JPS6019691B2 - television receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a television receiver having a surface acoustic wave filter in an intermediate frequency amplification circuit.

When using a surface acoustic wave (hereinafter abbreviated as SAW) filter in a television receiver, a triple transhot echo (hereinafter abbreviated as TTE) unique to the SAW filter is used.
This has the drawback that a ghost image is generated on the screen, degrading the image quality.

The reason for this will be explained below using FIGS. 1 and 2.

FIG. 1 is a plan view showing the structure of the SAW filter, and FIG. 2 is a waveform diagram for explaining the TTE generation process.

In Figure 1, 30 is a SAW propagation board (hereinafter abbreviated as the board)
A piezoelectric crystal or the like is usually used.

On this substrate 30, an input transducer 31 and an output transducer 32 are formed which are made of conductive comb-shaped electrodes. Input terminal 1,
The television signal input during 1' is input to the input converter 31.
is transformed into a SAW and propagates on the substrate 30 in the direction of arrow 40. The propagated SAW is sent to the output converter 3 on the propagation path.
At step 2, the signal is converted back into an electrical signal and output between output terminals 2 and 2'. The frequency characteristics of the SAW filter 3 from the input terminals 1, 1' to the output terminals 2, 2' are determined by setting in advance the logarithm of the comb teeth of the input converter 31 and the output converter 32, the overlapping width of the comb teeth, etc. The desired characteristics can be obtained by Furthermore, the delay time 7 for the signal to reach the output terminal 2, 2' from the input terminals 1, 1' is determined by the speed v of the SAW determined by the physical constants of the substrate material, and the input transducer 31.
is determined by the distance 1 of the output converter 32, and can be expressed as 7=1/v. The above is a powerful SAW filter.
Next, TTE will be explained.

FIG. 2a shows a case where a burst-like input waveform 41 is input as an input signal between input terminals 1 and 1' to help understand the process of TTE generation.

For simplicity, the center of the input waveform 41 is set at time t=0.
Now, the input waveform 41 input to the input converter 31 is SAW
The signal propagates through the substrate 30, is converted back into an electrical signal by the output converter 32, and is output.

This output signal is shown in FIG. 2b as the desired signal 42. This time t is 7. What should be noted here is that, as shown by arrow 5M in FIG. 1, a portion of the propagated SAW is reflected by the output transducer 32 and propagates in the direction of the input transducer. A part of this reflected and propagated SAW is reflected again at the input transducer 31 and propagated in the direction of the output transducer 32, and at time t=3, the SAW is converted back into an electrical signal by the output transducer 32, as shown in Fig. 2b. It is outputted from the output terminals 2 and 2' as the TTB signal 51 shown in FIG. TTE signal 51 is at time t=3
The reason why the desired signal 42 propagates has a propagation path length of 1, and the TTE signal 51 is output after propagating one and a half round trips, or three times, so the propagation path length of the TTE signal 51 is 9. Therefore, the delay time of the TTE signal 51 is also three times that of the desired signal 42, that is, 37 times.

When using SAW Filter 3 in a television receiver,
This TTE signal 51 has the disadvantage that it appears on the screen as a delayed signal with respect to the desired signal 42, that is, as a ghost, and degrades the image quality.

There are two types of TTE: one is generated by discontinuity in acoustic impedance, and the other is generated by electrical re-radiation in relation to a driving source or load impedance. The present invention relates to the latter type of TTE. It should also be noted that all of the following explanations are based on the center frequency of the intermediate frequency. Next, the relationship between the gain (desired signal power to input signal power ratio) of the SAW filter 3 and the TTE suppression degree (desired signal power to TTE signal power ratio) will be explained. Figure 3 shows the equivalent circuit of the SAW filter 3 and its peripheral circuits, where the SAW filter 3 input admittance 61 is set to y, = g,
1J0, output admittance 62 of SAW filter 3 is y
o=go+ibo, drive source admittance 71, ys=g
s + jq, load admittance 72 = gL + ibL
Then, the gain Q of SAW filter 3 on the input side is
1. The Qo of the SAW filter 3 on the output side, the TTE suppression degree on the input side is 8, and the TTE suppression degree Bo on the output side is Q,=open g,no ys+y,l2
...[1].

. =25g. /lyL+y. l2...[2
}8,=lys 1y,l2/g,2...(3
}8. =lyL+y. l2/&2...
・'4). Regarding these formulas, see K. Hanm
aetal,,'' A Triple Tr
aship it SupeSSionTechnique,
''1976 U/trasonlcs Simp. P
roc. mEECat. M. 76,CHI1201$U
．． Please refer to Now, from the formula {1' to '4}, Q
, 2 8, = heat rhombus 2/lys 1y, l2 < 4...(
5) Q Yu80=SasaL2/lyL+yol2<4...
{6} is derived, and the gain QIQo of the entire SAW filter is
If Q is Q, TTE suppression degree is 8, and 8o is 3, then Q and 8
In between, Q28; Q, 28, Q Ku B.

<16...[71Q28<16
It can be seen that there is a relationship of {71. {
From Equation 7', for example, TTE suppression degree 3 can be changed to the old value of 4, that is, 1
When the gain Q of the SAW filter 3 is set to 0.04 or less, that is, -14 dB or less. Conventionally, in order to ensure a TTE suppression degree of 4WB, a mismatch was caused in the output of the SAW filter 3, and the gain of the SAW filter 3 was therefore reduced. SUMMARY OF THE INVENTION An object of the present invention is to provide a television receiver that can minimize SAW filter loss without impairing TTE suppression when a tuner and an intermediate frequency amplifier circuit are connected via a distributed constant line.

The present invention will be explained below with reference to FIG. FIG. 4 shows an equivalent circuit of a SAW filter and its peripheral circuits when a tuner and an intermediate frequency amplifier circuit are coupled through an almost lossless distributed constant line 82. In the figure, 81 is the tuner output admittance, 83 is the input circuit admittance, and 91 is the tuner output admittance.
, 92 is a signal source. The following general properties of distributed constant lines are known.

1 When one end is terminated with characteristic impedance Z, the impedance seen from the end is equal to Z and does not change depending on the line length L.

11 When one end is terminated with an impedance having a value different from the characteristic impedance Z, the impedance seen from the other end changes depending on the line length L'.

Generally, the output terminals 5 and 5' of the distributed constant line 82 are terminated with a characteristic impedance Z. In this case, the line 82
The impedance looking into the line 82 from the input terminals 4, 4' is equal to Z, regardless of the line length L'. Therefore, the power gain in the cascade connection of the tuner and the SAW filter 3 is independent of L. Also, generally the tuner output impedance is not equal to Z.

In this case, the impedance looking into the line 82 from the input terminals 5, 5' of the line 82 changes depending on L. Therefore, S.A.
The admittance looking into the tuner side from the input terminals 1 and 1' of the W filter 3 changes with L, and ys in the '33 formula changes with L, so the TTE suppression degree on the input side of the SAW filter 3 is 8. It changes more than anything. That is, if the output terminals 5 and 5' sides of the distributed constant line 82 are terminated at the characteristic impedance Z of the line 82, and the output impedance of the tuner is not equal to Z, the line 8
By changing the length L of 2, the power generation in the cascade connection of the tuner and SAW filter 3 does not change, but
The degree of TTE suppression 31 on the input side of the SAW filter changes. The key point of the present invention is to reduce the mismatch that was intentionally caused on the SAW filter output side to ensure the TTE suppression degree of 4WB by determining L so that The aim is to reduce the loss of Next, a method for setting the line length L will be specifically explained. In FIG. 4, the output terminals 5, 5' of the line 82
If the admittance looking toward the tuner side is yc, and the admittance 83 of the input circuit is yN, then the equation regarding the degree of TTE suppression on the SAW filter input side [3', ys is the parallel admittance of yc and yN, and ys=yc+y
N...[It becomes 81. Also, from the condition that the output terminals 5 and 5' sides of the line 82 are terminated with the characteristic impedance Z, yN+y,=g
...{91. Here, g is the reciprocal of Z. ■ Substituting equation '9' into equation '3', the TTE suppression degree on the SAW filter input side is 8, = lyc + gl2
/g,2...00.

Therefore, if the line length L is set so that lyc+gl is maximized, the TTE suppression degree 8 at the SAW filter input end is maximized. The admittance yC of the nearly lossless distributed constant line 82 looking toward the tuner side from the output terminals 5, 5' is yC=g(yT cos8+igsin8)/(g
cos80ka, sino) …(
11) Here yT: Tuner output admittance 81
It is known that this is the wavelength of the signal line 82 (wavelength that takes into account the shortening rate).

For example, in Figure 4, y,=shame=(0.005-i0.001)○yT=0.
Considering the case of 00260, let g=0.0130^=4m.

At this time (from the 110,000 formula, L = 1m When lyc gl has a maximum value of 0.0650.

Therefore, according to the above equation, the degree of TTE suppression B, on the SAW filter input side is 170, and when 3 and 8o are considered to be 4 times old, that is, 1 person, 8o becomes 59. Therefore, from formula '4', yL is (0.0330io.001), [
Gain Qo on the SAW filter output side from equation 2
becomes Qo =0.22.

If the line length L in the secondary example is L=0.8h, lyc+gl is 0.020 by the same calculation.
0,31 is 16, 3o is 630, yL is (0.12 jo.001)○, ○○ ni〇. It becomes 〇77.

Therefore, in the above embodiment of the present invention, S
The gain of AW filter 3 is approximately 2. Needle sound (loss is approximately 0.3
4 times), which is approximately 4. MB is improved.

[Brief explanation of the drawing]

FIG. 1 is a plan view showing the structure of the SAW filter, FIG. 2 is a waveform diagram for explaining the state of TTB generation, and FIG. 3 is a circuit diagram of the equivalent circuit of the SAW filter and its peripheral circuits.
FIG. 4 is a circuit diagram of an equivalent circuit of a SAW filter and a peripheral circuit including a distributed constant line. 82...Distributed constant line. Figure 1 Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] 1 The input circuit of the surface acoustic wave filter and the output circuit of the tuner are coupled by an almost lossless distributed constant line, and the input circuit side of the distributed constant line is terminated at approximately the characteristic impedance of the distributed constant line at the center frequency of the intermediate frequency. In a television receiver in which the output impedance of the tuner is not equal to the characteristic impedance, the length of the distributed constant line is determined by the admittance looking into the tuner from the input circuit and the reciprocal of the characteristic impedance. A television receiver characterized in that the absolute value of the sum is approximately maximum at the center frequency of the intermediate frequency.