JP2001333014A - Fm detector circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an FM detector circuit which is externally provided with a muting detector circuit and also has a muting function that is not affected when an optical fiber cable is disconnected or by the sensitivity of a photodetector with a simple means using the level detection output of an integrator for FM IF amplification detection. SOLUTION: This FM detector circuit for performing optical-to electrical conversion of an optical modulation signal Lm including a sound signal transmitted by an optical fiber cable with the avalanche photodiode of a receiver side and reproducing the optical modulation signal, controls an externally provided muting detector 1g to generate a muting signal, mutes the sound FM detection output Vo of the receiver and also amplifies a sound signal subjected to FM detection with an FM IF amplification detector 2f up to a required level with an amplifier 2g to be outputted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM detection circuit having an audio muting function which is suitable when an optical modulation signal including an audio signal transmitted by an optical fiber cable is interrupted by a disconnection of the optical fiber cable or the like. Things.

[0002]

2. Description of the Related Art Conventionally, an FM receiver for receiving an optical modulation signal including an audio signal generally uses an FM detection circuit having a configuration as shown in FIG. Also, FM
The detection circuit generally has a muting function,
FM IF Amplifying and Detecting Integrator 2 with Configuration shown in FIG.
f 'was used. This FM detection circuit amplifies an electric signal i optically / electrically converted by an avalanche photodiode 2a by a transimpedance preamplifier 2b and limits noise generated when an optical modulation signal Lm is input to the optical receiver side. 2c, demodulator 2
d, the baseband signal modulated in advance is restored, and only the modulated wave frequency band of the FM audio signal included in the baseband signal demodulated by the demodulator 2d is extracted via the bandpass filter 2e, and the FM IF amplification detector 2f And the audio signal Vm FM-detected by the FM IF amplification detector 2f
Is amplified to a required level by an amplifier 2g and output.

[0003]

By the way, in the FM detection circuit having the conventional configuration, the input level IN of the FM signal is detected, and if there is no carrier, the detection output VO is attenuated by the muting function to reduce the influence of noise. I didn't come out. However, when the optical modulation signal Lm including the audio signal is transmitted through the optical fiber cable, noise due to dark current can be avoided depending on the sensitivity of the avalanche photodiode 2a of the light receiving element used in the FM receiver. Did not. That is, in the optical-electrical conversion circuit using the avalanche photodiode 2a on the receiver side,
A self-bias system that automatically amplifies the light receiving signal level when the light receiving amount decreases is adopted. Therefore, since the dark current is also amplified at the same time, the voltage level of the noise component may be higher than the signal component, and when there is no light input such as when the optical fiber cable is disconnected, furthermore, There was a drawback that the noise level increased.

Accordingly, an object of the present invention is to control a muting detection circuit to generate a muting signal by a simple means using an externally provided muting detection circuit and using a level detection output of an FMIF amplification detection integrated circuit. Accordingly, it is an object of the present invention to provide an FM detection circuit having a muting function that can surely mute an FM detection output and is not affected by disconnection of an optical fiber cable or sensitivity of a light receiving element. The FM detection circuit of the present invention performs muting in a receiver in an optical fiber transmission system by not only detecting a carrier wave level as in the related art but also detecting a dark current noise level of an avalanche photodiode. It is a feature.

[0005]

According to an aspect of the present invention, an FM detection circuit according to the present invention converts an optical modulation signal Lm including an audio signal transmitted through an optical fiber cable into an optical signal through an avalanche photodiode on the receiver side. This is an FM detection circuit for electrical conversion and reproduction, which is generated when an optical / electrically converted electrical signal i is amplified by a transimpedance preamplifier 2b and an optical modulation signal Lm is input to an optical receiver. The noise is suppressed by the limiting amplifier 2c, the demodulator 2d restores the baseband signal modulated in advance for transmission by the optical fiber cable, and is included in the baseband signal demodulated by the demodulator 2d via the bandpass filter 2e. Only the modulated wave frequency band of the FM audio signal to be extracted and input to the FM IF amplification detector 2f,
The FM IF amplification detector 2f detects the dark current noise level of the avalanche photodiode,
The external muting detector 1g is controlled to generate a muting signal, and the sound FM detection output V
o, and the audio signal FM-detected by the FM IF amplification detector 2f is amplified to a required level by an amplifier 2g and output.
An M detection circuit is provided.

According to the aspect of the present invention, since the external muting detector 1g is controlled by the level detection output Vs of the FM IF amplification detector 2f to generate a muting signal, a carrier wave is detected in a normal state. In the event of abnormalities in which the modulated wave audio signal is demodulated and the amount of received light is reduced, the dark current noise level of the avalanche photodiode 2a is detected and muting of the audio FM detection output of the receiver is performed. It can be attenuated to a specified level.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment Hereinafter, an FM detection circuit according to the present invention will be described in detail with reference to an embodiment shown in the drawings. FIG. 1 is a block diagram showing a configuration of a main part of the present invention, and FIG. 2 is a block diagram showing a configuration of a receiver side in an embodiment of the present invention. FIG. 3 is an explanatory diagram showing a muting function in the FM detection circuit according to the present invention. In addition, this does not limit the present invention.

Referring to FIG. 1, an electric signal i optically / electrically converted by an avalanche photodiode 2a is amplified by a transimpedance preamplifier 2b.
The noise generated when the optical modulation signal Lm is input to the optical receiver side can be suppressed by the limiting amplifier 2c. A demodulator 2d demodulates a baseband signal that has been modulated in advance for transmission by an optical fiber cable, and modulates the frequency of a modulated wave of an FM audio signal included in the baseband signal demodulated via a bandpass filter 2e. Only the band is taken out and input to the FM IF amplification detector 2f. It should be noted that a band-pass filter 2e for extracting only a modulated wave frequency band required as a pre-processing for inputting the modulated wave to the detector 2f is externally used.

In the FM detection circuit according to the present invention, the muting detector 1g is externally connected to the FM IF amplification detector 2f, and the level detector 1e detects the dark current noise level of the avalanche photodiode 2a in addition to the carrier wave. The muting detector 1g is controlled by the output Vs corresponding to the dark current noise level to generate a muting signal. When a muting signal is generated,
Muting of the audio FM detection output, which is the modulated signal reproduced by the last-stage quadrature demodulator 1c, is performed via the muting buffer 1d.
The audio signal FM-detected by the FM IF amplification detector 2f is amplified to a required level by the amplifier 2g and output.

FIG. 3 shows an FM input level IN and a detection output Vo.
It is explanatory drawing which shows the relationship. Now, when the optical fiber cable is disconnected and does not enter the receiver, the level detector 1e
Is increased, but the output of the muting detector 1g is set in advance to the operating point of the muting buffer 1d. Therefore, when the output Vs of the level detector 1e exceeds the threshold value, a comparator built in the muting detector 1g generates a muting signal, and the muting buffer 1d operates. As a result, F of the demodulation unit 1c
Since the M detection output Vo is instantaneously attenuated from the detection output voltage Vo to the specified voltage level VL, no noise is generated from the audio output even when the optical fiber cable is disconnected.

As described above, the FM detection circuit of the present invention can perform muting at the time of disconnection of an optical fiber cable with a simple circuit configuration simply by externally attaching the muting detector 1g, and can reduce noise due to dark current. , The detection output can be muted to prevent generation of noise from the audio output. It is needless to say that a carrier wave can be detected and an audio signal which is a modulated wave can be demodulated when the transmission by the fiber cable is normal and there is an optical input.

[0012]

According to the FM detection circuit of the present invention, when an audio signal is transmitted through an optical fiber cable, the FM detection output when the optical fiber cable is disconnected can be instantaneously muted with a simple circuit configuration. Noise can be prevented, and only high-quality audio signals can be reproduced, which has a great practical effect.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a main configuration of the present invention.

FIG. 2 is a block diagram illustrating a configuration of a receiver according to the embodiment of the present invention.

FIG. 3 is an explanatory diagram showing a muting function in the FM detection circuit according to the present invention.

FIG. 4 is a block diagram showing a configuration of a main part of a conventional example.

FIG. 5 is a block diagram showing a configuration of a conventional receiver.

[Explanation of symbols]

 1a IF amplifier 1b Filter 1c FM detector 1d Muting buffer 1e Level detector 1f Phase shifter 1g Muting detector 2a Avalanche photodiode 2b Preamplifier 2c Limiting amplifier 2d PFM demodulator 2e P. F 2f, 2f ‘FM IF Amplification detector 2g Amplifier Vo Detection output voltage Vs Level detection output voltage i Electric signal Lm Optical modulation wave input IN FM input

Claims (1)

[Claims] 1. An FM for optical / electrical conversion of an optically modulated signal including an audio signal transmitted through an optical fiber cable by an avalanche photodiode on a receiver side and reproducing the signal.
A demodulation circuit, which amplifies the optical signal i which has been optically / electrically converted by a transimpedance preamplifier 2b and suppresses noise generated when an optical modulation signal is input to the optical receiver side by a limiting amplifier 2c to demodulate. A demodulator 2d restores a baseband signal that has been modulated in advance for transmission over an optical fiber cable, and a modulated wave frequency band of an FM audio signal included in the baseband signal demodulated by the demodulator 2d via a bandpass filter 2e. The FM IF amplifying detector 2f detects only the dark current noise level of the avalanche photodiode, and outputs the dark current noise level to the FM IF amplifying detector 2f.
The external muting detector 1g is controlled to generate a muting signal, and the sound FM detection output V
A FM detection circuit, characterized in that the FM signal is muted and the audio signal FM-detected by the FM IF amplification detector 2f is amplified to a required level by an amplifier 2g and output.