JPH0218596Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention is a surface acoustic wave filter (hereinafter referred to as
Regarding the improvement of the printed circuit board on which the SAW filter (hereinafter abbreviated as "SAW filter") is attached, in particular the generation of unnecessary signals due to electromagnetic induction between the input terminal and output terminal of the SAW filter (hereinafter referred to as "feedthrough", hereinafter abbreviated as FT).
This paper proposes a printed board suitable for reducing this.

In recent years, SAW filters that utilize surface acoustic waves propagating on dielectric materials such as piezoelectric ceramics have begun to gain attention as new high-frequency filter elements.
Among them, those specifically designed for video intermediate frequency circuits of television receivers are already commercially available.

That is, FIGS. 1A and 1B show the appearance and back side of this type of SAW filter for a video intermediate frequency circuit.

Such a SAW filter is advantageous because it can achieve the frequency characteristics of a video intermediate frequency circuit, which conventionally required about five coils, with just one filter, and requires no adjustment.However, the filter itself is small and Because the spacing between input terminal 1, ground terminal 2, 2', and output terminal 3, 3' is narrow (approximately 5 mm), electromagnetic coupling occurs between the input and output terminals, and output terminals 3, 3' An unnecessary signal due to electromagnetic coupling appears before the normal signal propagated as a surface acoustic wave from the input terminal 1 arrives. That is,
When the pulse modulated wave a shown in Fig. 2 is applied between terminals 1 and 2, in addition to the regular signal b, the above-mentioned electromagnetic coupling, ie, feedthrough (FT) occurs between terminals 3 and 3'.
Therefore, an unnecessary signal c will appear. This unnecessary signal c appears when the propagation speed of electromagnetic waves is approximately 3
This is because surface acoustic waves are slower at only several ^{10 5} cm/sec, compared to 10 10 cm/sec. The unnecessary signal c caused by such FT appears as a front ghost in a television receiver, resulting in a decrease in image quality. In addition, in Figure 2, after the normal signal are an unnecessary signal d due to reflection at the end face of the piezoelectric body, a signal e due to an unnecessary wave passing through the piezoelectric band called a bulk wave, and a tribble transition echo (TTE). An unnecessary signal f due to reflection within the output electrode portion also appears.

By the way, among the above unnecessary signals c to f, especially
Conventionally, in order to eliminate the effect caused by FT, a shield plate was provided on the back side of the printed circuit board on which the SAW filter was attached to shield between the input terminal and output terminal of the filter. One effective method is proposed in Utility Model Application Publication No. 53-29247.

However, since such a method requires the preparation of a shield plate and the work to attach it, it cannot be said to be a good idea in terms of cost and workability.

Therefore, the present invention takes this point into consideration and reduces the unnecessary signals caused by the above-mentioned FT by devising the shape of the conductive foil pattern of the printed board to which the SAW filter is attached. An example of this will be explained. Note that the unnecessary signals d to f that appear after the regular signal b in Fig. 2 appear as ghosts after the television receiver.
This can be electrically eliminated or reduced by considering the degree of matching between the SAW filter and the video intermediate frequency circuit, so the present invention eliminates the need for FT, which is difficult to eliminate by electrical methods. It is intended only for signal reduction.

Figure 3 shows SAW on a printed board on which this invention was implemented.
This is a view from the back side of the printed board with the filter attached. In this figure, a printed circuit board 4 is formed with an input side conductive foil 6 and an output side conductive foil 7, 7' which are soldered to the input terminal 1 and output terminals 3, 3' of the SAW filter 5 , respectively. Through these conductive foils, the terminals are connected to a tuner of a television receiver, an IC for a video intermediate frequency circuit, and the like.
A conductive foil 8 for grounding is also formed on the printed board 4 , and a pair of grounding terminals 2, 2' of the SAW filter 5 and the IC for the video intermediate frequency circuit are mounted on this conductive foil. The grounding pins, etc. of the SAW filter 5 are connected to each other, but in this case, the grounding conductive foil 8 is not continuous from side to side, especially in the part between the input terminal 1 and the output terminals 3 and 3' of the SAW filter 5 . It is characterized in that it is divided into two parts 9 and 9', and is formed so as to be continuous at a sufficient distance from the SAW filter 5 .

FIG. 4 shows the response characteristics of the SAW filter configured in this manner. That is, when a pulse modulated wave in the video intermediate frequency band is applied between terminals 1 and 2 in Fig. 3, the signal waveform appearing between terminals 3 and 3' is actually measured. 3 shows the case where the grounding conductive foil 8 is divided and formed, and the broken line shows the case where the grounding conductive foil 8 is not divided and formed continuously as shown in FIG. 5. The structure shown in FIG. 5 is exactly the same as that shown in FIG. 3, except that the grounding conductive foil 8 is formed continuously.

According to FIG. 4, the unnecessary signal c due to FT is reduced by about 4 dB when the grounding conductive foil 8 is formed separately as shown in FIG. 3, compared to when it is formed continuously as shown in FIG. It can be seen that it is attenuated as much as possible.

Further, in FIG. 4, symbols a to f represent the same signals as in FIG. 2, respectively.

It should be noted that the reason why unnecessary signals due to FT are reduced when configured as shown in Fig. 3 is not necessarily clear (however, this effect was confirmed through experiments as shown in Fig. 4), but as follows. It can be considered.

That is, in either case of FIG. 3 or FIG. 5, the grounding conductive foil 8 is not completely grounded for relatively high frequencies such as the video intermediate frequency band. In this case, the FT
is mainly connected to the grounding conductive foil 8 and the output terminals 3, 3'.
This will occur between the side conductive foils 7 and 7',
In this case, if the configuration is as shown in FIG. 3, a part of the signal component on the grounding conductive foil 8 will be transmitted along the conductive foil 8 to the area between the input terminal 1 and the output terminals 3 and 3'. After being slightly attenuated by bypassing (the distance passing through the grounding conductive foil becomes longer), it reaches the vicinity of the conductive foils 7 and 7' on the output terminal side.
In this case, FT occurs between the grounding conductive foil 8 and the conductive foils 7, 7' on the output terminal side via the part between the input terminal 1 and the output terminals 3, 3' in the case of Fig. 5. F.T.
This is because it is considered to be weaker than the

As described above, the printed board of the present invention can be used with SAW
When forming the grounding conductive foil to be connected to the grounding terminal of the filter, by forming the grounding conductive foil to be separated at a portion between the input terminal and the output terminal of the filter, Since the structure is designed to reduce the feedthrough that occurs between the ground conductive foil and the conductive foil on the output terminal side, there is no need to separately install a shield plate to prevent feedthrough, which reduces cost and workability. It is advantageous for use in television receivers and the like that use SAW filters.

Although the above explanation has been about the use of SAW filters in the video intermediate frequency circuit of television receivers, this invention can also be applied to other general applications.
Needless to say, it can be applied to printed boards for mounting SAW filters.

[Brief explanation of drawings]

Figure 1 A and B show the SAW filter, Figure 2
The figure is a diagram explaining the response characteristics of the SAW filter, Figure 3 is a diagram showing the state in which the SAW filter is attached to the printed board on which the present invention is implemented, from the back side, and Figure 4 is the response characteristic in the case of Figure 3. The measured waveform diagram, FIG. 5, is a diagram similar to FIG. 3, showing a printed board before implementing the present invention. 1: Input terminal, 2, 2': Ground terminal, 3, 3':
Output terminal, 4 : Printed board, 5 : SAW filter,
8: Conductive foil for grounding.

Claims (1)

[Scope of utility model registration request] Forming a ground conductive foil connected to the ground terminal of the filter on the back side of the printed board so as to substantially surround at least one of the conductive foils connected to the input terminal and output terminal of the surface acoustic wave filter, respectively, This grounding conductive foil is separated at a portion between the input and output terminals, and is continuous at a portion away from the input terminal and output terminal to the extent that unnecessary signals passing on the grounding conductive foil are sufficiently attenuated. A printed board for mounting a surface acoustic wave filter, characterized by forming the above-mentioned conductive foil for grounding.