Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2002-03-05
2004-04-06
Ponomarenko, Nicholas (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C029S025350
Reexamination Certificate
active
06717328
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric resonator and an FM detection circuit incorporating the same.
2. Description of the Related Art
A phase shifter of an FM detection circuit which detects changes in the frequency of an FM wave by detecting changes in voltages has conventionally been used in a discriminator. Generally, to achieve a wide demodulation output bandwidth, piezoelectric materials that produce a low Q and a wide bandwidth &Dgr;F (=Fa−Fr) are used for the discriminator. However, the relationship of various temperature coefficients of the piezoelectric materials has not been appropriate, resulting in a relatively large temperature coefficient (fo
TC
) of the finished product. Thus, the guaranteed operation temperature range of a device incorporating the discriminator has been narrower than discriminators that incorporate a ceramic filter, which prevents the use of the discriminator in various devices.
The temperature coefficient (fo
TC
) of a discriminator in a finished product has been on the order of 25 ppm/° C., which corresponds to a frequency change of approximately 28 kHz in a temperature range of 100° C. and approximately 40 kHz in a temperature range of 150° C. Furthermore, in discriminators which have previously been available, frequency change has tended to be greater at temperatures above 20° C. Thus, the upper limit of the guaranteed operation temperature range has often been set to 60° C. in order to meet general specifications for fo
TC
which assume a change of ±300 kHz in fo.
In order to counter the problems described above, Japanese Unexamined Patent Application Publication No. 63-283215 discloses a device in which a capacitor is connected in series to a discriminator (piezoelectric resonator) and the temperature coefficient of the capacitance of the discriminator and the temperature coefficient of the capacitance of the capacitor satisfy a predetermined relationship, such that a change in the frequency-impedance characteristics associated with a temperature change in the discriminator is cancelled by the temperature characteristics of the capacitor, thus reducing a shift in frequency.
Furthermore, Japanese Registered Utility Model No. 2501521 discloses a bridge circuit with resistors connected respectively on three sides thereof and a discriminator (piezoelectric resonator) connected on the remaining side, in which a capacitor having temperature characteristics equivalent to those of the discriminator connected in parallel to one of the resistors.
However, each of the proposals requires use a capacitor in addition to a discriminator and thus requires control of the temperature characteristics of the capacitor which increases the uncertainties. Thus, it has been difficult to provide an FM detection circuit which achieves desired temperature characteristics.
SUMMARY OF THE INVENTION
To overcome the above-described problems, preferred embodiments of the present invention provide a piezoelectric resonator and an FM detection circuit incorporating the same, in which various temperature coefficients of the piezoelectric material are optimized, such that a finished product has stable temperature characteristics and a guaranteed operation temperature range that is greatly increased.
According to a preferred embodiment of the present invention, piezoelectric resonator is provided, wherein the temperature coefficient &egr;
TC
of the capacitance of the piezoelectric material, the bandwidth ratio &Dgr;f/fo, the temperature coefficient Fr
TC
of the resonance frequency, the temperature coefficient Fa
TC
of the anti-resonance frequency, and a target value &agr; for the temperature coefficient of the center frequency satisfy the following expression:
|(
Fr
TC
+Fa
TC
)/2
+K×&egr;
TC
×(&Dgr;
f/fo
)|≦&agr; (1)
where K=a coefficient determined according to the impedance at the midpoint between Fr and Fa; &egr;
TC
=A×(the amount of change in capacitance in a measured temperature range)/(the capacitance at a reference temperature×the measured temperature range); &Dgr;f/fo=(Fa at the reference temperature−Fr at the reference temperature)/(fo at the reference temperature); Fr
TC
=A×(the amount of change in Fr in the measured temperature range)/(Fr at the reference temperature×the measured temperature range); Fa
TC
=A×(the amount of change in Fa in the measured temperature range)/(Fa at the reference temperature×the measured temperature range); and A=a coefficient of +1 for a positive temperature coefficient and −1 for a negative temperature coefficient.
In accordance this preferred embodiment, because the piezoelectric material is selected such that the temperature coefficient of the capacitance and the temperature coefficient of the anti-resonance frequency cancel each other, the amount of change in the center frequency fo associated with a temperature change is greatly reduced, i.e., the temperature coefficient fo
TC
of the center frequency fo is reduced. Thus, the piezoelectric resonator has a wider guaranteed operation temperature range which produces a wider guaranteed operation temperature range in a device incorporating the piezoelectric resonator. Furthermore, because a capacitor for improving temperature characteristics need not be connected separately, the structure is greatly simplified and desired temperature characteristics are achieved.
In a piezoelectric resonator that is sealed by a packaging resin, in addition to the temperature coefficients of the piezoelectric resonator, the temperature coefficient Rfo
TC
of the center frequency related to a stress of the packaging resin is also taken into consideration such that the following expression is satisfied:
|(
Fr
TC
+Fa
TC
)/2
+K×&egr;
TC
×(&Dgr;
f/fo
)+
Rfo
TC
|≦&agr; (2)
Accordingly, the effects of the temperature coefficient of the packaging resin are eliminated, such that stable temperature characteristics are achieved in a piezoelectric resonator sealed by a packaging resin.
Another preferred embodiment of the present invention provides a method of calculating a temperature coefficient of a piezoelectric resonator, wherein the temperature coefficient fo
TC
of the center frequency is calculated according to the following approximate expression from the temperature coefficient &egr;
TC
of the capacitance of the piezoelectric material, the bandwidth ratio &Dgr;f/fo, the temperature coefficient Fr
TC
of the resonance frequency, and the temperature coefficient Fa
TC
of the anti-resonance frequency:
fo
Tc
=(
Fr
TC
+Fa
TC
)/2
+K×&egr;
TC
×(
&Dgr;f/fo
) (3)
where K=a coefficient determined according to the impedance at the midpoint between Fr and Fa; &egr;
TC
=A×(the amount of change in capacitance in a measured temperature range)/(the capacitance at a reference temperature×the measured temperature range); &Dgr;f/fo=(Fa at the reference temperature−Fr at the reference temperature)/(fo at the reference temperature); Fr
TC
=A×(the amount of change in Fr in the measured temperature range)/(Fr at the reference temperature×the measured temperature range); Fa
TC
=A×(the amount of change in Fa in the measured temperature range)/(Fa at the reference temperature×the measured temperature range); and A=a coefficient of +1 for a positive temperature coefficient and −1 for a negative temperature coefficient.
In a piezoelectric resonator sealed by a packaging resin, in addition to the temperature coefficients of the piezoelectric resonator, the temperature coefficient Rfo
TC
of the center frequency of a stress of the packaging resin is also taken into consideration so that the temperature coefficient fo
TC
of the center frequency is calculated according to the following expression:
fo
TC
=(
Fr
Ikeda Yoshihiro
Sawai Kunio
Aguirrechea J.
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Ponomarenko Nicholas
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