in Claim 1 or Claim 2 or Claim 3 which enables it to operate in an electrically balanced circuit.