JPS5928088B2 - filter device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a filter device in which the passband width of a thickness vibration multimode filter is easily expanded.

In general, energy trapping multimode filters have advantages such as simple structure and less unnecessary vibration response, so they are effective components as filters for high frequencies of several MHz or higher.

FIG. 1 shows an example of the basic configuration of such an energy confinement multimode filter.

In the figure, 1 is a piezoelectric plate that performs thickness vibration such as vertical thickness vibration or thickness shear vibration, 2 is an input electrode, and 3
4 is an output electrode, 4 is a counter electrode, 5 is an input terminal drawn out from the input electrode 2, 6 is an output terminal drawn out from the output electrode 3, and γ is a common terminal drawn out from the counter electrode 4.

The piezoelectric plate 1 is made of a piezoelectric ceramic made of quartz, a single crystal such as niobium lithium, or a polarized piezoelectric ceramic.

Still, in FIG. 1, S is a charge distribution of vibration called a symmetric mode excited by a multimode filter, and its polarity is in phase at the input electrode 2 and the output electrode 3.

Further, A is a charge distribution of vibration called an oblique symmetric mode, and its polarity is opposite in phase between the input electrode 2 and the output electrode 3.

The energy-trapped multi-mode Urahami device utilizes these two vibrations simultaneously.

Here, in order to excite only symmetric mode vibration, terminals 5 and 6 should be used in common, and terminal 7 should be used as shown from the charge distribution.
It is sufficient to apply a voltage between the two.

Still, the oblique symmetric mode vibration floats terminal γ and terminals 5 and 6
It can be excited by applying a voltage between them.

The energy-trapped multi-mode Uraha device has an equivalent circuit in which two types of resonators excited as described above are configured in a lattice shape as shown in FIG. 2A.

In the figure, Za indicates a symmetric mode resonator, and zb indicates an oblique symmetric mode resonator.

Figure 2B shows the resonator Z required when configuring the Urahami device.
The frequency relationship between a and Zb is shown.

In the figure, f, , f2 are the resonant frequency and anti-resonant frequency of the symmetric mode resonator, f3 . f, are the resonant frequency and anti-resonant frequency of the obliquely symmetric mode resonator, respectively.

In designing the energy confinement filter, the antiresonant frequency f2 of the symmetric mode and the resonant frequency f3 of the oblique symmetric mode are made to match.

Generally, a resonator becomes an inductance component between a resonant frequency and an anti-resonance frequency, and becomes a capacitance component in other regions.

The bandwidth of the energy-confined Uraha device with the equivalent circuit shown in Figure 2A corresponds to the frequency width at which the resonators Z a p Z b are in opposite phase, which is the same as the frequency width f1 to f4 in Figure 2B. Become.

This bandwidth is determined by the voltage-mechanical coupling coefficient of the piezoelectric plate 1.

Considering the fractional bandwidth obtained by dividing the bandwidth by the operating frequency, the upper limit of the fractional bandwidth of energy confinement filters that have been put into practical use to date is about 4%.

Normally, the fractional bandwidth required for the video intermediate frequency filter of a television receiver is approximately 6.5%, and in order to satisfy this with an energy confinement type Uraha filter, an appropriate method of expanding the bandwidth is required. It becomes necessary.

FIG. 3 shows an example of the configuration of a conventional broadband filter device.

In the figure, 101d is an energy confinement type Urawa device, and 8 and 9 indicate coils having the same value and connected in series to the input electrode 2 and the output electrode 3, respectively.

Other details are the same as in FIG. Problem 1 of this conventional example
One is that it is uneconomical because two coils are required.

Another problem is that it is difficult to obtain desirable filter characteristics, which will be discussed in more detail below.

By converting the circuit, the filter shown in FIG. 3 becomes equivalent to the equivalent circuit of the Uraha filter shown in FIG. 2A, in which a coil 8 is connected in series to each of the resonators Z a ) Z b.

This makes terminals 5 and 6 in Figure 3 common, and terminal 7.
Considering the impedance between
The impedance between .

When a coil is connected in series with a resonator, the antiresonant frequency of the resonator remains unchanged, but the resonant frequency decreases.

FIG. 4 shows the frequency relationship of the filter device of FIG. 3.

In the figure, Za indicates a series circuit of a symmetric mode resonator and a coil, and Zb indicates a series circuit of an oblique symmetric mode resonator and a coil.

f′1. f'2 represents the resonant frequency and anti-resonant frequency of the resonant circuit Z'a, f'3. f'4 represents the resonant frequency and anti-resonant frequency of the resonant circuit Z'b, respectively.

Although the frequency relationship of only the energy trapping type Urahami device satisfies FIG. 2B, by connecting a coil, the resonant frequency decreases and the frequencies f2 and f3 no longer match.

In Figure 4, the upper and lower limit frequencies of the passband are the boundary frequencies f'1 and f where the impedances of Z'a and Z'b are in opposite phase.
'4.

However, between the frequencies f2 and f3, the impedances of Z'a and Z'b are in phase, and are in an attenuation region according to the theory of the lattice type ura wave device.

As described above, in the conventional example shown in FIG. 3, the bandwidth is widened by lowering the resonant frequency, but there is a problem in that an attenuation region occurs in the passband, resulting in ripples.

The present invention provides a simple and economical wideband filter device that solves the problems of the conventional example.

Hereinafter, the present invention will be explained based on illustrated embodiments.

In FIG. 5, reference numeral 20 indicates the entirety of the energy trapping type Urawa device, which is composed of a piezoelectric plate 11 that performs thickness vibration, an input electrode 12, an output electrode 13, and a counter electrode 14.

is and is are an input terminal and an output terminal drawn out from the input electrode 12 and the output electrode 13, respectively.

The above configuration is essentially the same as the configuration of the filter shown in FIGS. 1 and 3.

18 indicates a two-terminal circuit including an inductance component connected in series to the counter electrode 14, and 17 indicates the two-terminal circuit 1.
This is a common terminal pulled out from 8.

Experiments have confirmed that with this configuration, the bandwidth can be expanded without causing ripples in the passband.

This can be explained qualitatively as follows.

In FIG. 5, if terminals 15 and 16 are connected and the impedance between them and terminal 17 is considered, the symmetric mode resonator and the two-terminal circuit 18 are connected in series, while the impedance between terminals 15 and 16 is This is only the obliquely symmetrical mode resonator of the wave filter 20.

Comparing these two types of impedance with the similar two types of impedance of only the energy confinement filter in Figure 1, the equal number circuit of the filter in Figure 5 corresponds to the symmetrical mode resonator in Figure 2A. It is thought that it will be a lattice type in which only the part is replaced with a two-terminal circuit and a series circuit of symmetric mode resonators.

From the above, the frequency relationship of each component of the equivalent lattice circuit in the embodiment of FIG. 5 is as shown in FIG. 6.

In the figure, Zc indicates a series circuit of a two-terminal circuit 18 including a symmetrical mode resonator and an inductance component, and Zd indicates a diagonally symmetrical mode resonator.

f5. f6 is the resonant frequency and anti-resonant frequency of the series circuit Zc, respectively, and f7 and f8 are the resonant frequency and anti-resonant frequency of the resonator Zd, respectively.

The difference from the frequency relationship of only the energy confinement filter shown in Figure 2A is that Zc is a series circuit of a two-terminal circuit and a symmetric mode resonator, so its resonance frequency f5 is equal to fl in Figure 2B. The other frequencies f6. f7 and f8 are respectively f2.
f3. Matches f4.

The passband width of the embodiment shown in FIG. 5 is the interval between frequencies f5 and f8 at which the impedances of the series circuit Zc and the resonator Zd are in opposite phases, and is wider than the bandwidth of only an energy trap filter.

Further, since frequencies f7 and f6 are kept the same, no ripple occurs in the passband.

FIG. 7 shows a specific embodiment of the present invention.

This uses one coil L as the two-terminal circuit 18 in FIG.

Here, the energy trapping filter 20 is constructed with a piezoelectric plate 11 made of lead titanate-lead zirconate-magnesium/lead niobate polarized in the thickness direction, and its shape and dimensions are 10 x 10 x 0.5 mm3. .

The dimensions of input electrode 12 and output electrode 13 are 0
．． Provided at an interval of 2 mm, the counter electrode 14 is 2 x 2.2 gj
The input electrode 12 and the output electrode 13 were provided as opposed to each other as i.

When the coil L was connected in series to the counter electrode 14 of this energy trap filter 20 and the change in bandwidth was investigated, the results shown in FIG. 8 were obtained.

In FIG. 8, curve A is the characteristic when a 33 μH coil is used as L in the embodiment shown in FIG. 7, and curve B is the characteristic when coil L is not added.

The bandwidth of curve B at the time of 3 dB reduction is 150 KHz, but the bandwidth of curve A is doubled to 300 KHz.

In addition, in FIG. 7, it was confirmed that the larger the value of the coil L, the greater the degree of broadening of the bandwidth.

FIGS. 9 and 10 are diagrams showing other specific embodiments of the present invention, respectively.

In FIG. 9, a series circuit of a resistor R and a coil L is used as the two-terminal circuit 18.

Although the figure shows a two-terminal circuit in which a resistor R and a coil L are connected in series, a two-terminal circuit in which a coil L and a resistor R are connected in parallel may be used instead.

In this embodiment, the resistor R was effective in improving the ripple in the passband that occurs when the resonance sharpness Q of the coil L and the Q of the energy trap filter 20 are unbalanced.

In FIG. 10, a series circuit of a capacitor C and a coil L is used as a two-terminal circuit 18.

To increase the bandwidth by the same amount between this embodiment and the embodiment of FIG. 7, the value of the coil L of FIG. 10 must be larger than the value of the coil L of FIG.

This can be easily understood from the fact that the apparent inductance of the series circuit of the coil L and capacitor C is smaller than that of the coil alone.

Therefore, the configuration shown in FIG. 10 is effective when it is difficult to obtain the required bandwidth with a single coil because the inductance value becomes too small.

Although a two-terminal circuit in which the capacitor C and the coil L are connected in series is used in FIG. 10, the same effect can be obtained by using a two-terminal circuit in which the capacitor C and the coil L are connected in parallel.

As is clear from the above description, the present invention is designed to match the anti-resonance frequency of the symmetric mode resonator and the resonant frequency of the oblique symmetric mode resonator in an energy trapping multimode filter using thickness vibration. At the same time, a two-terminal circuit including an inductance component is connected in series to the opposing electrode of the multimode filter, so that the anti-resonant frequency of the symmetric mode resonator and the resonant frequency of the oblique symmetric mode resonator can be adjusted. The resonant frequency of the symmetrical mode resonator is lowered while the resonator remains fixed, and the bandwidth of the multimode filter can be expanded without causing ripples in the passband. There is something very big.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the basic configuration of an energy confinement filter, Figures 2A and B are diagrams showing its equivalent circuit diagram and frequency relationship, Figure 3 is a diagram showing the configuration of a conventional filter device, and Figure 4 is a diagram showing the basic configuration of an energy confinement filter. 5 is a diagram showing the basic configuration of an embodiment of the present invention, FIG. 6 is a diagram showing the frequency relationship, FIG. 7 is a diagram showing the configuration of a specific embodiment of the present invention, and FIG. This figure shows the characteristics of the embodiment of FIG. 7 in comparison with the conventional example, and FIGS. 9 and 10 are configuration diagrams of still other specific embodiments of the present invention. DESCRIPTION OF SYMBOLS 11... Piezoelectric plate, 12... Input electrode, 13... Output electrode, 14... Common electrode, 18... 2-terminal circuit,
L...Coil, R...Resistor, C...Capacitor.

Claims (1)

[Claims] 1. A multi-mode Oki wave device using thickness vibration, in which an input electrode and an output electrode are provided on one main surface of a piezoelectric plate, and a common counter electrode is provided on the other main surface of the same piezoelectric plate. , the anti-resonant frequency of the symmetric mode resonator of the multimode filter is made to match the resonant frequency of the oblique symmetric mode resonator, and an inductive 2 including an inductance component is connected to the counter electrode. By connecting the terminal circuits in series, the resonant frequency of the symmetric mode resonator is fixed while the anti-resonant frequency of the symmetric mode resonator and the anti-resonant frequency of the oblique symmetric mode resonator of the multimode F-wave device are fixed. A filter device characterized in that it is configured to reduce. 2. The filter device according to claim 1, wherein the inductive two-terminal circuit is constituted by one coil. 3. The filter device according to claim 1, wherein the inductive two-terminal circuit is constituted by a series circuit of a coil and a resistor. 4. The filter device according to claim 1, wherein the inductive two-terminal circuit is constituted by a parallel circuit of a coil and a resistor. 5. The filter device according to claim 1, wherein the inductive two-terminal circuit is constituted by a series circuit of a coil and a capacitor. 6. The filter device according to claim 1, wherein the inductive two-terminal circuit is constituted by a parallel circuit of a coil and a capacitor.