Pulse or digital communications – Receivers – Automatic baseline or threshold adjustment
Reexamination Certificate
2000-12-08
2004-11-16
Chin, Stephen (Department: 2654)
Pulse or digital communications
Receivers
Automatic baseline or threshold adjustment
C455S355000
Reexamination Certificate
active
06819722
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an offset control circuit, an optical receiver using the same and an optical communication system. More particularly, the invention relates to an offset control circuit controlling transitionally varying offset level according to an elapsed time when a wide dynamic level is required in an optical receiver for receiving a burst signal, the optical receiver and an optical communication system.
2. Description of the Related Art
In general, an optical receiver employing an optical access system, an optical LAN, an optical interconnection and so forth, is constructed with a light receiving element receiving a digital light signal and converting into a current signal and a reception circuit amplifying the current signal and converting into a digital electric signal having a given amplitude. For an input current signal of the reception circuit in the optical receiver of this kind, a given level of an offset current is generated due to influence of an extinction ratio failure, reflected return light and a dark current generated in a light receiving element of a receiver, and so forth. At the same time, among carriers generated in response to incidence of light, a carrier generated in a portion in the light receiving element where field intensity is low, spreads gradually in a long period to cause offset current varying large time constant in comparison with a clock frequency of the signal.
If the current signal having offset is amplified, a duty ratio of an output waveform is significantly fluctuates to cause difficulty in accurate discrimination of logical “0” level and logical “1” level. Among the problems of fluctuation of the duty ratio due to offset current, the various prior arts suppressing fluctuation of duty ratio due to a given level of offset current have been proposed. For example, as shown in
FIG. 19
, an optical receiver circuit disclosed in Japanese Unexamined Patent Publication No. Heisei 8-84160 (corresponding to U.S. Pat. No. 5,612,810) proposes cancellation of a given level of offset respectively adding peak values of a positive-phase output and a negative-phase output of a differential pre-amplifier
20
and peak values of the negative-phase output and the positive-phase output, and can restrict fluctuation of duty ratio by setting a half value of a signal pulse amplitude as a threshold value for discriminating logical “0” level and logical “1” level irrespective of signal amplitude. In
FIG. 19
, the reference numeral
10
denotes a light receiving element,
40
denotes an automatic discrimination level control circuit,
31
denotes a peak-hold circuit,
32
denotes a peak-hold circuit and
37
,
38
denote adder circuits, respectively.
However, the shown optical receiver circuit cannot suppress offset transitionally varying according to elapsed time to significantly vary the duty ratio of the output waveform as shown in FIG.
19
. Influence of transitional variation of offset according to elapsed time for fluctuation of duty ratio of the output waveform is particularly significant when a reception dynamic range having large level difference (about 30 dB in optical level) of respective burst signals and wide dynamic range is required upon receiving a burst form optical signal in optical access system, optical LAN, optical interconnection and so forth.
An example of a passive Optical Network (PON) to be employed when such wide reception dynamic range is required, is shown in FIG.
22
. In
FIG. 22
, respective optical transmission signal from a plurality of optical subscriber line terminal units (Optical Network Unit: ONU)
1
to
3
are synthesized by a star coupler
4
and then received by an optical receiver
6
in an optical subscriber line Terminal Station Unit (Optical Line Termination:OLT)
5
. As shown, in the PON system, levels of the optical signals transmitted from respective ONUs
1
to
3
in burst manner are different respectively. Thus, in the optical receiver
6
, wide dynamic range is required as set forth above.
Now, consideration is given for the case where the optical receiver
6
shown in
FIG. 22
receives small level burst signal B
2
after reception of large level burst signal B
1
. By offset due to influence of the carrier having low spreading time constant generated while the large level burst signal B
1
is received, this offset may reside even upon starting of reception of the small level burst signal B
2
to slowly attenuate offset at low speed time constant while small level burst signal B
2
is received. In this case, a ratio of variation in elapsed time of an offset level with respect to the signal amplitude becomes non-ignorably large to cause significant fluctuation of the duty ratio of the output waveform.
As a prior art for restricting transitionally varying offset according to elapsed time, there has been proposed an optical receiver disclosed in Japanese Unexamined Patent Publication No. Heisei 10-22521 (corresponding to U.S. Pat. No. 5,737,111). A construction of the optical receiver circuit and waveform at respective portion therein are illustrated in FIG.
21
. In this optical receiver, by an offset detection circuit constructed with a resistor and a capacitor, a voltage value corresponding to an offset current varying transitionally according to elapse of time, is detected. In a current drawing circuit, the detected voltage value is converted into a current to withdraw from the output current of the light receiving element. Then, the output current withdrawn the current is fed to a pre-amplifier
20
. By this, an offset varying transitionally according to elapse of time can be canceled to restrict variation of the duty ratio of the output waveform.
However, in general, since the waveform of the offset current according to elapsed time which varies transitionally according to elapsed time, is different per each individual light receiving element. Therefore, in the prior art disclosed in Japanese Unexamined Patent Publication No. 10-22521, the resistance value of the resistors forming offset detecting circuit has to be adjusted individually per each individual light receiving element. Thus, the resistor has to be variable of the resistance value. For this reason, a variable resistor has to be mounted out of the chip and thus is not suited for integration of the receiving circuit into one chip. Furthermore, the resistor mounted outside of the chip inherently hinder down-sizing of the optical receiver. On the other hand, since adjustment is required for each optical receiver to incur adjustment cost for every optical receiver and thus is not suited for lowering of cost. Furthermore, since the offset detecting circuit is inserted between the light receiving element and the maximum potential power source, a bias value to be applied to the light receiving element is lowered in the extent corresponding to potential drop by the offset detecting circuit.
SUMMARY OF THE INVENTION
An object of the present invention to provide an offset control circuit which can obtain an output waveform free of fluctuation of duty ratio by canceling offset transitionally varying according to elapsed time even upon reception of an optical signal in burst form significantly variable of level difference, an optical receiver employing the same and an optical communication system.
Another object of the present invention is to provide an offset control circuit which can be realized in a form suited for integration of a receiving circuit into one chip without requiring individual adjustment by an external element, an optical receiver employing the same and an optical communication system.
According to the first aspect of the present invention, an offset control circuit comprises offset canceling means for canceling an offset component included in a pair of positive-phase and negative-phase signals and varying transitionally according to elapse of time by using at least one of a peak value and a bottom value of the positive-phase and negative-phase signals.
In the preferre
Chin Stephen
Dickstein Shapiro Morin & Oshinsky LLP.
NEC Corporation
Vartanian Harry
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