Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – By integrating
Reexamination Certificate
1999-07-28
2002-11-05
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
By integrating
C327S344000, C327S345000
Reexamination Certificate
active
06476660
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a fully integrated long time constant integrator circuit which finds application in long time constant feedback loop arrangements such as control circuits in optical receivers.
BACKGROUND TO THE INVENTION
The ever increasing demands for high capacity communications systems has seen the wide spread employment of optical fibre networks across the world. A fundamental component for such systems is a means of converting optical pulses comprising a digital bit stream into electrical signals. This component of such a system is commonly known as an optical receiver.
The operational requirements of such a receiver are very demanding. The receiver is required to exhibit a very low noise characteristic, such that it is capable of detecting very low levels of optical input in systems employing maximum optical fibre lengths, thus requiring high gain amplification for maximum sensitivity, but is conversely required to cope with high levels of optical input in systems employing short fibre lengths, thus requiring low gain amplification. As such, the optical receiver is required to have a wide dynamic range which can only be practically achieved with some form of automatic gain control (AGC). A typical integrated circuit (IC) optical receiver
10
is illustrated in block schematic form in FIG.
1
. This comprises an IC (denoted by broken line
12
) including a transimpedance amplifier stage (denoted by broken line
14
) with an integrator in a control loop providing AGC.
As illustrated by
FIG. 1
, optical input power OP
IN
is converted into an electrical current I
IN
by a PIN diode photodetector
16
. This current I
IN
is applied as an input to the IC optical receiver
10
. The input current I
IN
is amplified by a transimpedance amplifier (Tz Amp)
18
which converts the input current I
IN
into an amplified voltage output signal V
OUT
. To meet the requirement of wide dynamic range, the output voltage V
OUT
of the Tz Amp
18
which is in the form of a broadband data signal and may be considered as an ac, multi-frequency signal, is rectified or peak detected by a rectifier/peak detector
20
to provide a dc signal level V
REC
for comparison with a pre-determined dc reference voltage V
REF
. The difference between the rectified/peak detected output voltage V
REC
and the reference voltage V
REF
is considered as an error signal which is amplified and integrated by a Miller Integrator
22
to provide a control signal V
CONTROL
. A Miller Integrator is a well known form of integrator incorporating an active device such as a transistor amplifier. The Miller Integrator
22
is required to have a high gain, in order to ensure that the error signal approaches zero (ie in order to ensure that the difference between the rectified/peak detected output voltage V
REC
and the reference voltage V
REF
becomes zero) by means of controlling the gain of the Tz Amp by varying the impedance of a feedback resistor
24
.
If the rectified/peak detected output voltage V
REC
is smaller than the dc reference voltage V
REF
, then the Tz Amp
18
must operate at high gain to provide high sensitivity of the optical receiver. When the rectified/peak detected output voltage V
REC
becomes just greater than the dc reference voltage V
REF
, then the on-set of AGC occurs and continues whilst the input channel I
IN
increases. When the feedback resistor
24
is at a minimum the Tz Amp is operating at very low gain and approaches an overload condition.
In addition, the Miller Integrator is required to have a long time constant (Tp) so that the effect of the AGC action of the control loop does not compromise data embedded in the voltage output signal V
OUT
of the Tz Amp.
The requirements for the time constant Tp of the Miller integrator can be better understood with reference to
FIG. 2
which identifies the fundamental gain stages of the typical optical receiver of FIG.
1
.
To understand the effects of the Miller Integrator time constant Tp, each gain block of the optical receiver
10
must be considered. For a first order approximation, the Tz Amp gain of the optical receiver
10
is proportional to V
OUT
/I
IN
and can be considered as the value Rf of the feedback resistor
24
. This assumes that the Tz Amp gain is constant for all frequencies up to an upper −3 dB point. This assumption is only true if the AGC is not operating which is often the case at low optical input levels. Once the AGC begins to operate to prevent the output signal V
OUT
from increasing further, this has a significant effect on the Tz Amp gain. Using standard feedback control theory, the presence of a pole in the control loop feedback path (ie Tp of the Miller Integrator) presents a zero in the forward Tz Amp gain path, reduced by a factor of the loop gain. To illustrate the above, consider the loop gain of the optical receiver as:
Loop Gain=Tz
o
.A
r
.A
o
/Rf
where Tz
o
=Open loop gain of the Tz Amp
Rf=Value of the feedback resistor
A
r
=Rectifier gain
A
o
=Open loop gain of the Miller Integrator
The forward closed loop transimpedance gain of the Tz Amp is given by:
Tz
CL
=(1+sTz).Rf
where S=Laplace operator
and Tz=Tp/(Loop Gain)
∴Tz=Tp.Rf/(Tz
o
.A
r
.A
o
)
Consequently, the time constant Tz in the forward Tz Amp gain path is greatly reduced by the loop gain of the control circuit. In the typical arrangement, the Miller Integrator pole position (ie Tp of the Miller Integrator) is such that it results in a transmission zero in the MHz region. This can have the undesirable effect of generating pattern dependant jitter in the broadband data stream.
In a typical scenario, for a 155 Mbit/sec data stream, the transmission zero should be at 25 KHz or below to prevent jitter in the broadband data stream, representing a time constant Tz=6.36 &mgr;secs. Typically, Tz
o
=4 M&OHgr;, A
r
=2, A
o
=100 and Rf=50 K&OHgr;. Consequently, the loop gain is 16000(or 84.1 dB). This requires a very long, relatively speaking, Miller Integrator time constant Tp of approximately 0.1 secs.
Using present bipolar IC technology, the maximum practicable size of resistors than can be manufactured “on chip” are in the M&OHgr; region. For example, if a 1.5 M&OHgr; resistor is fabricated on chip, the required value of capacitor to provide a 25 KHz high pass cut-off needs to be in the order of 67 nF. However, present bipolar IC technology allows a maximum practicable value of capacitors in the region of tens to hundreds of pF to be formed on chip. Therefore, it can be seen that to achieve the necessary high pass cut-off frequency of 25 KHz would require connection of a large size discrete component capacitor to the optical receiver integrated circuit. This normally comprises a lumped silicon device which is mounted on pads on the silicon substrate containing the integrator IC. The optical receiver IC is normally contained in a DIL package which is hermetically sealed. Experience has shown that it is the connections of the discrete component capacitor which provide the most likely points of failure of the device under test. Failed devices are normally discarded, it being extremely difficult and expensive to recover any of the constituent parts of the device for reuse. A known alternative is to connect a combination of discrete resistor and capacitor components to the optical receiver IC but this is equally undesirable for the same reasons as aforesaid.
It is also known to fabricate IC transimpedance amplifiers using BiCMOS technology. In such a case, a Field Effect Transistor (FET) can be used to provide very low current leakage of an on-chip capacitor which has been charged from the peak detection circuit thus providing the necessary long time constant. This technology allows a fully integrated IC optical receiver to be provided but at a higher cost than one provided using bipolar technology.
OBJECTS OF THE INVENTION
The invention seeks to provide a long time constant IC integrator without requiring exte
Flett Robin M
Visocchi Pasqualino Michelle
Whittaker Edward J W
Lam Tuan T.
Lee Mann Smith McWilliams Sweeney & Ohlson
Nguyen Hiep
Nortel Networks Limited
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