JPH0223707A - TV signal demodulation input filter - Google Patents

Abstract

PURPOSE:To reduce phase distortion and cross modulation by connecting a capacity coupling serial/parallel circuit between the connection part of a TV tuner and an elastic filter and a ground when a TV signal is demodulated through the use of the surface acaustic wave filter. CONSTITUTION:A capacitor coupling serial circuit in which a coil 5 and a capacitor 4 are connected in parallel and one end of the connection is connected with a capacitor 3 in serial is set between the connection part of the TV tuner 1 and the surface acaustic wave filter 2 and the ground. Thus, the group delay characteristic of the capacitor coupling serial/parallel circuit can steeply be changed by the frequency of an attenuation pole. As a result, the frequency of the attenuation pole can appropriately be set to a frequency lower than a sound IC and therefore the phase distortion of the TV signal which is accompanied by wide band demodulation can be reduced. Since the capacity coupling serial parallel circuit operates with respect to the frequency lower than the sound IC as a trap for the deterioration of the selectivity characteristic of the surface acaustic wave filter 2, the interruption of cross modulation can be prevented.

Description

[Detailed Description of the Invention] Industrial Application Field The present invention is used for television receivers, VTRs, etc.
The present invention relates to a television signal demodulation input filter using a surface acoustic wave filter for demodulating an RF multiplied signal modulated by a television signal.

2. Description of the Related Art A conventional television signal demodulation input filter of this type has a configuration as shown in FIG. Reference numeral 1 denotes a television tuner which converts the RF multiplied signal modulated by the television signal into an intermediate frequency, amplifies it, and inputs it to the surface acoustic wave filter 2.

Since the surface acoustic wave filter 2 is used as a television signal demodulation input filter, the group delay characteristic in the band from the video intermediate frequency to the audio intermediate frequency has become flatter than when a helical filter is used. In order to minimize the chip size of the surface acoustic wave filter 2, it is necessary to flatten the group delay characteristic within the band.

Problems to be Solved by the Invention Page 3 When attempting to demodulate a television signal over a wide band using such a conventional configuration, the demodulation output of 3 MHz
The group delay characteristics of the above components are ringing.

The influence of overshoot, preshoot, etc. on image quality becomes stronger.

Furthermore, the TV signal transmitter side applies group delay characteristic correction to the components higher than 3MH2 to compensate for the phase distortion generated inside the TV signal receiver, and this When the signal is demodulated by passing through a flat surface acoustic wave filter, a phase distortion of a component higher than 3M) (Z) occurs, and the wider the band is demodulated, the more noticeable ringing becomes.

In addition, by widening the band of the surface acoustic wave filter,
The selectivity of the surface acoustic wave filter in a frequency band lower than the audio intermediate frequency and higher than the upper adjacent video frequency tends to deteriorate, and cross-modulation interference increases.

Therefore, the present invention aims to improve the phase distortion and cross-modulation that occur when demodulating a television signal into a wide band, and to suppress the overshoot and preshoot of the wide band demodulated output to obtain the optimum image quality. The purpose is to provide filters.

Means for Solving the Problem In order to solve this problem, the present invention provides a capacitively coupled series-parallel circuit in which a coil and a first capacitor are connected in parallel, and a second capacitor is connected in series to one end of the coil. It is connected between the signal line connecting the television tuner and the surface acoustic wave filter and the ground, and the attenuation pole of this series-parallel circuit is set to be a circle around the audio intermediate frequency. Phase distortion and cross modulation are improved by performing phase compensation in the high frequency range.

In the capacitively coupled series-parallel circuit, a circuit in which a voltage variable capacitance diode and a DC blocking capacitor are connected in series to the first capacitor is connected in parallel, and the attenuation pole of the series-parallel circuit is connected to the control voltage of the voltage variable capacitance diode. control with
The overshoot and preshoot of the wideband demodulation output are optimized.

Operation Page 5 With such a configuration, the group delay characteristic of the capacitively coupled series-parallel circuit can be made to change sharply at the frequency of the attenuation pole. Therefore, by appropriately setting the frequency of the attenuation pole to a frequency lower than the audio intermediate frequency, it is possible to reduce the frequency by approximately 17 m at a frequency 3.5 a MH2 lower than the video intermediate frequency.
Since it can be delayed by 0 nSeC, phase distortion of the television signal due to wideband demodulation can be accounted for.

Furthermore, in response to the deterioration of the selectivity characteristics of the broadband surface acoustic wave filter, capacitively coupled series-parallel circuits have to act as traps at least at frequencies higher than the audio intermediate frequency. It is possible to improve the selectivity in a band higher than the adjacent video frequency and to improve cross-modulation interference.

On the other hand, in order to optimize the overshoot and preshoot in the broadband demodulated output, the attenuation pole of the series-parallel circuit is appropriately set and the flattened group delay characteristic is slightly adjusted to obtain the optimum image quality. In order to advance the delayed phase, the capacitance of the voltage variable capacitance diode, which can be controlled externally, is controlled, and the attenuation pole is varied to optimize overshoot and pre-displacement.

EXAMPLE An example of the present invention is shown in FIG. 1 below. In the figure, 3.4 is a capacitor, and 5 is a coil, forming a capacitively coupled series-parallel circuit. A television tuner 1 receives an RF multiplied signal modulated by a television signal, and its output is connected to a surface acoustic wave filter 2. In this embodiment, the capacitor 3 has a capacitance value of 18 pF, the capacitor 4 has a capacitance value of 220 pF, and the coil 6 is an air-core coil made of polyurethane copper wire with an inner diameter of 3'' and a wire diameter of 0.3 mm and three turns.

The group delay characteristic of the broadband demodulated output is as shown in Figure 3, when the attenuation pole of the series-parallel circuit is set to a sufficiently low frequency.
Phase distortion occurs as shown in the curve (■), and when the frequency of the attenuation pole is increased to the optimum attenuation pole compared to the setting of the curve (■), the curve becomes flat over a wide band as shown in the curve (■). As the frequency increases, phase distortion occurs as shown in the curve (■). If the frequency of the attenuation pole is set as shown in the 7H curve in ①, the frequency is set to approximately 53M1-IZ, and the selectivity characteristic at that time is as shown in the curve ② in Figure 4. This is an improvement compared to the curve (2) in which the series-parallel circuits are not connected, and cross-modulation interference can be improved.

Another embodiment of the invention is shown in FIG. In the figure, 6 is a DC blocking capacitor, 7 is a voltage variable capacitance diode, and 8 is a resistor for controlling the capacitance of the diode 7. With these configurations, the frequency of the attenuation pole is slightly varied with respect to the frequency of 53 MHz, and the high frequency It is possible to optimize overshoot and preshoot by correcting group delay characteristics.

Effects of the Invention As described above, according to the present invention, the phase distortion and cross-modulation that occur when demodulating a television signal over a wide band using a surface acoustic wave filter can be solved by connecting a capacitively coupled series-parallel circuit. This can be improved by

Moreover, by introducing a capacitively coupled series-parallel circuit that can optimally fine-tune overshoot and preshoot, a TV signal demodulation input filter using a surface acoustic wave filter that is easy to manufacture and has a small chip size has been realized. It can be done, and its industrial value is great.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing the configuration of a television signal demodulation input filter in one embodiment of the present invention, FIG. 2 is a circuit diagram showing the configuration of another embodiment of the present invention, and FIG. 3 shows its group delay characteristics. Figure 4 is a characteristic diagram showing its selectivity characteristics, Figure 5 is a characteristic diagram showing its selectivity characteristics.
The figure is a circuit diagram showing a conventional television signal demodulation input filter. 1...Television tuner, 2.Old...Surface acoustic wave filter, 3.4...Capacitor, 5.Old...Coil.

Claims (2)

[Claims] (1) A signal line connecting a television tuner and a surface acoustic wave filter through a capacitively coupled series-parallel circuit in which a coil and a first capacitor are connected in parallel, and a second capacitor is connected in series to one end of the parallel circuit. and ground,
A television signal demodulation input filter configured to set the attenuation pole of the series-parallel circuit to be lower than the audio intermediate frequency to attenuate the audio intermediate frequency and to compensate for the phase of the high frequency portion within the band. (2) A circuit in which a voltage variable capacitance diode and a DC blocking capacitor are connected in series is connected in parallel with the first capacitor of the capacitively coupled series-parallel circuit, and an attenuation pole is connected to a part of the voltage variable capacitance diode through a resistor. 2. The television signal demodulation input filter according to claim 1, wherein a DC voltage is applied to control the input voltage.