JPS6298905A - Filter device - Google Patents

Abstract

PURPOSE:To obtain a desired filter characteristic by so designing that a temperature coefficient of a coil connected in parallel between an input or an output of a filter element and ground and a temperature coefficient of a capacitor connected in series with the input or the output of the filter element satisfy a special condition. CONSTITUTION:The temperature coefficient KL of the coil La connected in parallel between the input or the output of the monolithic crystal filter element 10 and ground and the temperature coefficient Kc of the capacitor Ca connected in series with the input or the output are designed to satisfy the relation of Kc=(1/KL)+ or -10%. When temperature rises, the inductance of the coil La is varied to the positive direction and the insertion loss of the filter is increased, while the capacitance of the capacitor Ca is varied to the negative direction and the insertion loss of the filter is decreased. That is, the variation in the filter characteristic is cancelled together and the range where the insertion loss variation is less than 0.2dB is the range Po, and the variation in the coil La and the capacitor Ca is within + or -0.1% respectively.

Description

[Detailed Description of the Invention] [4 Already Required] In a filter device including a monolithic crystal filter element and a capacitor and a coil connected to an input/output part of the filter element, the temperature coefficients of the capacitor and the coil to be inserted are set in a predetermined relationship. By regulating the filter characteristics, fluctuations in filter characteristics due to temperature changes are suppressed.

[Industrial application field]

The present invention relates to a filter device, and particularly to a filter device for high frequency bands using a monolithic crystal filter element.

The monolithic crystal filter has a steep cutoff characteristic and is easy to downsize and reduce cost, so it can be used in several M f
It is widely used as a narrow band filter in the frequency band above iz.

Further, with the development of digital transmission technology, the use of monolithic crystal filters as filters for timing extraction has been further expanded.

Such a filter device has a center frequency of 50M11z
If the problem exceeds this level, it becomes difficult to obtain desired characteristics with a single monolithic crystal filter element, so a capacitor and a coil are connected to the input/output section to cope with the problem.

Coil on monolithic crystal filter element.

When designing a filter device with a capacitor added,
It should be noted that the variation in element value due to temperature changes in the coil and capacitor is much larger than the variation in element value of a monolithic crystal filter element.

(Prior art) In the equivalent circuit diagram, a monolithic crystal filter element 10 includes a crystal resonator 1 cut in a desired cutting direction.
and a holder that holds the crystal resonator 1.

The crystal resonator 1 is provided with a pair of electrode films 21 that correspond to each other on the front and back surfaces in one semicircular area, and a pair of electrode films 2□ that correspond to each other on the front and back surfaces in the other semicircular area. The electrode film 21 and the electrode film 2□ are formed symmetrically opposite to each other about the center line, and one serves as the input side and the other serves as the output side, forming a filter.

The holder includes a pair of spring supports that are connected to the lead film portions of the respective electrode films 21 and 2□, and engage with symmetrical points on the peripheral edge of the crystal resonator 1 to sandwich the crystal resonator 1 therebetween. line 4
The supporting wire 4 is hermetically sealed and is composed of a holding plate 6 that passes through the supporting line 4 vertically.A case (not shown) is attached to the holding plate 6, and a crystal oscillator is attached. ) It is designed to seal J child 1.

In addition, the earth electrode part 3 is led out from the electrode films 21, 2□,
The structure is such that a ground terminal 5 is connected and connected to the outside below the holding plate 6.

In the equivalent circuit of this filter device, as shown in Figure 5, an oscillating circuit formed by an inductance L and a capacitor C6, and an oscillating circuit formed by an inductance L2 and a capacitor ff1cz are connected by a coupling capacitor C0.

Furthermore, the input/output terminal side includes the stray capacitance of the retainer in addition to the stray capacitance of the vibrator. There is $ free capacity cd.

The filter device shown in FIG. 4 has a center frequency of 50 M I
If it is about 1z or less, the filter has satisfactory filter characteristics as long as there are no major restrictions on the input/output impedance of the device circuit and the spurious at higher mode frequencies.

However, when the center frequency of the filter becomes about 50 MIIz or more, the capacitance ratio (Cd /C,) of the monolithic crystal filter element 10 becomes large due to the influence of the stray capacitance of the retainer, etc., and the monolithic crystal filter element 10 alone becomes It becomes difficult to configure the filter.

Therefore, as shown in the equivalent circuit diagram of Fig. 1, a coil La is inserted between the input/output section of the monolithic crystal filter element and the ground to reduce the apparent capacitance ratio of the oscillator, thereby constructing a filter. I have to.

On the other hand, the impedance of the filter output section is often required to have a predetermined low impedance due to requirements on the device circuit side.

For this purpose, a capacitor Ca (shown in FIG. 1) is connected to the input/output portion of the monolithic crystal filter element, and an impedance conversion circuit is provided as the above-mentioned coil La.

That is, as shown in FIG. 1, a filter device having a filter center frequency of about 50 Ml1z or higher has a monolithic crystal filter element and at least one of the input/output portions of the monolithic crystal filter element with a condenser C.
A and coil La are required to be connected as desired.

As described above, the filter characteristics of the filter device in which the coil La and capacitor Ca are inserted are as shown in FIG.

In FIG. 3, the horizontal axis shows frequency, the vertical axis shows the insertion and loss of coils and capacitors, and the center frequency of this bandpass filter is f. It is.

The curve shown by the solid line A is the filter characteristic expected at the time of design, and the solid line A is the center frequency of the passband ".±Δf)
It is a curve that is almost flat and has a small insertion loss, and 9 on both sides, which rises steeply and has a large insertion loss.

The curve shown by the dotted line B is the coil La and the capacitor Ca.
Because the element value of deviates from the design value within the tolerance,
It shows the filter characteristics of an actual filter device that deviates from the solid line A at several places and has unevenness within the passband.

That is, the dotted line B deviates from the solid line A near (ro-Δf) and has a smaller loss by 6 m2, a larger loss by 6 m1 near f0, and a smaller loss by 6 m3 near (f0+Δf).

This characteristic shows the case of the reference temperature, Δml.

Δmz and Δm3 are desirably small, and the deviation from the dotted line B to the solid line is within a predetermined value range.

[Problem that the invention seeks to solve]

However, as described above, the filter device constituted by the monolithic crystal filter element and the capacitor Ca and coil La connected as desired has the problem that the filter characteristics fluctuate due to temperature changes, as described below.

That is, while the monolithic crystal filter element 10 is composed of a mechanical vibrator, the coil La,
Because capacitor Ca is made of ferrite, ceramic, etc., coil La and capacitor C
The element value fluctuation due to temperature change etc. of a is as large as 10 to 100 times the element value fluctuation of the monolithic crystal filter element 10.

Therefore, variations in insertion loss of the capacitor and coil occur with temperature changes, and there is a possibility that desired filter characteristics may not be obtained.

[Means for solving problems]

In order to solve the above-mentioned conventional problems, the present invention, as shown in FIG. In the filter device, a coil La is connected in parallel between the input part or the output part of the monolithic crystal filter element 10 and the ground.
The temperature coefficient KL of the capacitor Ca and the temperature coefficient Kc of the capacitor Ca connected in series to the input section or the output section are set to have a relationship of Kc = (1/KL) ±10%.

[Effect]

Since the temperature coefficient KL of the coil La and the temperature coefficient Kc of the capacitor Ca are almost inversely proportional, the coil La has a temperature coefficient of Kc - (1/KL) ±10%, By selecting the capacitor Ca, fluctuations in filter characteristics caused by fluctuations in each element value due to temperature changes are canceled out.

That is, fluctuations in filter characteristics due to temperature changes can be suppressed within a desired range.

〔Example〕

FIG. 1 is an equivalent circuit diagram of a filter device according to the present invention, and FIG. 2 is a sensitivity characteristic diagram of the effect of fluctuations in element values due to temperature changes in the coil and capacitor on insertion loss.

In FIG. 1, an oscillating circuit formed by an inductance L1 and a capacitor C-1 and an oscillating circuit formed by an inductance L2 and a capacitor C2 are connected by a coupling capacitor lc, and constitute a monolithic crystal filter element 10. A stray capacitance Cd exists in the input/output section.

Coils La each having a temperature coefficient KL selected as desired are connected in parallel between the input section and the ground of the monolithic crystal filter element 10, and between the output section and the ground.

Furthermore, capacitors Ca each having a temperature coefficient Kc selected as desired are connected in series to the input section and the output section of the monolithic crystal filter element 10, respectively.

As mentioned above, the filter characteristic of such a filter device is indicated by the dotted line B in FIG.

Within the temperature range in which the filter device operates, the element value of coil La with temperature coefficient KL fluctuates due to temperature changes, and therefore the value at which the insertion loss fluctuates, and the element value of capacitor Ca with temperature coefficient Kc fluctuates due to temperature changes. However, the relationship between the values at which the insertion loss varies can be illustrated as shown in FIG. 2.

In Fig. 2, the horizontal axis shows the amount of variation (expressed in %) in the element value of the coil La, and the vertical axis shows the amount of variation (expressed in %) in the element value of the capacitor Ca.
(expressed in %).

In FIG. 2, for example, when the temperature rises, the element value of the coil La changes in the positive direction, which acts to increase the insertion loss of the filter. However, on the other hand, it is shown that the element value of the capacitor Ca changes in the negative direction, which acts to reduce the insertion loss of the filter.

That is, fluctuations in filter characteristics caused by fluctuations in the values of the respective elements due to temperature changes act in a direction that cancels each other out.

The range in which the insertion loss fluctuation amount is smaller than 0.2 dB is a range P0 indicated by a substantially elliptical shadow line downward to the right.

In this range P0, the fluctuations (2) of the coil La and capacitor Ca are each within ±10%.

In a range P1 shown in a diagonal band shape on both sides of the range Po, the insertion loss fluctuation amount is 0.2 dB to 0.4 dB. Further, in a diagonally band-shaped range P2 on both sides of range 1) 1, the insertion loss fluctuation amount is 0.4 d [1 to 1 dB].

Therefore, the present invention provides a temperature coefficient KL of the coil La;
The temperature coefficient Kc of capacitor Ca is Kc = (1/
Since the relationship is selected to have a relationship of KL ) ±10%, fluctuations in filter characteristics due to temperature changes can be suppressed to a predetermined small value of 0.2 dB or less.

〔Effect of the invention〕

As explained above, the present invention regulates the temperature coefficients of the capacitor and coil additionally connected to the monolithic crystal filter element to a predetermined relationship, thereby suppressing fluctuations in filter characteristics due to temperature changes to below a desired value. This has an excellent practical effect.

[Brief explanation of drawings]

Fig. 1 is an equivalent circuit diagram of the filter device according to the present invention, Fig. 2 is a sensitivity characteristic diagram of the effects of changes in element values due to temperature changes in the coil and capacitor on insertion loss, and Fig. 3 is a diagram of the sensitivity characteristics of the coil and capacitor. Fig. 4 is a one-view diagram of a conventional filter device used when the center frequency is approximately 50 Mllz or less, and Fig. 5 is an equivalent circuit diagram of the filter device shown in Fig. 4. It is. In the figure, 1 is a crystal oscillator, 2. , 22 is an electrode film, 4 is a support wire, 5 is a ground terminal, La is a coil,
Ca is a capacitor, Cd is a stray capacitance, and 10 is a monolithic crystal filter element.
Figure 32 sword

Claims (1)

[Claims] A monolithic crystal filter element (10) and at least one of the input and output parts of the filter element (10),
In a filter device in which a capacitor (Ca) and a coil (La) are connected, the temperature coefficient KL of the coil (La) connected in parallel between the input part or the output part of the filter element (10) and the ground. and the temperature coefficient Kc of the capacitor (Ca) connected in series to the input section or the output section of the filter element (10) have a relationship of Kc=(1/KL)±10%. filter device.