JPS5830767B2 - filter chain - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention consists of a number of filter units made of quartz crystal or its height measurement quality piezoelectric material and vibrating in a thickness shear vibration mode, these units being electrically coupled to each other and The invention relates to a filter system in which each unit passes a frequency band comprising two primarily acoustically coupled resonators.

This kind of filter/naene is available in a patent application filed in 1984-2043.
It is stated in No. 6.

As the development of telecommunication technology has led to higher and higher carrier current frequencies, filters with very small relative bandwidths have become necessary to filter narrow frequency bands from these high frequencies.

In a filter consisting of two acoustically coupled resonators, the Q factor is large, so a narrow pass band can only be achieved by small acoustic coupling.

However, the disadvantage of such filter units is that
There is high passband attenuation and high sensitivity to signal levels.

Additionally, these filter units are difficult and expensive to manufacture and reproduce.

Furthermore, a disadvantage is that the bandwidth after such a filter has been manufactured cannot be adjusted externally, or can only be done by very difficult methods.

The present invention solves the above problem by an alternative processing method, namely, the filter system includes one or more filter sets, each of which has at least one equal resonant frequency. The problem was solved by having two electrically coupled filter units with - the acoustic coupling coefficient in the filter set being set much larger than the electrical coupling coefficient.

This embodiment has a four-tuned circuit with three couplings, two of which have high coupling coefficients and one with an adjustable low coupling coefficient.

two of the four resonant frequencies are within the desired passband;
The other two are outside this passband.

Within a filter set, the symmetrical resonant frequency of one of these filter units is preferably
equal or nearly equal to the antisymmetric resonant frequency of the other filter unit.

These sound tube coupling coefficients must be extremely large, but are not necessarily equal to each other.

Next, the present invention will be explained with reference to the drawings.

A standard dual (dipole oscillator) with two resonators with a very strong acoustic coupling is used to construct the filter nano.

The second important thing is the absolute value of the coupling coefficient.

The symmetric or anti-symmetric resonant frequencies of these standard duals are the same.

This will be further explained below.

Strong acoustic coupling is achieved by (1) extremely thin electrodes, (2) small dimensions of the electrodes along the acoustic coupling axis (le in Figure 1), and (3) extremely small spaces between the resonators (Fig. 1). Obtained by d) in Figure 1.

A dual with constant resonant frequency is made in the same way as a unipolar crystal.

These standard duals are therefore easy to manufacture and their properties can be reproduced correctly.

As stated in the above patent application, the two resonant frequencies of one dual are represented by .

Similarly, from Figure 2, the acoustic coupling coefficient is 1 Ka2□ is obtained.

The coupling coefficient is extremely dog I2 In this case, the two resonant frequencies are significantly different.

FIG. 3 shows the frequency attenuation characteristics of such a dual.

FIG. 4 shows two standard duals connected in tandem by a coupling capacitor C23.

In this specification, such a circuit is referred to as a filter set.

One filter set has four resonant frequencies.

Figure 5 shows the frequency attenuation characteristics of the filter set, where the symmetric and anti-symmetric resonances are almost equal and each form a pair (
frl and ff3. ff2 and ff4).

The difference between frequencies frl and ff3 and between frequencies fr2 and fr+ is determined by the value of capacitor C23.

It is approximately represented by two fills.

In FIG. 6, the ratio between the electrical coupling coefficient and the acoustic coupling coefficient is plotted against frequency, and the four resonant frequencies are parameters.

This ratio X=xe is approximately such that Ka is much larger than the electrical coupling coefficient, and therefore, when - is large, both frl and frs and ff2 and ff become close to each other.

Changing the value of capacitor C23, and therefore the value of -, changes these distances.

As is well known, the electrical coupling coefficient is constantly affected by changes in ambient temperature.

As a result of the change in the electrical coupling coefficient (Ke), the ratio of the electrical coupling coefficient (Ke) to the acoustic coupling coefficient (Ka), X-Ke, changes, but as shown in Figure 6, the change in X depends on the frequency. This causes a change in fr3.

However, in the case of Sat≧4, as shown in Figure 6,
A change in (and thus a change in 7) results in only a small change in frequency fr3.

For this reason, the condition for the stability of the filter passband formed by frequencies fr1 and ff3 should be above ≧4, in other words the acoustic coupling coefficient should be at least 4 times the electrical coupling coefficient.

From the circuit described above, it is possible to construct a filter having the characteristics shown in FIG. 5, in which frequencies fr1 and ff3 are within the passband of the filter, and frequencies fr2 and ff4 are outside the passband of the filter.

As shown in Figure 5, the distance between the frequencies within the filter's passband (frl and frs) and the frequencies outside the filter's passband (ff2 and fr+) is very large, so the latter frequency (ff2 and ff4)
has no effect on the filter passband and is therefore negligible for the filter band.

This is true regardless of the absolute value of the distance between the frequencies fr1 and frs (which is very small compared to the distance between the in-pass band frequencies fr1 and ff3 and the out-of-pass band frequencies f2 and f4).

In a similar manner, a filter can be constructed in which two duals are coupled and the symmetrical resonant frequency of one dual is equal to the anti-symmetrical resonant frequency of the other.

Therefore, a frequency attenuation characteristic as shown in FIG. 1 is obtained.

Since the value of capacitor C23 can be changed to change the difference between frequencies fr2 and ff3, the bandwidth of the filter set can be adjusted externally.

As deduced from FIG. 8, this filter set has a very narrow passband.

To improve edge robustness, several filter sets can be connected in tandem (FIG. 9).

In view of arriving at the invention according to the above-mentioned patent application, self-inductance is chosen as the coupling impedance for the purpose just mentioned.

FIG. 10 shows the frequency attenuation characteristics of three filter sets connected in tandem.

This filter has no secondary resonance, and the ripple is less than 0.3 dB. Next, an embodiment of the present invention will be described.

A filter chain as claimed in the claims, characterized in that the symmetrical resonant frequency of one filter unit in the filter set is equal or almost equal to the anti-symmetrical resonant frequency of the other filter unit. chain.

[Brief explanation of the drawing]

Figure 1 shows a single structure that forms a filter unit and has two pairs of resonators, Figure 2 shows the equivalent circuit of the filter unit or dual according to Figure 1, and Figure 3 shows the frequency attenuation characteristics of the dual. Figure 4 shows a filter set consisting of two equal duals according to Figure 1 with capacitively coupled impedance, Figure 5 shows four resonant frequencies fr1.
y fr2 'fr3+fr4; FIG. 6 is a graph showing the relationship between frequency and the ratio of acoustic coupling coefficient to electrical coupling coefficient; FIG.
The figure shows the frequency attenuation characteristic of the circuit according to figure 4, where the symmetrical resonance frequency of one dual is almost equal to the antisymmetrical resonance frequency of the other, figure 8 shows the frequency attenuation characteristic of the actual filter set, and figure 9 shows the frequency attenuation characteristic of the circuit according to figure 4. FIG. 10 shows the frequency attenuation characteristics of the filter chains according to the invention, respectively, taken from the filter set according to the figure. In the figure, C is a capacitor and fr is a frequency.

Claims (1)

[Claims] 1 In a bandpass filter that passes frequencies within a predetermined frequency band, one or more filters
each of these filter sets consists of two electrically coupled filter units made of high grade piezoelectric material vibrating in a thickness-shear vibration mode, and each filter unit has two acoustic each filter unit has at least one common natural resonant frequency, and the acoustic coupling coefficient within the filter units of the set is equal to the electrical coupling coefficient between the filter units of the set. A filter naehn characterized in that it is at least 4 times as large.