KR920001479Y1 - Noise deleting circuit of catv - Google Patents

Abstract

No content.

Description

Bit Noise Reduction Circuit of Cable TV Tuner

1 is a diagram showing an intermediate frequency generation circuit of a cable television tuner according to the prior art.

2 is a diagram showing an intermediate frequency generating circuit of a cable television tuner employing a bit noise canceling circuit according to the present invention;

3 is a diagram showing operating characteristics according to the oscillation signal level of the pre-oscillator.

* Explanation of symbols for main parts of the drawings

10: band pass filter 20: mixer

30 ': pre-oscillator 40: buffy

50: prescaler 60: phase comparator

70 low pass filter 80 mutual inductive coupling means

90: printed circuit board ground line

The present invention relates to a beat noise cancellation circuit of a cable television (CATV) tuner. In particular, in an intermediate frequency generating circuit of a CATV tuner, an oscillation frequency for frequency conversion to be combined with a high frequency input signal is variably generated according to channel selection. The present invention relates to a circuit for removing bit noise generated by a prescaler coupled to a pre-oscillator.

The intermediate frequency generator circuit of the CATV tuner usually has the same configuration as that of the first diagram, and the high frequency input (RF) in the 50 to 500 (or 550) MHz band is first passed through the bandpass filter 10 to remove the mixer 20. A pre-oscillator 30 whose output is controlled by a PLL (Phase Locked Loop) composed of a prescaler 50, a phase comparator 60 and a low pass filter 70, is applied to one end of In order to convert the frequency band of the signal to a higher band, an oscillation signal of 660 to 1180 MHz is output. The output of pre-oscillator 30 is connected to the other end of mixer 20 via buffer 40 and mixed with the RF input. The mixer 20 generates the intermediate frequency output IF by performing an up-conversion that mixes an oscillation signal having a frequency higher than the intermediate frequency output IF to the high frequency input RF. .

The pre-oscillator 30 of the CATV tuner has a varactor diode VD having a different capacitance value according to a change in the applied voltage, a capacitor C1 connected in series thereto, and an inductor L1 connected in parallel thereto. It consists of).

Here, the oscillation output signal of the pre-oscillator 30 is extracted at the connection point of the varactor ion VD and the inductor L1 and applied to the buffer 40 via the capacitor C2. In addition, a part of the oscillation signal of the oscillator 30 is extracted from the midpoint of the inductor L1 and applied to the prescaler 50 via the DC blocking capacitor C3. The output of the low pass filter 70 is applied through the capacitor C4 at the connection point between the varactor diode VD and the capacitor C1.

Here, the pre-oscillator 30 of the CATV tuner is configured to output different oscillation frequencies depending on the channel selection of the desired channel. In order to satisfy this operation, the above-described PLL circuit is adapted to control the pre-oscillator 30 having the voltage controlled oscillation characteristic. That is, the oscillation frequency generated by the pre-oscillator 30 is divided into a sawtooth wave through the prescaler 50 and applied to one end of the phase comparator 60. The phase comparator 60 inputs a signal fi obtained by dividing an oscillation frequency of a crystal oscillator (not shown) and compares it with the signal output from the prescaler 50. The error signal output from the phase comparator 60 corrects the output frequency of the pre-exciter 30 in a direction to reduce the frequency difference between the signal fi and the output signal of the pre-oscillator 30.

However, when the oscillation frequency of the pre-oscillator 30 enters the input terminal of the prescaler 50, if the signal level of the oscillation frequency is too large, a part of the oscillation frequency signal input to the prescaler is input to the prescaler in the prescaler. By returning back to the channel, it causes beat noise when tuning a specific channel, which causes trouble on the screen.

As a remedy for this, it is known to install a plurality of resistors to reduce the signal level applied to the input side of the prescaler. However, there is a problem in that a satisfactory effect is not obtained, but rather, an increase in the number of parts leads to a decrease in productivity.

Accordingly, the present invention aims to provide a circuit capable of removing bit noise by reducing the oscillation signal of the pre-oscillator applied to the prescaler by using an element having an impedance of reactance component to solve the above problem. I put it.

Next, the present invention will be described in more detail with reference to the drawings of FIG.

In FIG. 2, the same reference numerals are added to the same parts as those in FIG.

In the tuner according to the present invention, the pre-oscillator 30 'also includes a varactor diode (VD), a capacitor (C1), and an inductance (L1), and the output of the pre-oscillator (30') is an inductor. It is supplied to the buffer 40 side via the mutual inductive coupling means 80 composed of (L1) and (L2), and is applied to the ground line 90 made of a printed circuit board (PCB). The PCB ground line 90 electromagnetically separates the pre-oscillator 30 'and the prescaler 50. The input end of the prescaler 50 is connected to the PCB ground line 90 via a capacitor C3.

Under this configuration, the signal output from the pre-oscillator 30 'is weakened by the mutual inductive coupling means 80, and the signal is connected to the PCB ground line 90 in the course of passing to the prescaler 50. It is weaker while passing. In addition, the oscillation frequency of the pre-oscillator 30 'may be transmitted to the prescaler 50 by the Gauss law, but the frequency component traced from the prescaler 50 may be a PCB surrounding the pre-oscillator 30'. The ground line 90 is not transmitted to the next preamplifier 30 'side.

Here, if the level of the oscillation frequency rises above D point as shown in Fig. 3, bit noise is generated, and if it falls below A point, the input scale of the prescaler is low and becomes inoperable. The relative adjustment of the mutual inductive coupling means and the PCB ground line is pre-determined and fixed so that the signal level comes in.

As described above, according to the circuit according to the present invention, it is possible to eliminate the bit noise generated during the tuning of a particular channel to improve the quality of the product, and to improve the yield by adjusting the adjustment in mass production.

Claims (2)

A bandpass filter 10 for removing noise from the high frequency input RF, a pre-oscillator 30, 30 'generating a frequency converted oscillation signal for converting the high frequency input into a higher frequency band, CATV including a buffer 40 for amplifying the pre-oscillator output, a mixer 20 for mixing the output of the bandpass filter 10 and the buffer 40 to generate an intermediate frequency signal IF, and the like. In the tuner, the output of the preamplifier is supplied to the buffer 40 through the mutual inductive coupling means 80 and at the same time the input terminal of the prescaler 50 through the mutual inductive competition means 80 and the PCB ground line 90. A bit noise canceling circuit of a cable television tuner, characterized in that it is configured to be supplied to. The method of claim 1, wherein the PCB ground line 90 is disposed so as to surround the pre-oscillator (30 ') so as to electromagnetically separate between the pre-oscillator (30') and the prescaler (50). Bit noise reduction circuit of cable TV tuner.