JPS5824973B2 - Dansei Hyōmen Hagata Filter - Google Patents

Description

[Detailed description of the invention] The present invention relates to a surface acoustic wave filter.

Video synchronous detection circuits are being introduced as video detection circuits at intermediate frequencies in television receivers.

This video synchronous detection circuit generally has a configuration in which a video carrier wave component obtained from a video intermediate frequency band using a narrowband LC filter is applied to a transistor to perform switching.

Regarding the AFC circuit, a carrier wave having a phase difference of π/2 radian with respect to the video carrier component used for video detection is extracted using an LC filter, and this is used for switching of a circuit having the same configuration as video coherent detection. It is applied to a transistor to perform phase synchronized detection.

Since these detailed circuit configurations and operations are well known, they will be omitted here.

Such a synchronous detection circuit requires an LC resonant circuit tuned to the video carrier frequency, but this is a very serious obstacle to simplifying the circuit and implementing it into an IC.

Therefore, if you use a surface acoustic wave filter to combine the image and AF
If C synchronous detection can be realized, the circuit can be simplified and integrated into an IC.

Therefore, an object of the present invention is to provide a surface acoustic wave filter that can achieve synchronous detection of video and AFC.

FIG. 1 is a block diagram of an intermediate frequency video detection circuit using an acoustic wave type filter of the present invention.

That is, the VIF signal a supplied to the input terminal is
The signal is amplified by an amplifier 1 and supplied to a surface acoustic wave element 2.

When this surface acoustic wave element 2 outputs a detected wave signal, a video detection output C is obtained.

This output C is supplied as a detection signal d through an amplifier 3 and a limiter 4 to a video detection circuit 5 along with outputting a detection signal.

This detection circuit 5 can perform synchronous detection and obtain a detected video signal.

In FIG. 1, if the video detection output C is a video carrier output and the video detection circuit 5 is a phase detection circuit, the circuit in FIG. 1 becomes a frequency discrimination circuit for AFC.

FIG. 2 is a configuration diagram of the surface acoustic wave filter 2 used in FIG. 1.

That is, the broadband filter input electrode 6 is configured to have intermediate frequency filter characteristics in combination with the output comb-shaped electrode 7, and is supplied with the input signal a amplified by the VIP amplifier 1.

The comb-shaped electrodes 8 form a narrow band filter whose center frequency is the image carrier frequency, and its output is provided as a detection signal.

When obtaining a video carrier signal used for AFC synchronous detection using the output comb-shaped electrodes 8, these electrodes 7 and 8
The positional deviation L is determined as follows.

The center frequency f of the output characteristic having the frequency response shown in FIG.

In this case, the relative phase difference between the electrodes 7 and 8 is π/2 as shown in FIG. 4, and the frequency discrimination range required for AFC is f.

If the relative phase difference is not within π/2 at ±Δf,
In AFC detection, multiple center points of the discrimination frequency are generated, so that the normal function cannot be achieved.

From the above conditions, λ.

:Wavelength of surface acoustic wave at fo (image carrier frequency) Wavelength of surface acoustic wave at λ”fo+Δf Since the relative phase difference due to the frequency shift is expressed as N≦fo/4Δf N: Positive integer becomes.

The case where a video carrier wave signal used for video synchronous detection is obtained by the comb-shaped electrode 8 is also performed in the same manner as described above.

Here, Δf is at least 2 of the band of the intermediate frequency filter.
It is advisable to choose twice as many.

Next, when configuring the intermediate frequency filter of a conventional television receiver with a surface acoustic wave filter, it is coupled to the front stage of the intermediate frequency amplifier to obtain its characteristics, and in order to obtain the video carrier frequency, an LC resonant circuit is used. In a configuration using a surface acoustic wave filter, a difference in temperature characteristics between the surface acoustic wave filter and the LC resonant circuit causes a deviation between the center frequency of the LC resonance and the image intermediate frequency characteristics of the surface acoustic wave filter.

This problem can be solved by forming both the filter for obtaining the video carrier signal and the intermediate frequency filter using surface acoustic wave filters on the same substrate.

FIG. 5 shows a surface acoustic wave filter constructed in this manner, and FIG. 6 shows a frequency discrimination circuit for intermediate frequency image detection and AFC using the surface acoustic wave filter.

In FIG. 6, a VIF signal e supplied to an input terminal is amplified by a VIF amplifier 9 and supplied to a surface acoustic wave filter 10.

The filter 10 provides a test wave signal output f, a video carrier output g for video detection, and a video carrier output for phase detection.

The output g passes through the amplifier 11 and limiter 12 and becomes the detected signal i.
, and is supplied to the video detection circuit 13 together with the detected wave signal output f.

The output passes through the amplifier 14 and limiter 15 and becomes the detected signal j.
and is supplied together with the output f to the frequency discrimination circuit 16 for phase detection.

The video detection circuit 13 and the frequency discrimination circuit 16 perform synchronous detection to obtain a detected video signal and a control signal for AFC.

Next, in FIG. 5, the input electrode 17 of the surface acoustic wave filter 10 is supplied with an amplified intermediate frequency signal.

The comb-shaped electrode 17 and the comb-shaped electrode 18 form an intermediate frequency filter.

Further, the comb-shaped electrodes 19 and 20 are formed to have filter characteristics in which the center frequency is the image carrier frequency in a narrow band.

When obtaining a detection signal for video detection with the electrode 19, the electrodes 18 and 19 are L-λ.

The electrodes 18 and 20 are arranged offset by N
Ten-! It is sufficient to place them offset by -).

At this time, the output of the electrode 18 is the test wave signal f1 from the electrode 19.
Then, the video carrier wave signal g1 for video detection is used at the electrode 20 to obtain the video carrier wave signal for AFC detection.

As described above, the surface acoustic wave filter of the present invention can achieve synchronous detection of video and AFC, and can simplify the circuit and implement IC.

Furthermore, since the intermediate frequency filter and the video carrier extraction filter are formed on the same substrate, it is possible to eliminate relative changes in characteristics between the filters due to temperature changes.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an intermediate frequency video detection circuit using the surface acoustic wave filter of the present invention, FIG. 2 is a block diagram of an embodiment of the surface acoustic wave filter of the present invention, and FIG. Fig. 4 is a diagram explaining its characteristics, Fig. 5 is a block diagram of another embodiment of the present invention, and Fig. 6 is a block diagram of a frequency discrimination circuit for intermediate frequency video detection and AFC using the filter of Fig. 5. It is a diagram. 2.10...Surface acoustic wave filter, 6.17
...Input electrode? , 8.1B, 19.20...
...Output electrode.

Claims (1)

[Scope of Claims] 1. A first electrode having a wideband filter characteristic, a second electrode having a wideband filter characteristic whose center frequency is located within the frequency band, and a third electrode having a narrowband filter characteristic; the first and second electrodes form a broadband surface acoustic wave filter; the first and third electrodes form a narrowband surface acoustic wave filter; and a surface acoustic wave filter formed on the same substrate so as to have a propagation phase difference of an integral multiple of 2π radians or an integral multiple of 2π radians plus π/2 radians. 2 further comprising a fourth electrode having narrowband filter characteristics with a center frequency within the frequency band of the first electrode,
2. The surface acoustic wave filter according to claim 1, wherein the fourth electrode is formed on the same substrate so as to have a propagation phase difference of π/2 radians from the third electrode at its center frequency.