Clipping distortion controlling circuits

Details Clipping distortion controlling circuits Clipping distortion controlling circuits

1997-03-13

2001-04-03

Negash, Kinfe-Michael (Department: 2733)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C375S296000

Reexamination Certificate

active

06211984

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a clipping distortion controlling circuit, and in particular to a clipping distortion circuit for controlling generated from a light-emitting element in an optical transmitter used for an optical CATV (Cable Television) system in which both a digital signal and an analog signal are transmitted simultaneously.
DESCRIPTION OF THE PRIOR ART
When both a digital signal and an analog signal, for example including a 64 QAM (Quadrature Amplitude Modulation) signal, are transmitted simultaneously using a multi-carrier transmission system like an optical CATV system, problems result from an impulse-attributive clipping distortion generated from a light-emitting element such as a laser diode and, caused by the analog signal, causes the error-rate of the digital signal to become floor-state. In other words, the error-rate of the digital signal cannot be improved beyond a certain value.
The clipping distortion will be described in reference to FIG.
1
. As shown in
FIG. 1
, the waveform x denotes an input-current waveform of the laser, the waveform y denotes an optical-output waveform corresponding to the waveform x, and curve z denotes an I-L (laser driving current-optical-output) characteristic of the laser. In the case of the multi carrier transmission scheme using a plurality of carriers, generally an input waveform x of the laser becomes superpositioned with a plurality of sinusoidal waves. For this reason, in cases where some carriers all in phase with each other, and carrier amplitude thereof is also in phase, the amplitude thereof grows, so that the input waveform of the laser has a peak value greater than the oscillation threshold value T of the laser. In this case, the input waveform x is clipped. This is shown in
FIG. 1
as the clipping C of the waveform y. An impulse-attributive large distortion is generated by this clipping. This is the clipping distortion.
The bandwidth of the clipping distortion extends over the large bound. If the level of the distortion is large, a problem results in that the error rate of a digital-channel becomes floor-state with high-level (for example, 1×10
−3
). In order to eliminate this problem, the level of the input signal can be reduced. However, other problems arise in that the CNR (Carrier to Noise Ratio), which has a minimum required level, is too low in this case where the level of the input signal is diminished.
The literature cited: Clipping Noise Suppression by Limiter Circuit (1955; Electronic Intellingence Communication Society Meeting B-682), includes a technique which resolves the problems. In this technique, the analog signal passes through both a limiter and a low-pass filter before the analog signal is multiplexed with the digital signal.
A circuit which suppresses the clipping distortion will be described by referring to FIG.
2
. In
FIG. 2
, a conventional clipping distortion circuit has a light-output
300
, wherein both the analog RF (Radio Frequency) signal
100
and the digital RF (Radio Frequency) signal
200
are input signals. The circuit shown in
FIG. 2
comprises an amplifier
1
for amplifying the analog RF signal
100
, a pre-clipping circuit
11
which operates as the limiter, a filter
3
that can be a low-pass filter which eliminates a radio-frequency component of the output thereof, a multiplexer
10
for superimposing the digital RF signal
200
on the filtered-output, and a light-emitting element
5
for converting the multiplexed output into a light-signal.
The light-emitting element
5
is a semiconductor laser element such as a laser diode and Vee is a power-supply voltage. C
1
and C
2
are DC-blocking capacitors.
In this circuit, the pre-clipping circuit
11
causes the signal outputted from the amplifier
1
to first clip by means both of the diode
111
whose anode side is grounded through a DC power supply
113
, and the diode
112
whose cathode side is grounded through the DC power supply
114
. Regarding the output signal of the pre-clipping circuit
11
, the amplitude of the output signal is restricted and a lot of distortion-components are incorporated. The distortion, which increases the high-frequency component rather than the carrier component of the analog RF signal
100
, affects the digital signal. For this reason, the low-pass filter
3
is provided for blocking the distortion. The distortion component is eliminated by the low-pass filter before the digital RF signal
200
is superimposed by the multiplexer
10
. As a result, generation of the clipping distortion caused by the laser element is prevented, because the analog RF signal
100
is subjected to the amplitude restriction by the pre-clipping circuit
11
.
However the above-described conventional circuit cannot cope with the level-fluctuation of the clipping distortion caused by gain-fluctuation of the amplifier or characteristic degradation of the laser element or the like. In a worst case, this defect can cause communication-failure.
Japanese Patent Application Laid-Open No. Hei 1-277368 relates to optical information record reproducing and, accordingly, does not resolve the above defects.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide a novel and improved circuit that eliminates the above-described defects of the conventional art by controlling clipping distortion, in order to stabilize the amount of generation of the clipping distortion.
According to one aspect of the present invention, for achieving the above-mentioned object, there is provided a clipping distortion controlling circuit that comprises limiting means for restricting an analog input signal below a required threshold value before outputting thereof, a semiconductor laser element for converting the restricted-output signal together with a digital input signal into an optical signal and transmitting the converted optical signal, and controlling means for changing the threshold value in accordance with a distortion-component of the optical signal.
As stated above, a clipping distortion controlling circuit according to the invention in which in a level of the clipping distortion generated by a light-emitting element, a semiconductor laser element, is monitored through a light-receiving element and a band-pass filter, a limiter-level of a pre-clipping circuit is changed in accordance with the distortion level. For this reason, the circuit is capable of coping with the level-change of the clipping distortion caused by gain-change of an amplifier or by characteristic degradation of the light-emitting element, and stability is obtained.
The above and further objects and novel features of the invention will be more fully understood from the following detailed description, read in connection with the accompanying drawings. It should be expressly understood, however, that the drawings are for purpose of illustration only and are not intended as a definition of the limits of the invention.


REFERENCES:
patent: 5680238 (1997-10-01), Masuda
patent: 1-277368 (1989-11-01), None
patent: 2-207585 (1990-08-01), None
Kishimoto et al., “Suppression of clipping-induced distortion by means of a limiting circuit technology”,Electronic Intelligence Communication Society Meeting B-682, 1955, p. 349.

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