JPS6160484B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video signal reproducing device, and particularly to a capacitor for detecting changes in capacitance from a video disk on which video and audio signals are recorded as changes in capacitance and demodulating them into electrical signals. Regarding the detector, its purpose is the carrier noise ratio (C/N ratio)
It is an object of the present invention to provide a capacitance detector for a video disk which has improved signal-to-noise ratio (S/N ratio) characteristics and simplified assembly work.

Generally, this type of detector is formed by two blocks inside a metal casing, with one block containing a fixed oscillator and the other block containing a disk whose frequency changes in response to changes in the capacity of the video disk. A stylus resonance circuit and a detection circuit are housed therein. The loop that inductively couples the fixed oscillator and the disk stylus resonant circuit is connected through a shield plate that partitions both blocks, and as shown in Figure 1, the loop is pre-shifted slightly lower than the oscillation frequency of the fixed oscillator ( A disk stylus resonant circuit having a resonance point at a frequency (which may be shifted slightly higher) changes the resonance frequency in accordance with the change in capacitance recorded on the disk, so that the oscillation frequency signal transmitted from the coupling loop is The amplitude changes in response to changes in the resonant frequency of the stylus resonant circuit, and AM modulation is performed. This AM modulated signal is passed through a detection circuit and detected, and the signal recorded on the video disc is obtained as an output signal. was.

The above-mentioned capacitance detector is conventionally constructed as shown in FIGS. 2 and 3, with FIG. 2 showing a circuit diagram of the conventional example, and FIG. 3 showing a circuit diagram of the conventional example shown in FIG. FIG. 3 is a perspective view showing a mounting state. In the figure, reference numeral 1 denotes a metal external casing, and the metal external casing 1 is divided into two blocks by a shielding plate 2 into a UHF oscillation circuit 3 side serving as a fixed oscillation circuit and a disk stylus resonant circuit 4 side. ing. The UHF oscillation circuit 3 generates an oscillating force using a transistor 5, a feedback capacitor 6, and a capacitor 7. The capacitor 7 and inductance 8 constitute a main resonant circuit, and oscillates at a predetermined frequency (91MHz). . The UHF oscillation circuit 3 and the disc stylus resonance circuit 4 are inductively coupled through a coupling loop 9 passing through a hole 10 in the shielding plate 2, and a UHF oscillation signal is transmitted from an inductance 8 to a stylus resonance inductance 11. The capacitance change detected from the video disk and the stray capacitance are connected to one end of this stylus resonance inductance 11, and the resonance frequency of the disk stylus resonance circuit 4 changes as shown in FIG. 1 in response to the capacitance change. , thereby making the oscillation frequency signal from the fixed oscillator
The AM modulated signal is taken out by the coupling loop 12 and then passed through the AM detection diode 1.
3 to obtain the detected output.

However, in the conventional capacitance detector as mentioned above, the transistor itself becomes a load and the Q
(sharpness of the resonant circuit) could not be obtained, and the coupling loop 9 and stylus resonance inductance 1
1 becomes weaker, and the detection output becomes smaller. Therefore, in order to obtain a large detection output, it is necessary to strongly couple the inductance 8 and the coupling loop 9 and the stylus resonant inductance 11 and the coupling loop 12, and as a result, the load on each increases and the Q also decreases. Therefore, the C/N ratio is 55dB,
The drawback was that the S/N ratio was 36 dB, making it impossible to obtain a satisfactory value.

The present invention improves the C/N ratio and S/N ratio by obtaining a high Q oscillation frequency,
This will be explained below with reference to the drawings.

FIG. 4 shows an embodiment of the present invention, and FIG. 5 shows a perspective view of the circuit mounting state in the embodiment, in which the metal external casing 1 is divided into two by a shielding plate 2. A fixed oscillation circuit that oscillates with an inductance 8, a transistor 5, a feedback capacitor 6, and a capacitor 7 is placed on one block side, and the oscillation frequency is set 20 to 30MHz lower than the predetermined frequency (91MHz). A fixed capacitor 14 and a variable capacitor 15 are connected to both ends of adjacent coupling coils 16 to form a coupling loop 9 forming a λ/2 resonant circuit, and the resonance frequency of the coupling loop 9 is set to a point 1 to 2 MHz lower than a predetermined frequency. Then, the coupling loop 9 is installed in one block together with the fixed oscillation circuit, and the inductance 8 and the coupling loop 9 are inductively coupled to cause the Chien phenomenon, and the self-resonant frequency of the coupling loop 9 becomes the dominant oscillation frequency. A predetermined oscillation frequency (91MHz) is generated by determining the frequency. Then, the signal at the oscillation frequency (91MHz) is AM modulated in the disc stylus resonant circuit by the capacitance change of the video disc, and this modulated signal is taken out by the coupling loop 12, and the AM
AM detection is performed by the detection diode 13, and a video signal and an audio signal recorded on the video disc are obtained.

As described above, according to the present invention, since the coupling loop inductively coupled to the inductance connected to the active element is configured as a λ/2 resonant circuit, high Q oscillation with reduced load influence by the transistor 5 can be obtained. Since the coupling loop 9 is a λ/2 type resonant circuit, the node point of the loop can be appropriately set, and by locating the vicinity of the node point in the through hole 10 of the shielding plate 2, the stylus resonance inductance 4 can be easily set. can be inductively coupled to obtain a large detection output.

Therefore, there is no need for strong coupling that would increase the load on each resonant circuit, so a reduction in the Q of the resonance characteristics is prevented, and the carrier noise ratio (C/N ratio) and signal-to-noise ratio (S /N ratio) is improved, and in the embodiment according to the present invention, the C/N ratio is improved.
The improvement was 64 dB and the S/N ratio was 50 dB, which was a fully satisfactory result.

In addition, because the magnetic flux generated at the node point of the λ/2 coupling loop can strengthen the inductive coupling at that point, the through hole 10 can be made smaller, the degree of shielding in each block increases, and the Q of resonance within each block is also reduced. It can be made small. moreover,
When the coupling loop 9 is attached to the capacitance detector, it is not necessary to pass through the coupling through hole 10 of the shielding plate 2 as in the conventional case, so the installation time is shortened and productivity can be improved. It has an extremely large effect.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing the detection characteristics of a capacitance detector, Fig. 2 is a circuit diagram showing a conventional capacitance detector, and Fig. 3 shows the mounting state of the conventional example shown in Fig. 2. 4 is a circuit diagram showing a capacitance detector according to an embodiment of the present invention, and FIG. 5 is a perspective view showing a state in which the embodiment shown in FIG. 4 is mounted. 1... Metal external casing, 2... Shielding plate, 3...
UHF resonance circuit, 4... Disk stylus resonance circuit, 5... Transistor, 6... Feedback capacitor, 7, 14... Capacitor, 8, 16... Inductance, 9, 12... Coupling loop, 10...
...Through hole, 11...Stylus resonance inductance, 13...Detection diode, 15...Variable capacitor.

Claims (1)

[Claims] 1 Consisting of a fixed oscillation circuit, a disc stylus resonance circuit for detecting capacitance changes, and a detection circuit, a coupling coil is provided for inductive coupling with a resonance inductance connected to an active element of the fixed oscillation circuit, and the coupling A coupling loop is formed by connecting capacitors to both ends of the coil to form a λ/2 resonant circuit, and the coupling loop is placed in a block that houses the fixed oscillation circuit, and the coupling loop is used to reduce the amount of noise from the fixed oscillation circuit. A capacitance detector for a video disc, characterized in that an oscillation signal is transmitted to the disc stylus resonant circuit.