JPS6057708A - Fm demodulator - Google Patents

Abstract

PURPOSE:To prevent generation of noise by providing an FM carrier level detecting means to an FM demodulator having a PLL circuit so as to stop the operation of the PLL circuit when a carrier level is missing. CONSTITUTION:An FM demodulation signal A is inputted to a terminal 5. The signal A is compared with an output of a voltage controlled oscillator VCO2 at a phase comparator 1. An output of the comparator 1 is inputted to a low-pass filter 3 via a switch 7. An output of the filter 3 is amplified by a buffer amplifier 4 and fed to the VCO2 and also fed to an output terminal 6 as a demodulated output. The signal A is fed also to a carrier level detector 8. When the carrier level is missing, the switch 7 is interrupted by a switching control circuit 9. The switch 7 is interrupted by a mute command.

Description

[Detailed Description of the Invention] Technical Field> The present invention provides an FM suitable for use in video tape recorders, etc.
This invention relates to a demodulator.

<Prior Art and Problems> In general, in video playback devices such as video tape recorders and video disc players, the FM carrier wave in the playback signal often rapidly attenuates or disappears during playback.
In such a case, spike-like noise occurs in the output of the FM demodulator.

Therefore, in the conventional video reproducing apparatus of this type, a noise removing device was inserted after the FM demodulator to counteract such noise, but in this case, the circuit configuration became complicated and the cost increased.

<Objective> The present invention has been made in view of the above points, and it is an object of the present invention to provide an FM demodulator that can suppress the generation of noise when a carrier wave is missing and does not require the insertion of a noise removal device at a subsequent stage.

Furthermore, the present invention provides an FM demodulator that can mute the demodulated output at any time as desired, not only when a carrier wave is missing.

<Examples> The present invention will be described below according to examples shown in the drawings. The drawing shows one embodiment of the device of the present invention, where 1 is a phase comparator, 2 is a voltage controlled oscillator (VCO), 3 is a low-pass filter consisting of a resistor R and a capacitor C, 4 is a buffer amplifier, These constitute a general FM demodulator that employs a PLL (phase-locked loop) system.

As is well known, the demodulation operation of this device is such that when an FM modulated signal is supplied to the input terminal 4, this FM modulated signal is applied to the phase comparator 1, where the phase is compared with the oscillation signal from the VCO 2. The 0 comparison output is supplied to the buffer amplifier 4 through the low-pass filter 3, where the low frequency component is amplified, and then applied to the VCO 2 as a control voltage and output to the output terminal 6 as a demodulated output. In addition, an on/off switch 7 and a carrier wave level detector 8 are added to the FM demodulator using the PLL method as described above.
Although the on-off switch 711''j is shown as a mechanical switch, it is actually preferable to use an electronic switching circuit.

In the FM demodulator configured as described above, the OFF switch 7 is in the ON state during normal operation, and in this case, the phase comparator 1. A PLL circuit is formed by the VCO 2, buffer amplifier 4, low-pass filter 3, and on/off switch 7, and a demodulated output from the output terminal 6Vc is derived by the same circuit operation as described above.

However, when the carrier wave of the FM modulation signal supplied to the input terminal 5 temporarily decreases or disappears, this is detected by the carrier wave level detector 8, and the switch control circuit 9 is activated. is in the off state. Therefore, in this case, the input impedance of the buffer amplifier 4 is high, and the charge charged in the capacitor C in the low-pass filter 3 is held without being discharged, so that the control voltage to the VCO 2 is held at a constant value from then on, and The demodulated output delivered to the output terminal 6 also maintains the previous output. Result input F like this
No spike-like noise is generated at the output terminal 6 when the M carrier wave is missing.

In the above embodiment, if you want to mute the demodulation output during steady FM demodulation operation, use another input terminal 10.
When a mute command is given to the VCO 2, the switch control circuit 9 is activated, the on/off switch 7 is turned off, and the same operation as described above suppresses the change in the control voltage to the VCO 2, resulting in the demodulated output being muted. In the above embodiment, the buffer 1 amplifier 4 with large input impedance is inserted into the PLL circuit, but the VCO
It is also possible to omit the buffer amplifier 4 by increasing the input impedance of the control input to 2.

Effect> As described above, according to the present invention, the F
In the M demodulator, there is no possibility that spike-like noise will occur in the out-of-key output of the input FM carrier.

Furthermore, according to the present invention, it is possible to mute the demodulated output at any time by temporarily stopping the operation of the PLL circuit when desired.

[Brief explanation of drawings]

The drawing is a block diagram of one embodiment of the device of the present invention.

Claims (1)

[Claims] 1. In an FM demodulator having a PLL circuit consisting of a phase comparator, a voltage-controlled oscillator, a low-pass filter, etc., an FM carrier level detection means for detecting an input FM carrier level; Based on the detection output, the P
1. An FM demodulator comprising: switching means for controlling on/off operation of an LL circuit, and stopping operation of the PLL circuit when an input FM carrier wave is lost, etc. 2. The FM demodulator according to claim 1, wherein the switching means is actuated whenever PJr is desired to temporarily stop the operation of the PLL circuit.