JPH01221031A - Optical receiver - Google Patents

Abstract

PURPOSE:To reduce the circuit scale by omitting an LPF of a 1st demodulator and a pulse width constant circuit of a 2nd demodulator and constituting the demodulation system by a pulse width constant circuit of the 1st demodulator and an LPF of the 2nd demodulator. CONSTITUTION:An input signal is converted into a pulse frequency modulation wave by the 1st PFM modulator 9, inputted to the 2nd PFM modulator 10, where it is further subject to PFM modulation. An output of the 2nd PFM modulator 10 is given to a light emitting element drive circuit 11, where the light emitting element 12 is subject to intensity modulation and its optical output is sent via an optical fiber 13. On the other hand, the reception side converts the optical signal from the optical fiber 13 into an electric signal by a photodetector 14, and amplified to a specified level by a pulse amplifier 15. Its output is demodulated into an original signal by a demodulator 5. The demodulator 5 consists of the pulse width constant circuit 1 and the LPF 2. Moreover, the demodulation signal is amplified by a signal amplifier 16.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is applied to an optical transmission system. In particular, the present invention relates to a double PFM modulation method in which a transmission signal is pulse frequency modulated and then subjected to electro-optical conversion (hereinafter referred to as PFM modulation).

〔overview〕

The present invention provides a means for demodulating an electrical signal related to an optical signal obtained by converting a doubly premodulated electrical signal, in which the low frequency range of a first demodulator is fixed and the pulse width of a second demodulator is made constant. By omitting the circuit from the conventional example, the circuit scale can be reduced and the occurrence of nonlinear distortion can be reduced.

[Conventional technology]

Conventionally, in this type of dual PFM modulation method, the transmitting side has a first PPM modulator that modulates the transmission signal, and a first PPM modulator that modulates the transmission signal.
It consists of a second PPM modulator (or FSX modulator) that modulates the output of the modulator again, and an electro-optic converter, and the receiving side includes the opto-electric converter and the first PPM modulator that corresponds to the second PPM modulator.
PFMi modulator 6 and a second PPM modulator corresponding to the first PPM modulator
It consists of an FM demodulator 7. Further, a PFM demodulator is generally configured with a pulse width constant circuit and a low-pass filter (hereinafter referred to as LPF).

FIG. 3 shows the configuration of a conventional demodulating means. Here, the first
In the pulse width constant circuit l included in the PPM demodulator 6--
The chemotactic pulse width τ is smaller than the pulse width τ2 that is made constant by the pulse width constant circuit 4 included in the second P-FM demodulator 7. That is, r,<r2, and LPF3 included in the first PPM demodulator 6
The cutoff frequency f is higher than the cutoff frequency f2 of the LPF 2 included in the second PFM demodulator 7. That is, f,>f2.

[Problem that the invention seeks to solve]

On the receiving side of such a conventional dual PFM modulation system, the output signal of the second PPM modulator is demodulated by the first PPM demodulator, and then the output signal of the first PFM modulator is demodulated by the second PPM modulator.
Since it has a two-stage configuration in which demodulation is performed using a demodulator, it has the disadvantage of requiring a large number of components.

The present invention aims to eliminate such drawbacks, and aims to provide an optical receiver having demodulation means with a simple circuit configuration.

[Means for solving problems]

The present invention includes a light receiving unit that receives an optical signal obtained by converting an electrical signal obtained by doubly modulating an electrical input signal, and converts the optical signal into an electrical signal, and demodulates the electrical signal converted by the light receiving unit. and demodulating means for generating an electrical signal corresponding to the electrical input signal, wherein the demodulating means converts the pulse width of the electrical signal converted by the light receiving means into the doubly modulated electrical signal. a pulse width constant circuit that maintains the pulse width at a substantially constant pulse width of 2 or less of the period corresponding to the highest frequency of the signal; and a cutoff frequency near the upper limit of the frequency band of the electrical input signal, and the output of the constant pulse width circuit. It is characterized in that the low range to which electrical signals are applied is equipped with a wave generator.

[Effect]

On the transmitting side, the transmission signal is modulated by a first PPM modulator,
Furthermore, the signal output from the first PPM modulator is
It is modulated by a PM modulator (or FSK modulator), and this pre-modulated electrical signal is converted into an optical signal and sent out.

On the receiving side, a light receiving means converts an incoming optical signal into an electrical signal of a specified level, and this electrical signal is demodulated into an original signal by a demodulator including a pulse width constant circuit and an LPF.

〔Example〕

Hereinafter, one embodiment of the present invention will be described based on the drawings.

FIG. 1 is a block diagram showing the configuration of this embodiment. As shown in FIG. 1, this embodiment includes an optical receiver of the present invention and an optical transmitter that supplies an optical signal to the optical receiver. The optical receiver includes a light receiving element 14 and a pulse amplifier 15, which are light receiving means for receiving an optical signal obtained by doubly modulating an electrical input signal and converting this optical signal into an electrical signal, and a pulse amplifier 15 for receiving the light. The demodulator 5 is a demodulating means for demodulating the electric signal converted by the means to generate an electric signal corresponding to the electric human signal, and the demodulator 5 is the light receiving means. a pulse width constant circuit 1 that keeps the pulse width of the electrical signal to be converted substantially constant to a pulse width of 2 or less of the period corresponding to the highest frequency of the doubly modulated electrical signal; and a frequency of the electrical input signal. A cutoff frequency is set near the upper limit of the band, and a wave generator 2 is provided in the low frequency range to which the electrical signal output from the pulse width stabilization circuit 1 is applied.

The conventional demodulation circuit has a first pulse width constant circuit 1 and a first LP.
The demodulation circuit of the present invention is composed of a first PPM demodulator 6 consisting of an F3, a second pulse width constant circuit 4, and a second PPM demodulator 7 consisting of a second LPF2.
First pulse width constant circuit 1 and second PFM of PM demodulator 6
The feature is that it is configured in combination with the second LPF 2 of the demodulator 7.

Now, the level adjuster 8 adjusts the modulation degree of the first PPM modulator 9. The degree of modulation is not set too large to ensure good distortion characteristics. The human power signal is converted into a pulse frequency modulated wave by the first PPM modulator 9. The amount of frequency shift of the pulse frequency modulated wave here shifts continuously in proportion to continuous level changes of the human input signal. That is, the amount of deviation becomes multivalued on the frequency axis. First PPM modulator 9
The output of is input to the second PPM modulator 10 and further PF
The amount of frequency shift of the second PPM modulator 10 is two values of high frequency fI (and low frequency ft) on the frequency axis. This is due to the fact that even though the output (output of the converter) is multivalued on the frequency axis, it is binary on the amplitude axis.Second PP
The modulation degree of the M modulator 10 is set sufficiently large so that no distortion occurs.

The output of the second PPM modulator 10 is given to a light emitting element drive circuit 11, which intensity modulates the light emitting element 12, and its optical output is sent out via an optical fiber 13.

On the receiving side, on the other hand, the optical signal from the optical fiber 13 is converted into an electrical signal by the light receiving element 14, and amplified by the pulse amplifier 15 to a specified level. The output is demodulated into the original signal by a demodulator 5. The demodulator 5 includes a pulse width constant circuit 1 and an LPF.
It consists of 2. Further, the demodulated signal is sent to a signal amplifier 16.
is amplified.

FIG. 2 shows examples of waveforms at various parts of this embodiment.

〔Effect of the invention〕

As explained above, the present invention eliminates the LPF of the first demodulator and the pulse width constant circuit of the second demodulator, and constructs a demodulation system using the pulse width constant circuit of the first modulator and the LPF of the second demodulator. This has the effect of reducing the circuit scale. Furthermore, since non-linear distortion generated in the omitted circuit can be removed, transmission quality can be improved.

[Brief explanation of the drawing]

FIG. 1 is a block configuration diagram showing the configuration of an embodiment of the present invention. FIG. 2 is a waveform diagram showing the operation of the embodiment of the present invention. FIG. 3 is a block configuration diagram showing the configuration of main parts of a conventional example. 1.4... Pulse width constant circuit, 2. LPF. 5... Demodulator, 6... First demodulator, 7... Second demodulator, 8%... Level adjuster, 9... First PPM modulator, 10... Second PFM modulator , 11... Light emitting element drive circuit, 12... Light emitting element, 13... Optical fiber, 14... Light receiving element, 15... Pulse amplifier, 16
...Signal amplifier.

Claims (1)

[Claims] 1. A light receiving means for receiving an optical signal obtained by converting an electrical signal obtained by doubly modulating an electrical input signal by two modulators connected in cascade, and converting this optical signal into an electrical signal. , a demodulating means for demodulating the electrical signal converted by the light receiving means to generate an electrical signal corresponding to the electrical input signal, wherein the demodulating means is configured to generate the electrical signal converted by the light receiving means. a pulse width constant circuit that keeps the pulse width of the signal almost constant to a pulse width of 1/2 or less of the period corresponding to the highest frequency of the doubly modulated electrical signal; and an upper limit of the frequency band of the electrical input signal. An optical receiver comprising: a low-pass filter having a cutoff frequency in the vicinity thereof and receiving an electrical signal output from the constant pulse width circuit.