JPH026667Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to the structure of a surface acoustic wave filter, and particularly relates to the structure of a surface acoustic wave filter that uses a TO can type package.

A surface acoustic wave filter has an input/output electrode of an interdigital electrode on one surface of a propagation medium having piezoelectric properties, converts an input electrical signal into a surface acoustic wave at the input electrode, propagates through the surface of the propagation medium, and outputs it. It converts the surface acoustic waves that reach the electrodes into output electrical signals. Utilizing its frequency amplitude characteristics and delay time characteristics in electrical-mechanical signal conversion, it is used as filters, delay lines, etc.

In this surface acoustic wave filter, not all of the input electrical signals applied to the input electrodes are converted into surface acoustic waves; some are converted into leaky bulk waves propagating inside the substrate, and some are propagated through space. It becomes a leaked electromagnetic wave. When these particles reach the output electrode and are converted into electrical signals, unnecessary signals are added to the output electrical signals, causing a large amount of noise that degrades the passband characteristics and guaranteed attenuation characteristics of the surface acoustic wave filter. It becomes a factor.

The present invention relates to a structure for shielding electromagnetic waves that cause deterioration of the characteristics of surface acoustic wave filters as described above, and thereby prevents electromagnetic waves from reaching the output electrode and being coupled to the output signal. It is intended to prevent

The noise level of the output signal due to electromagnetic waves increases as the frequency increases. It is said that the noise in phase with the main signal is mainly caused by capacitive coupling, and the noise in the opposite phase is mainly caused by mutual inductive coupling. The former is dominant below 100MHz. Therefore, when used as a filter, the output waveform near the attenuation pole at frequencies above and below the center frequency differs significantly from the designed value, and the amount of attenuation is insufficient.
In order to remove the effects of these electromagnetic waves, conventionally,
It has been considered to form a shield electrode on the propagation path of the surface acoustic wave, or to add a shield electrode arranged with a gap on the propagation path. However, it is difficult to achieve a sufficient shielding effect with these methods, and if the shield electrodes are made larger or more in number in order to improve the shielding effect, the surface acoustic wave filter becomes larger and costs increase. It will rise.

The present invention aims to solve the above-mentioned problems and provide a surface acoustic wave filter that has a simple structure and can provide the most effective shielding.

In the surface acoustic wave filter according to the present invention,
Using the most reliable TO can type package for surface acoustic wave filter elements,
The above objective is achieved by improving the structure of the TO can.

The surface acoustic wave filter of the present invention improves frequency attenuation characteristics by significantly attenuating the noise level relative to the output level of the center frequency.For example, when used in a 57MHz VIF filter for color TV, the noise level can be reduced by The purpose is to attenuate the wave by 70 dB or more from the output level of the center wave number and obtain a surface acoustic wave filter that meets the design value.

The surface acoustic wave filter according to the present invention was obtained through research on the factors that determine the strength of signals that cause noise coupled between input and output electrodes. FIG. 1 shows an equivalent circuit representing the noise level in a surface acoustic wave filter. Here, the capacitance Co indicates the capacitance of the interdigital electrode and its bus bar. Capacity Cs ₁
denotes the capacitance between the interdigital electrode and the stem, which is obtained as a function of the spacing t between the interdigital electrode and the stem and the spacing D between the input and output electrodes. The capacitance Cs ₂ represents the capacitance between the interdigital electrode and the cap and is obtained as a function of the spacings L and D between the interdigital electrode and the cap. Capacitance Cs' indicates the capacitance between input and output electrodes, and is obtained as a function of t, L, and D.

Ls indicates the inductance of the lead portion and finger. If Ls is small enough,
Assuming that the noise level shown in the above equivalent circuit is A, A changes so as to substantially satisfy A∞Cs'/Cs ₁ +Cs ₂ +C ₀ . In other words, it is proportional to the capacitance Cs' between the input and output electrodes, and inversely proportional to the sum of the capacitance Cs ₁ between the interdigital electrode and the stem, the capacitance Cs ₂ between the interdigital electrode and the cap, and the capacitance C ₀ between the interdigital electrode and the busbar. I found out.

In this way, when CS′ is made small and Cs ₁ , Cs ₂ , and C ₀ are made large, not only the noise component but also the entire signal component passes through to ground, but this is compared to the attenuation of the signal within the passband. Therefore, the degree of attenuation of the noise level signal in a region with a large amount of attenuation outside the passband becomes relatively large.

That is, A can be made smaller by reducing Cs' and increasing Cs ₁ , Cs ₂ , and C ₀ . however,
Since C ₀ is determined by the design of the interdigital electrode pattern, other Cs′, Cs ₁ ,
By changing Cs ₂ , A can be made smaller. Of course, the lead portion is taken into consideration so that Ls is sufficiently small.

As mentioned above, Cs', Cs ₁ and Cs ₂ are obtained as functions of t, L, and D, and the relationship between these variables is as follows. Regarding Cs′,
As t becomes smaller, as L becomes smaller, it decreases, and conversely, as D becomes smaller, it increases.
Regarding Cs ₁ , it increases as t becomes smaller,
Even if D is made smaller, it does not change much. Regarding Cs ₂ , it increases when L is decreased, and it does not change much even when D is decreased.

From the above, Cs′, which determines the noise level,
Determine the values of the three elements Cs ₁ and Cs ₂ , t, L,
By appropriately selecting the three variables of D,
The gist of the present invention is to lower the noise level, that is, to increase the amount of attenuation of the noise signal transmitted as an electromagnetic wave with respect to the main signal of the elastic wave.

FIG. 2 shows a front sectional view of an example of a surface acoustic wave filter according to the present invention. stem 1
A lead 11 is implanted in the stem 10, and a surface acoustic wave propagation substrate 12 is fixed onto the stem 10 with a conductive adhesive or the like. An input electrode 13 and an output electrode 14 are provided on the surface of this propagation medium. The input electrode 13, the output electrode 14, and the lead 11 are connected by a wire 15, and these are covered and sealed with a cap 16 by welding or the like. Here, the distance between the input electrode 13, the output electrode 14, and the stem 10 is t, the distance between the input electrode 13, the output electrode 14, and the cap 16 is L, and the distance between the input electrode 13 and the output electrode 14 is D.

As mentioned above, in order to reduce the noise level, it is necessary to reduce t and L and increase D. However, as D becomes larger, the propagation distance of the surface acoustic wave becomes longer, and the propagation medium must be made larger. Therefore, D
It is not preferable to make it too large. In order to reduce t, the propagation medium can be made thinner, but
In order to maintain mechanical strength, a certain degree of thickness is required, and at least two wavelengths of surface acoustic waves are required. Also, L is for wire bonding.
Approximately 300 μm is required. FIG. 3 shows the results of an experiment to find out how to set each value within these ranges to sufficiently lower the noise level. The horizontal axis represents L, and the vertical axis represents the amount of relative attenuation based on the maximum output of the surface acoustic wave.

Curve 21 shows an example when t is 0.7 mm and D is 533 μm, and in this case, even if L is made small, an attenuation of 70 dB cannot be obtained. Therefore, it can be said from this that D needs to be larger than t. In curve 22, t is 0.5 mm and D is 533 μm, and when L is large, sufficient attenuation cannot be obtained, but when L is 1 mm or less, attenuation of 70 dB or more is obtained.

Curve 23 shows that t is 0.7 mm and D is 877 μm, and that a sufficient amount of attenuation can be obtained even if L is 3 mm. D/t at this time≒1.25
In curves 24 and 25, the values are even higher. From this, D/t
It can be seen that when the value is 1.25 or more, a sufficient amount of attenuation can be obtained in the range where the value of L is up to 3 mm.

Although not shown in FIG. 3, if D is made larger than either t or L, the coupling between the input and output electrodes becomes smaller, and the degree of coupling between the input electrode and the stem and cap decreases. It has also been found that as the size increases, the shielding becomes more complete.

Further, FIG. 4 shows the frequency band characteristics of the 58 MHz VIF filter, with design values and experimental values, respectively. With respect to the design values shown by dotted lines, the thin solid line shows the characteristics of the conventional filter, and the thick solid line shows the characteristics of the filter according to the present invention.

Summarizing the above points, there are three conditions for lowering the noise level:

(1) L＜1(mm), D/t＞1 (2) 1(mm)≦L＜3(mm), D/t＞1.25 (3) D/t＞1, D/L＞1 or more If either of these conditions is met, it is possible to obtain attenuation of 70 dB or more, which is required for a normal filter. Therefore, it becomes easy to match the characteristics of the filter to the design values.

According to the present invention, a sufficient electromagnetic wave shielding effect can be obtained without increasing the distance between input and output electrodes. Furthermore, since the height of the cap is reduced, there is an advantage that the overall shape of the filter device can be made smaller.

[Brief explanation of drawings]

Fig. 1 is an equivalent circuit of noise of a surface acoustic wave filter, Fig. 2 is a front sectional view of an embodiment of the present invention, and Fig. 3 is a front sectional view of an embodiment of the present invention.
4 and 4 show characteristic diagrams of the surface acoustic wave filter according to the present invention. 10... Stem, 13... Input electrode, 14...
Output electrode, 16...cap.

Claims (1)

[Claims for Utility Model Registration] A surface acoustic wave propagation substrate is provided with input and output electrodes formed by interdigital electrodes on one surface, which is fixed to a metal stem connected to a ground terminal with a conductive adhesive, and the surface is connected to an air gap. In a surface acoustic wave filter using a can-type package, the surface acoustic wave filter is provided with a metal cap that is covered with a metal stem and is electrically connected to the metal stem, in which the distance between the input and output electrodes is D, the distance between the surface and the metal cap is L, A surface characterized in that, when the distance between the surface and the stem is t, the values of D, L, and t are determined so as to satisfy L<1 (mm) D>L, D>t. elastic wave filter.