JPS6145406B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an intermediate frequency surface wave filter for a vestigial sideband receiver. The present invention will be explained using the most popular color television receiver as a typical example of a vestigial sideband receiver, but the same thing can be applied to VSB receivers other than color televisions. It goes without saying that it can be done.

Figure 1 shows the ideal frequency characteristics required for video intermediate frequency filters for color television receivers in Japan. In FIG. 1, _p corresponds to the video carrier frequency, _s corresponds to the audio carrier frequency, and _Q corresponds to the adjacent audio carrier frequency. FIG. 2 shows an experimental example of a surface wave filter (filter A) designed to approximate the frequency characteristics shown in FIG. 1 as closely as possible. In this filter, the input electrode is an apodized type with 76 logarithmic pairs, and the output electrode is a normal type with 6 logarithmic pairs, and the phase characteristic is approximately linear. Furthermore, as the piezoelectric substrate, a LiTaO ₃ X-cut 112° propagation was used. FIG. 3 shows the experimental results of the pulse rise characteristics of the filter having the characteristics shown in FIG. 2.
As can be seen from FIG. 3, a spurious signal of about -26 dB occurs at the rising edge of the pulse and before the preshoot. This spurious can be discerned on a television's cathode ray tube, and is not desirable in practice. The cause of spurious noise is that the above filter does not have ideal VSB filter characteristics.
Particularly in FIG. 2, a major cause is that the level near _P +0.75 is about -4 dB lower than the desired value. In order to bring the level around _P +0.75 close to the desired value, it is necessary to increase the number of input electrode pairs to 76 or more, which has the drawback of increasing the electrode shape and, as a result, increasing the filter chip size. The present invention provides a filter that improves pulse response characteristics without increasing the input logarithm. That is, the absolute value of the short-circuit transmission admittance of the filter is set to take a maximum value between ±(0.7 to 1.6) MHz with the carrier frequency as a reference. For example, the maximum response point of the frequency response characteristic of the filter is set between ±(0.7 to 1.6) MHz with respect to the carrier frequency. This effect can in principle be explained by the video frequency after detection ()
This is equivalent to flattening the characteristic (so-called direct detectability), and therefore the detection characteristic corresponding to FIG. 4 is almost the same as the detection characteristic of the filter having the desired frequency characteristic shown in FIG. The differences between FIG. 1 and FIG. 4 are as follows.

In other words, the position of the frequency at which the insertion loss of the filter is minimized (hereinafter referred to as the maximum response frequency) is 57MHz relative to the carrier frequency of 58.75MHz in Figure 1.
The difference frequency is around _P −1.8 MHz, but in Figure 4, the carrier frequency is 58.75 MHz and _P − 57.8 MHz.
Exists around 0.9MHz. Regarding the level of _P +0.75MHz, Figure 4 is - compared to Figure 1.
The level is about 4 dB lower, and this is the reason why the characteristics shown in FIG. 4 can be achieved with a small logarithm when constructed using a surface wave filter.

A surface wave filter (Filter B) designed with weighting to approximate the frequency characteristics shown in Figure 4.
An experimental example is shown in FIG. The input, output logarithm, phase characteristics of filter B and the piezoelectric substrate used are as follows:
It is exactly the same as filter A, and the only difference is the weighting of the input electrodes. From Figures 2 and 5,
When comparing the frequency characteristics of filter A and filter B, the most different point is the position of the maximum response frequency, and other characteristics are almost the same for both filters.

FIG. 6 shows the pulse response characteristics when filter B is used. The spurious level before the preshoot is less than -43dB, and the filter A
When compared with the spurious at the pulse rise shown in FIG. 3, it can be seen that this is greatly improved.

As shown in the above explanation, it has been found that by bringing the maximum response frequency of the filter close to the frequency of the carrier wave, for example, the video carrier wave, the pulse response characteristics can be significantly improved. Based on various computer simulation and experimental results, the position of this maximum response frequency is ±1.6 with respect to the video carrier frequency.
It was found that good pulse response characteristics could be obtained at a position of ~±0.7MHz.

That is, if the position of the maximum response frequency deviates from the carrier frequency by more than ±1.6 MHz, the spurious response at the rise of the pulse response becomes large, making it difficult to put it into practical use. A specific numerical example is as follows. If the difference frequency between the maximum response frequency of the filter in question and the carrier frequency is △, |
The spurious level when △｜=1.6MHz is -
It becomes 34dB. Also, at |△|=1.7MHz, approximately -
3 dB, |△|=1.8 MHz, the filter characteristics are approximately as shown in FIG. 2, and as mentioned above, the spurious level in this case is -26 dB. It is generally known that in image transmission using the vestigial sideband method, it is desirable that the spurious level of the filter be -34 dB or less. This corresponds to the level at which the spurious in question can be clearly discerned by the human eye on a cathode ray tube. From this, when |△|1.6MHz, the permissible spurious level level of −34 dB or less is satisfied.

On the other hand, if the position of the maximum response frequency is brought closer than ±0.7 MHz to the carrier frequency, there will be a disadvantage that the number of electrode pairs will increase. To give a specific example, if |△|=0.6MHz, |△
Compared to the case of |=0.7MHz, the number of required electrode pairs increases by about 15%. An increase in the number of electrode pairs means an increase in chip length, which is not preferable in filter fabrication.

From the above, the range of ±1.6 to ±0.7 MHz is the range in which spurious signals can be suppressed in a practical sense. In addition, within this range, the input logarithm of the surface wave filter can be realized with a smaller number of input logarithms than the input logarithm required to achieve the ideal VSB characteristic (Figure 1), which is extremely difficult in practical situations. It can be said that it is an effective means.

[Brief explanation of the drawing]

Fig. 1 is a frequency amplitude characteristic curve diagram of a conventionally required video intermediate frequency filter, and Fig. 2 is a surface wave filter designed to approximate Fig. 1.
A) is a frequency characteristic curve diagram, FIG. 3 is a pulse rise characteristic curve diagram of FIG. 1, FIG. 4 is a frequency amplitude characteristic curve diagram for explaining an embodiment of the surface wave filter of the present invention, and FIG. FIG. 4 is a frequency characteristic curve diagram of a surface wave filter (Filter-B) designed to approximate it, and FIG. 6 is a pulse rising characteristic curve diagram of FIG. 5.

Claims (1)

[Claims] 1 In a surface wave filter that obtains residual sideband filter characteristics by weighting either the input or output interdigital electrodes provided on the surface of the piezoelectric substrate to change the electrode crossing width, the short-circuit transfer admittance of the filter is The absolute value is −1.6 to −0.7MHz (or +
1. An intermediate frequency surface wave filter for a vestigial sideband receiver, characterized in that electrode weighting is limited so as to take a maximum value between 0.7 and +1.6 MHz).