JPS5944803B2 - Surface acoustic wave filter for channel selection - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave filter for channel selection used in an automatic channel selection device of a television receiver.

Recently, an automatic channel selection device shown in FIG. 1 has been proposed.

In FIG. 1, an electronically tuned tuner includes a local oscillator 2 whose oscillation frequency is controlled by a control voltage, and 3 is a comb-shaped filter exhibiting a comb-shaped characteristic with a predetermined frequency interval corresponding to the channel interval of the television receiver. ,
4 is a detector for extracting the amplitude component of the signal output from the comb filter 3; 5 is an amplifier for amplifying the output of the detector 4; 6 is a detector for counting the number of times the output of the amplifier 5 reaches a peak. , and a programmable counter with functions such as up-counting, down-counting, and presetting; 7 is a channel number input device for setting the desired channel; 8 is a code for the channel number input to the channel number input device 7; An encoder 9 is a voltage sweep circuit that supplies the local oscillator 2 with a voltage that gradually increases or decreases the local oscillation frequency from the reference frequency.

This automatic channel selection device counts the number of times the local oscillation frequency corresponding to each channel is passed when sweeping the local oscillation frequency, and stops sweeping the local oscillation frequency after counting a predetermined number of times. It is.

If the comb filter 3 of such an automatic tuning device is realized by an LC filter consisting of a coil and a capacitor, there are disadvantages such as a large number of parts, a complicated circuit, and many adjustment points.

It is also possible to configure the comb filter 3 with a surface acoustic wave filter instead of an LC filter.

However, for example, in the case of a Japanese television receiver, the variation range of the local oscillation frequency is as wide as 150 to 824 MHz, so the fractional bandwidth required for the comb filter 3 is approximately 140%, whereas the normal Since the surface acoustic wave filter has a fractional bandwidth of only several tens of percent, it is difficult to realize the comb filter 3 with a surface acoustic wave filter.

SUMMARY OF THE INVENTION An object of the present invention is to provide a surface acoustic wave filter for channel selection that eliminates the drawbacks of the prior art described above and can obtain comb-shaped characteristics in a wide frequency band.

In the present invention, a plurality of surface acoustic wave filters exhibiting comb-shaped characteristics are provided in a relatively narrow frequency band obtained by dividing a wide frequency band, and these surface acoustic wave filters are connected in parallel.

The present invention will be explained below using FIGS. 2 to 4.

The table shows the local oscillation frequencies of Japanese television receivers, and the frequency band is the low band of the VHF band (1
Channel band consisting of 3 channels), channel band consisting of high band of VHF band (4 to 12 channels), U
It consists of three channel bands: the HF band (channels 13 to 62).

FIG. 2 is a plan view showing an embodiment of the surface acoustic wave filter for channel selection according to the present invention.

In FIG. 2, transducers 13 to 24 are provided on a piezoelectric substrate, and these transducers 13 to 24 generate surface acoustic waves when receiving electrical signals, and conversely, when receiving surface acoustic waves, sometimes generate electrical signals. do.

The transducers 13, 16, 19.degree. 22 act as input transducers and convert the electrical signals supplied between the input terminals 11.degree. 11' into surface acoustic waves.

Also, converters 14.15, 17, 18.20. 2
1.23, 24 act as output transducers, and output electrical signals obtained by converting surface acoustic waves from input transducers 13, 16, 19, 22 between output terminals 12, 12'.

25 is a shielding pattern for electrically shielding the input side and the output side on the piezoelectric substrate 10.

The input transducer 13 and the output transducer 14.15 connect channel borrowing comb filters of 1 to 3 channels to the input transducer 1.
6. The output converter 17.18 connects the channel borrowed comb filter with 4 to 12 channels to the input converter 19, and the output converter 20.21 connects the channel borrowed comb filter with 13 to 37 channels to the input converter 22 and the output converter. 23.
24 constitute channel borrowing comb filters of 38 to 62 channels, respectively.

Electrodes 13A, 13B of input transducer 13, output transducer 14
electrodes 14A, 14B, electrode 15A of the output converter 15,
The electrode spacing of 15B is determined to respond to a frequency of 150 to 162 MHz, and the spacing of output transducers 14 and 15 is such that the reciprocal of the difference in delay time of the electrical signals taken out from output transducers 14 and 15 is 6 MHz. It is determined as follows.

The configurations of the comb filters borrowed from other channels are similar, but to explain the configuration of each transducer in more detail, transducers 13, 14, and 15 are normal type with 10 pairs (20 electrodes) and have a response center frequency of is determined to be 156 MHz, and the center-to-center distance of output transducers 14 and 15 is 156 MHz.
The difference in delay time of the electrical signals extracted from 15 is 167
It is set so that it becomes 0S.

Since the output converters 14 and 15 are connected in parallel with a delay time difference of 167 ns, the frequency characteristic is Δt
= m = 6 It becomes a comb-shaped characteristic in which a peak appears every Mn2.

Channel band converter 16 consisting of high band of VHF band
, 17, 18 are normal forms of 5.5 pairs (11 pairs), the response center frequency is set to 252 MHz, and the output converters 17 and 1
The delay time difference between the electrical signals taken out from 8 is 500ns.
This comb filter is set to 2MHz.
It exhibits a comb-shaped characteristic in which a peak appears every time.

There are 4 pairs (8 channels) of channel borrowing converters 19, 20, and 21 consisting of UHF low band (channels 13 to 37).
The response center frequency is set to 602MHz in the normal form of
The delay time difference between the electrical signals taken out from the output transducers 20 and 21 is set to 500 ns, which allows 2 MH
A comb-shaped characteristic in which a peak appears for each z is obtained.

The channel borrowing converters 22, 23, and 24, which consist of high bands (channels 38 to 62) of the UHF band, are in the normal form of 5 pairs (10 channels) with a response center frequency set to 752 MHz, and are taken out from the output converters 23 and 24. The delay time difference of the electrical signals to be transmitted is set to 500 S, which results in a 2 MHz
A comb-shaped characteristic in which a peak appears every time is obtained.

The comb filters that respond to the frequencies of the respective channel bands described above are connected in parallel, thereby obtaining a comb filter that responds over a wide frequency band.

In other words, since a wide frequency band is divided and only a relatively narrow frequency band is required to respond, a surface acoustic wave filter with a small fractional bandwidth can be used.

Moreover, since the comb filters that respond to each narrow frequency band are formed by surface acoustic wave filters that exhibit large impedance outside the resonant frequency, a frequency band switching device is required when each comb filter is formed by an LC filter. Broadband characteristics can be obtained by simply connecting comb filters in parallel without using .

In Figure 2, the output transducers 14, 15, 17, 18°20
．． 21, 23, 24 are all input converters 13゜16.1
9, 22, but output transducers 14 and 15, 17 and 18, 120 and 21
．． 23 and 24 are input transducers 13, 16, 1, respectively.
9 and 22 may be placed between them.

However, in the case shown, the chip size can be made smaller, and therefore the manufacturing cost can be lowered, and the converters 13 to 2
4 and terminals 11, 11'.

This is desirable because direct coupling due to cross-linking of conductors connecting 12 and 12' does not occur.

FIG. 3 is a frequency characteristic diagram showing the comb characteristics of the comb filter shown in FIG.

In Fig. 3, a part of the peak characteristic 25 is omitted, only showing the envelope connecting the peak values, but in reality, the peak characteristic 25 as shown by the solid line repeats along the frequency axis. It is food.

When using a piezoelectric substrate 3 having a surface wave velocity of about 3500 m/s (for example, LiTa0s), 6×
The size is about 5n.

In order to obtain a frequency accuracy of about 100 KHz, the electrode arrangement requires an accuracy of about 0.2 μm, which is fully possible with current manufacturing technology and allows the desired characteristics to be obtained without adjustment. be able to.

FIG. 4 is a plan view showing another embodiment of the surface acoustic wave filter for channel selection according to the present invention.

In the comb filter shown in Figure 4, the VHF band low band comb filter and high band comb filter share the same propagation path, and the UHF band low band comb filter and high band comb filter share the same propagation path. The transducers 13 to 24 are arranged so as to have the same propagation path.

Therefore, compared to the comb filter shown in Fig. 2, the chip
The size can be reduced and the manufacturing price can be reduced.

In FIG. 2 and FIG. 4, all the transducers 13 to 24 are normal type electrodes, but for example, the input transducers 13, 16, 19, 2
If 2 is weighted as an electrode, the envelope in Figure 3 will become close to flat, and the level difference between each peak value will become smaller.
Furthermore, better combing characteristics can be obtained.

Further, although all the explanations have been made with reference to the local oscillation frequency of a Japanese television receiver, it goes without saying that the present invention is equally applicable to a foreign television receiver.

Furthermore, in the comb-shaped characteristic explained in the example, a peak value appears every 6 MHz in the channel band consisting of the low band of the VHF band, and every 2 MHz in other channel bands, but A comb-shaped characteristic in which a peak value appears in each channel can be used as an automatic channel selection device; for example, a comb-shaped characteristic in which a peak value appears in each IMHz may be used.

As described above, according to the present invention, it is possible to construct a comb filter that is small, inexpensive, and has a dead bandwidth that does not require adjustment.

Therefore, the automatic channel selection device, which conventionally had a complicated structure, can be simplified and lowered in price.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the principle of an automatic channel selection device, FIG. 2 is a plan view showing an embodiment of a surface acoustic wave filter for channel selection according to the present invention, and FIG. 3 is a block diagram showing the principle of an automatic channel selection device. FIG. 4 is a plan view showing another embodiment of the surface acoustic wave filter for channel selection according to the present invention. 1; electronic tuning tuner; 2; local oscillator; 3; comb filter; 10; piezoelectric substrate; 13-24: transducer.

Claims (1)

[Claims] 1. Provided on a piezoelectric substrate to convert electrical signals into surface acoustic waves,
Alternatively, the converter comprises first and second comb filters each consisting of three converters that perform the inverse conversion, and the above converter constitutes the first comb filter and the above converter constitutes the second comb filter. The filters are each set to respond in a first television channel band and a second television channel band different from the first television channel band, and the spacing between the two parallel-connected transducers of each comb filter is 1. A surface acoustic wave filter for channel selection, characterized in that it is configured to correspond to the channel spacing frequency of each responsive television channel band. 2. An elastic surface for channel selection according to claim 1, characterized in that the transducers constituting each comb filter are arranged in a line so that each comb filter shares the same propagation path. wave filter.