Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – By integrating
Reexamination Certificate
2001-08-07
2002-09-24
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
By integrating
C205S214000
Reexamination Certificate
active
06456141
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a current pulse receiving circuit that coverts an input current pulse to a corresponding logic level voltage pulse and outputs the voltage pulse with an accurate pulse width. More particularly, the invention relates to a current pulse receiving circuit used, in optical communications and similar application fields, after a received light pulse is converted to a corresponding current pulse by a photodetector.
2. Description of the Related Art
Recent developments in the telecommunications field has seen an infrared data communications (IrDA communications) function using infrared rays to connect spaces added to portable terminals, personal computers, and cellular phones. Fiber-optic communications networks are being established to build and expand a telecommunications infrastructure.
In optical communications systems such as those noted above, sunlight, illumination by fluorescent lamps and other lighting devices, and other disturbance light are also applied to a photodiode and other types of photodetectors depending on the brightness of the environment in which the equipment is used. These disturbance lights function to energize the photodetector and allow unwanted DC current components to flow through the current pulse receiving circuit. This makes it necessary to remove these DC current components.
The light pulse signals being transmitted and received are generally burst signals whose pulse width and duty ratio vary. Because of its characteristics, the photodiode PD tends to develop rounding on a rising edge and a falling edge of a current output, resulting at times in tail dragging. Furthermore, it is common, from the viewpoint of removing DC offset components, to use a differentiated waveform which is the result of differentiating the pulse signal, which contributes to tail dragging of the output signal. A region of such tail dragging is more conspicuous when the signal has a wider pulse width, which could cause a comparator circuit or amplifier circuit to malfunction so that it inverts its output state. As a means for producing an output of a logic pulse having an accurate pulse width by preventing this malfunction and maintaining the correct output state, it has been conventional practice to add to an input signal to the comparator circuit or the amplifier circuit a hysteresis voltage or other significant differential voltage. This means also contributes to improved noise resistance.
FIG. 26
shows a current pulse receiving circuit
100
as a first related art. A light pulse is received by a photodiode PD and converted to a current pulse IPD. The current pulse IPD is applied to an input node IN of a current-to-voltage converter circuit
101
A, in which it is converted to a corresponding voltage which, in turn, is applied to an inverting input terminal VM
2
of an amplifier circuit
102
A. Though the figure typically shows a single input terminal for the current-to-voltage converter circuit
101
A, the circuit may also be configured as a differential input by using a noninput terminal as a dummy terminal or applying a complementary current pulse. A DC cancellation circuit
105
A detects a DC voltage level of an output signal VM
2
from the current-to-voltage converter circuit
101
A and feeds it back to the input node IN of the current-to-voltage converter circuit
101
A, thereby canceling DC disturbance light, such as sunlight and illuminating light, which are to be converted to corresponding current values by the photodiode PD. It sets a time constant sufficiently large with respect to the input light pulse, thereby canceling only the disturbance light components whose input frequencies are less than several kHz. The DC cancellation circuit
105
A may be configured as a differential output to coincide with the differential input of the current-to-voltage converter circuit
101
A.
The pulse signal converted to a corresponding voltage by the current-to-voltage converter circuit
101
A is input to, and amplified by, the amplifier circuit
102
A. The amplifier circuit functions to improve response in a comparator circuit
103
at a later stage. A signal is provided from a DC feedback circuit
106
to a reference voltage terminal VP
2
of the amplifier circuit
102
A. The DC feedback circuit
106
uses an integrating circuit C
101
, R
101
, and R
102
to integrate a voltage developing at an output terminal VP
3
of the amplifier circuit
102
A and feeds back the resultant voltage to the reference voltage terminal VP
2
of the amplifier circuit
102
A, thereby improving an input offset voltage in the amplifier circuit
102
A. At the same time, it follows changes in a signal input to the inverting input terminal VM
2
with a lag, thus offering a function of adding hysteresis effect between input terminals.
An output from the amplifier circuit
102
A is applied to the comparator circuit
103
via the output terminal VP
3
and, through a comparison made with a reference voltage VTH coupled to a reference voltage terminal VM
3
, a positive logic pulse is output. This positive logic pulse is inverted at an inverter circuit
104
and a negative logic pulse is output from an output terminal RX as the output from the current pulse receiving circuit
100
.
FIG. 27
shows a current pulse receiving circuit
200
as a second related art. It has the same basic circuit configuration as the current pulse receiving circuit
100
shown in FIG.
26
. In the current pulse receiving circuit
200
, the output signal from a current-to-voltage converter circuit
101
B is subjected to capacitive coupling through capacitive components C
102
and C
103
to the input terminal of an amplifier circuit
102
B, thereby improving the input offset voltage in the amplifier circuit
102
B. Therefore, a DC cancellation circuit
105
B also has a differential input configuration.
Like the current pulse receiving circuit
100
, the DC cancellation circuit
105
B functions to cancel DC offset caused by disturbance light. In addition, the input offset voltage in the amplifier circuit
102
B is improved through capacitive coupling to the input terminal of the amplifier circuit
102
B and, at the same time, a significant differential voltage is added between input signals through the input of a differential signal.
However, due to the tail characteristic of a current output determined by the output characteristics of the photodiode PD and a difference in tail characteristics and others in the differentiated waveform of the pulse signal, in addition to widely varying light pulse widths involved with burst signals in IrDA communications and other optical communications, it is difficult to stably establish a hysteresis voltage width sufficient to prevent erroneous outputs from the comparator circuit
103
even in the tail region of current outputs. That is, the setting value deviates between photodiodes PD that have different tail characteristics in output currents. With the differentiated waveform, too, the tail region varies for different constants of the photodiode PD and differentiating circuit and input signal amplitudes. Moreover, the tail region occurs differently according to the strength of the light pulse received, the clamp level of the current pulse IPD, and other factors. The tail characteristic is thus variable depending on the characteristics of the parts used, circuit constant of the differentiating and other circuits, and operating environment, thus presenting a problem in which it is difficult to stably provide an output with a highly accurate pulse width for a light pulse having a long pulse width.
As an example,
FIG. 28
shows input and output waveforms of response of the comparator circuit
103
in the first related art. In this case, the amplifier circuit
102
A provides a single output supplied to the noninverting input terminal VP
3
and a signal having a predetermined hysteresis voltage width with respect to the reference voltage VTH is supplied to the reference voltage terminal VM
3
. A shorter current pulse
Kumano Tatsuo
Nishizono Kazunori
Arent Fox Kintner & Plotkin & Kahn, PLLC
Callahan Timothy P.
Fujitsu Limited
Nguyen Linh
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