JPS6276990A - High frequency circuit for converter tuner - Google Patents

Abstract

PURPOSE:To eliminate interference to a channel of high band by providing a low-pass filter that suppresses the higher harmonics of a fundamental wave to the final stage of an up-down tuner. CONSTITUTION:A signal converted to the second channel at the second mixer 23 in an up-down tuner 40 is supplied to a SAW filter 411. The output of the SAW filter 411 is amplified at a cascade amplifier 412, being supplied to a SAW filter 413. The output of an amplifier 414 is supplied to a low-pass filter 42 through a coupling capacitor 421 and the output terminal 43 of the low-pass filter 42 is connected to the antenna terminal of a television receiver. By providing the low-pass filter 42 through the capacitor 421 to cut a DC component, the higher harmonic components in the components of a frequency f2 and the components of a frequency f3 that has three-fold of the frequency included in the output components of an output circuit 41 are attenuated.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a high frequency circuit of a converter tuner, and is designed to prevent interference of an intermediate band channel reception signal of a cable television broadcast signal with a high band signal, for example. It is something that

[Technical background of the invention]

As shown in FIG. 5, a receiver that receives cheap television signals has two transmission lines that branch signals from a main line.

In the case of Japan, transmission line II is the first to third channels (
90 to 108 [Hz] (hereinafter referred to as Low Kundo) and the 4th to 12th channels (170 to 2201J
Hz) (hereinafter referred to as iband)).

Further, the transmission path 12 is a thread path that processes signals of the A-th channel to the J-th channel (110 to 170 uHz) (hereinafter referred to as midband).

The transmitted signal is divided by a bidirectional distributor and input to transmissions @11 and 12. To the transmission line 12 (d1
An up/down tuner 13 is provided, which converts the mid-band signal into a low frequency (for example, 2 channels) frequency T.
The frequency is changed to a frequency that can be received by the television receiver 14. The output signal of the up/down tuner 13 is mixed with the signal of the transmission line 11 (first to twelfth channels) by a coupler.

The up/down tuner 13 has a configuration as shown in FIG. The signal supplied to the input terminal 15 is connected to the input circuit 1
6 to the first mixer 17. The first mixer 17 mixes the input signal and the oscillation output from the first oscillator 18, and generates a first intermediate frequency signal (frequency 612 MHz).
) is output. The first oscillator 18 changes its oscillation frequency from 660 to 1160 in response to a tuning signal from the tuning device 19.
It is variable up to MHz. The first intermediate frequency signal is transmitted through a bandpass filter 20, an intermediate frequency amplifier 21, a bandpass filter 2
2 to the second mixer 23. The second mixer 23 mixes the oscillation output from the second oscillator 24 and the intermediate frequency signal, and converts it into a frequency signal of a desired channel. For example, if it is desired to convert to a second channel that can be received by a general television receiver, the oscillation frequency is set to 668 JHz by operating the switching circuit 25. As a result, from the second mixer 23, 55.7
5) A second channel frequency signal of JHz can be obtained, and this signal will be supplied to the antenna terminal of the television receiver via the output circuit 26 and terminal 27.

FIG. 7 shows the external appearance of a case housing the above-mentioned up-down tuner, and parts corresponding to the circuits in FIG. 6 are given the same numbers. A voltage VT for obtaining tuning is applied to the terminal TI. Further, the terminal T2 is for connecting a channel selection IC. Also, terminal T3. T5 is for applying the power supply voltage Vaa, and terminal T4 is for automatic frequency control AFC, which is for stabilizing the oscillation frequency of the oscillator 24.

As described above, the up-down tuner converts the mid-band signal into the second channel and outputs it, and its frequency characteristics are as shown in FIG. S, is the video carrier wave of the second channel CH2, and S is the audio carrier wave of the second channel.

The second channel signal obtained in this way is transmitted to the transmission line 11.
(Figure 5), the second channel is connected to the first,
Interference also begins to occur on the third channel. That is, as shown in FIG. 9, the solid line represents the original main channel 1st channel CH.
I and the third channel CI (J).

The dashed line is the carrier spectrum of the second channel CH2.

In order to eliminate such interference, it is possible to solve the problem by further providing a filter device as shown in FIG. 10 at the output stage of the up-down cheese.
(hereinafter referred to as SA'W filter) 31.33 and amplifier 3
This is a filter device composed of 2.34. According to this filter device, as shown in FIG. 11, a bandpass filter characteristic having a steep frequency cut characteristic can be obtained, and the above-mentioned interference with adjacent channels can be prevented. In other words, adjacent channels can be suppressed by 60 dB or more. Here, the output of the sAw filter 3z is increased in gain by 33 dB in the cascade amplifier 32, and the gain of the output of the SAW filter 33 is increased in gain by 22 dB in the post amplifier 34. But on the other hand, the amplifier 32.34
Distortion from the active element in , that is, 2 to the fundamental wave
Double or triple harmonics are generated.

[Problems with background technology]

In the up-down tuner described above, interference and interference with adjacent channels (1st and 3rd channels) can be improved by using the output circuit using the SAW filter described above. New problems are emerging.

That is, since SAW filters generally have attenuation characteristics, even signals that are originally required are attenuated to some extent, so an amplifier is always connected to the output stage of the SAW filter. Additionally, the SAW filter generates noise called triple transient echo. This is third-order distortion, which is an unnecessary component caused by the signal reflected on the output side returning to the input and appearing on the output side again. Unwanted components generated by the SAW filter 31 are amplified by an amplifier 32 and supplied to a SAW filter 33. Although unnecessary components outside the band are cut off by the 8AW filter 33, noise is also generated in the SAW filter 33, which is amplified by the amplifier 34 and output. Furthermore, the distortion due to the active elements in the amplifier 32, 34, i.e. the second-order distortion in the case of FETs, and 3 in the case of transistors.
Next-order distortion occurs, resulting in twice the frequency f1 of the fundamental wave,
There is a problem in that the triple harmonic component cannot be sufficiently suppressed. Then, the double harmonic component (frequency f2) appears at the frequency position of the eighth channel, as shown in FIG. 12, and acts as an interference wave for the eighth channel. PI-P9 is the video carrier spectrum of each channel.

[Purpose of the invention]

The present invention has been made in view of the above circumstances, and provides a converter capable of removing unnecessary harmonic signals from the high frequency output signal of an up-down tuner and preventing interference with high band channels. The purpose is to provide high frequency circuits for tuners.

[Summary of the invention]

In the present invention, as shown in FIG. 1, a low-pass filter 42 is connected to the high-frequency output circuit of the converter tuner, thereby attenuating unnecessary harmonic components.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings. 1st
The figure shows an embodiment of the present invention, which is used as an output circuit of the up-down chainer 40 shown in FIGS. 5 and 6.

The signal converted into the second channel by the updakunchena 40 (D second mixer 23) is supplied to the first SAW filter 411 constituting the output circuit 41 using the SAw filter. The output of
The signal is supplied to the W filter 413. Here, an even steeper cutoff is obtained, and its output is supplied to the next stage post-amplifier I14.

Here, in the present invention, the output of the amplifier 414 is supplied to the low-pass filter 42 via the coupling capacitor 421. A parallel circuit of a coil 424 and a capacitor 425 is further connected to the parallel circuit of the low-pass filter 42, a coil 422, and a capacitor 423. A capacitor 426 is connected between the connection point of the parallel circuit and ground. An output terminal 43 of this low-pass filter 42 is connected to an antenna terminal of a television receiver.

In addition to the output component (frequency fz) of the output circuit 4z, it includes a component 2 at twice the frequency and a component 3 at three times the frequency. The reason for this is, as explained in Figure 10, K, SAW
This is due to the filter's triple transient echo and the amplifier's second-order distortion component. In other words, the first stage amplifier 41
At the output section of 2, a harmonic of 50 dB (Fig. 4(a)) is generated at the frequency position of 2 x fI of the fundamental wave component, and 2
At the output section of the amplifier 414 in the second stage, the harmonics of the fundamental wave component are again mixed and superimposed on the harmonics of the first stage, resulting in a component with an 8N ratio of 40 ciB (FIG. 4(b)). Furthermore, a DC level is also added to these high frequency components, resulting in an image as shown in FIG. 2.

In order to reduce the above harmonic components, the first stage amplifier 4
It is possible to lower the high g8 wave level of 12, but 2
It is not possible to suppress harmonics generated in the stages. Also,
Although it is conceivable to lower the input level of the second-stage intensifier 414, this would deteriorate the original S/N ratio of the fundamental wave.

Therefore, in this invention, the capacitor 4z for DC cut
A low-pass filter 42 is provided via r, and the characteristics of this low-pass filter 42 are set as shown in FIG. 3 to attenuate high frequency components.

The characteristics of the low-pass filter 42 are, for example, 195MF (z
is set to suppress by 40 dB and allow components below this to pass. Note that, when the output channel of the up-down tuner is not changed, the low-pass filter 42 may be configured to pass only the band of that channel.

〔Effect of the invention〕

As explained above, the present invention provides a high-frequency circuit of a converter tuner that eliminates interference to high-band channels by providing a low-pass filter that suppresses harmonics of the fundamental wave in the final output stage of the up-down tuner. Can be provided.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing an embodiment of which invention, Figure 2 is a frequency characteristic diagram of the output circuit of the first box, Figure 3 is a characteristic diagram of the low-pass filter of Figure 1, and Figure 4 is a diagram of the output circuit of the first box. Figure 5 is an explanatory diagram of the output circuit high-phrase wave component, Figure 5 is an explanatory diagram of the CATv receiver, 8
Figure g6 is the configuration diagram of the up-down cheese in Figure 5, and Figure 7.
The figure shows an example of the circuit layout in Figure 6, Figure 8 is a frequency characteristic diagram of the output of the circuit in Figure 6, Figure 9 is an explanatory diagram of the spectrum of the output of the circuit in Figure 6, and Figure 10 is FIG. 11 is a diagram showing the high frequency output circuit of the circuit of FIG. 6, FIG. 11 is a frequency characteristic diagram of the circuit of FIG. 10, and FIG. 12 is a spectrum explanatory diagram showing the chaotic state of the system of FIG. 5. 40... Up/down tuner, 41... Output circuit, 42... Low pass filter. Applicant's agent Patent attorney Suzue Takehiko No. 5 Figure P2 52 PI 1,000 yen l111 Figure

Claims (1)

[Claims] a cable through which a first signal of a channel that can be directly received by a tuner of a television receiver and a second signal of a channel that cannot be directly received by the tuner are transmitted;
A receiver equipped with a converter tuner that receives the second signal on the cable, converts it into a third signal of a channel receivable by the tuner, and supplies the third signal to the tuner, the frequency conversion output of the converter tuner. A high-frequency circuit of a converter tuner, characterized in that a low-pass filter for attenuating harmonics of the fundamental wave of the third signal is connected to the final stage of the converter tuner.