Wave transmission lines and networks – Coupling networks – Electromechanical filter
Patent
1992-01-24
1994-04-26
Lee, Benny
Wave transmission lines and networks
Coupling networks
Electromechanical filter
310321, 310366, 310367, H03H 956
Patent
active
053070348
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a multimode quartz crystal filter element using an ultrathin AT cut quartz crystal substrate which is capable of oscillation at high frequencies greater than about ten megahertz through fundamental vibration.
BACKGROUND ART
A quartz crystal plate to be used for a conventional ordinary AT cut quartz crystal resonator is flat, and hence the highest fundamental frequency obtainable for practical use is around 40 MHz from present manufacturing techniques and mechanical strength. On this account there has been widely employed what is called overtone oscillation means which extracts a higher order harmonic mode vibration of an AT cut quartz crystal resonator to obtain a frequency which is an odd multiple of the fundamental resonance frequency, but its oscillation circuit requires for an LC tuning circuit including a coil and hence is not suitable for fabrication as an IC, besides the overtone oscillation circuit may sometimes be difficult to maintain at a stable oscillation because a resonator to be included in the overtone oscillator has a large capacitance ratio and consequently a high impedance level. On the other hand, a surface acoustic wave (SAW) resonator, whose resonant frequency is determined by the pitch of electrode fingers of an interdigital transducer, is able to resonate at a maximum of 1 GHz or so due to the progress in photolithography, but a piezoelectric substrate when used as the surface acoustic wave resonator is remarkably inferior to the AT cut quartz crystal in terms of temperature-frequency characteristic. To solve this problem, there has been proposed and is now under study an ultrathin piezoelectric resonator which has a cavity provided, by etching or mechanical grinding, in one side or in both sides of a block of AT cut quartz crystal to form an ultrathin vibratory portion surrounded with a support frame so that the fundamental resonant frequency may extend from ten megahertz to hundreds of megahertz while retaining the mechanical strength of the vibratory portion.
By fabricating a multimode filter element using such an ultrathin piezoelectric substrate as above, a filter with a center frequency in the range from about ten to about 100 megahertz could be easily obtained without utilizing the overtone techniques.
Incidentally, as regards a conventional multimode piezoelectric filter element, it is said to be preferable that when the center frequency of the filter is 10 MHz, the thickness of an electrode to be evaporated onto the piezoelectric substrate should be about 3000 .ANG. so there is entrapping or confinement of the vibration energy at an appropriate value and an ohmic loss which is sufficiently low. The mass of electrodes is one of the parameters in the analysis of the confinement of vibration energy, and the equivalent thickness of electrode films is defined as the sum of the thicknesses of the select films formed on both sides of the quartz crystal substrate to calculate the mass of electrodes. Here, this definition of the thickness of electrode films is employed.
On the other hand, in the case where a filter element which has a center frequency of 100 MHz and is similar to the above-mentioned filter element in the vibration energy entrapping characteristic and other basic characteristics is manufactured using an ultrathin AT cut quartz crystal plate, the electrode should be vapor-deposited to a thickness of 300 .ANG., i.e. 1/10 of the above-said value. However, it is evident that such an electrode thickness is so small that the ohmic loss will increase, making it impossible to obtain a sufficient attenuation of the filter.
If the thickness of the electrode is set to around 1000 .ANG. to keep the ohmic loss sufficiently low, the electrode is too thick as compared with the thickness of the AT cut quartz crystal substrate (about 17 .mu.m), and consequently, the amount of vibration energy entrapped or confined becomes excessive and satisfactory acoustic coupling between the electrodes cannot be obtained unless the inter-electro
REFERENCES:
patent: 5075651 (1991-12-01), Pradal
Ishii Osamu
Kurosaki Takebumi
Morita Takao
Lee Benny
Toyo Communication Equipment Co., Ltd.
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