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CROSS-REFERENCE TO RELATED APPLICATION
This application discloses subject matter disclosed and claimed in
co-owned, copending application U.S. Ser. No. 07/905836 which is U.S. Pat.
No. 5302922, patented Apr. 12,1994, (Atty Docket NO. 907-102/R.
Heidemann-23) filed on the same day as this application and hereby
expressly incorporated by reference.
TECHNICAL FIELD
The invention concerns a circuit designed to compensate for nonlinear
distortions in signals transmitted through optical communications systems.
BACKGROUND OF THE INVENTION
Such a circuit is known from Electronics Letters, Feb. 28, 1991, Volume 27,
No. 5, pages 421 to 423. It serves as a transit time component, whose
transit time is determined by the input voltage, therefore by the signal
voltage, since the variable capacitance diode, which operates in the
high-distance direction, changes its capacitance according to the blocking
voltage existing therein.
It was shown that the property of the known circuit to equalize signals, is
only available in one of the two possible polarities of the variable
capacitance diode. Whether or not the circuit, hereinafter simply called
"equalizer", produces the desired equalization, depends therefore on
whether the variable capacitance diode is connected to the input line by
its cathode or its anode.
Which polarity is the right one cannot be predicted, because in practical
communication systems, it is possible for the polarity of the signal to be
inverted in the transmission path between the laser on the sending side
and the location of the equalizer, e.g. by inverting amplifiers, which are
unknown to the operator and installer of the communication system, because
they play no role, aside from the nonlinear signal distortion of interest
in this instance. When such an equalizer is installed in a transmission
path, it is undesirable, for practical reasons, to have to determine, by
means of a test, the appropriate one of the two possible polarities of the
variable capacitance diode. It would rather be desirable to have an
equalizing circuit that functions in all instances.
DISCLOSURE OF THE INVENTION
It is therefore the task of this invention to introduce a circuit that
equalizes in the known manner, which can simply be adjusted or regulated
without repolarizing the variable capacitance diode, so that it performs
the desired equalization function in all instances.
According to the present invention, nonlinear distortions in signals
transmitted through optical communication systems are compensated by means
of an LC section, whose capacitive element includes a reverse-biased
varactor diode, wherein an additional reverse-biased varactor diode is
connected in inverse parallel with the first-mentioned varactor diode and
that bias voltages for biasing said varactor diodes are adjustable so that
essentially only one of the varactor diodes has a capacitance varying with
the applied signal voltage.
In further accord with the present invention, two additional varactor
diodes in series opposition, whose total capacitance is determined by an
adjustable bias, may be connected in parallel with the above-mentioned
capacitive element of the LC section.
Further according to the present invention, a control circuit may be
provide comprising a device for measuring at the output of the LC section
a second-order distortion product of one or more pilot signals as a
measure of the nonlinear distortions in the transmitted signals, and a
controller which adjusts the bias of the essentially only one of the
varactor diodes to obtain a maximum reduction of the measured distortion
products and maintains the bias of the other varactor diode at a constant
maximum value.
Further according to the present invention, a control circuit for
controlling the compensating circuit mentioned above comprises a device
for measuring second-order distortion products at the output of the LC
section first mentioned above, of one or more pilot signals as a measure
of the nonlinear distortions in the transmitted signals and a frequency
response measuring device responsive to the output of the LC section for
measuring the level difference of two pilot signals as a measure of the
frequency dependence of the attenuation of the LC section, and a
controller which adjusts the bias of the essentially only one of the
varactor diodes to obtain a maximum reduction of the measured distortion
products and, if necessary, maintains the measured level difference of the
two pilot signals constant during said adjustment by varying the bias of
the other varactor diode.
In a case where the two additional varactor diodes are provided in series
opposition, as mentioned above, in still further accord with the present
invention, a control circuit comprises a second-order distortion products
measuring device responsive to the output of the LC section, including the
two additional varactor diodes in series opposition, for measuring the
second order distortion products of one or more pilot signals as a measure
of the nonlinear distortions in the transmitted signals, and a frequency
response measuring device which measures at the output of the LC section,
including the two additional varactor diodes in series opposition, the
level difference of the two pilot signals as a measure of the frequency
dependence of the attenuation of the transmitted signals, and a controller
which adjusts the bias of the essentially only one of the varactor diodes
to obtain a maximum reduction of the measured distortion products,
maintains the bias of the other varactor diode at a constant maximum
value, and adjusts the bias determining the total capacitance of the
additional two varactor diodes in series opposition, such that the level
difference of the two pilot signals remains substantially independent of
the adjustment of the bias of the essentially only one varactor diode.
In still further accord with the present invention, a circuit for
compensating for nonlinear distortions in signals transmitted through
optical communication systems comprises an LC section whose capacitive
element includes a reverse-biased varactor diode wherein two additional
varactor diodes in series opposition whose total capacitance is determined
by an adjustable bias, are connected in parallel with the reverse-biased
varactor diode.
These and other objects, features and advantages of the present invention
will become more apparent in light of the detailed description of a best
mode embodiment thereof, as illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a principal wiring diagram of the equalizer according to the
invention.
FIG. 2 schematically shows the dependence of the capacitance of a variable
capacitance diode on the blocking voltage existing therein.
FIG. 3 is a further development of the equalizer according to the
invention, to keep the frequency response constant when the equalizer is
adjusted.
FIG. 4 shows an equalizer according to the invention, consisting of a chain
circuit with LC components according to FIG. 1,
FIG. 5 shows an equalizer according to the invention in the form of an LC
chain circuit, with LC components according to FIG. 3., and
FIG. 6 shows a control circuit which may be used to adjust the bias of the
varactors of FIG. 3, for example, according to the invention.
BEST MODE FOR CARRYING OUT THE INVENTION
The equalizer according to FIG. 1 has an inductance L and a first variable
capacitance diode C.sub.a, which is connected by its cathode to inductance
L, and has a bias voltage that is polarized in the high-resistance
direction, i.e., reverse-biased. So far it corresponds to the equalizer
shown in the above mentioned publication (FIG. 4b). The bias voltage for
the variable capacitance diode is provided by a DC circuit, which leads
from ground through a resistor R, the cathode of variable capacitance
diode C.sub.a, its anode, through a resistor R.sub.a, to the negative pole
of a DC voltage source U.sub.a, whose other pole is connected to ground.
With this polarity of the variable capacitance diode, the existing blocking
voltage increases with positive input voltages U.sub.i, thereby decreasing
its capacitance, so that more positive voltages pass through the equalizer
faster than less positive ones. However, the variable capacitance diode
C.sub.a, only produces an equalization when the existing transmission path
is able to delay more positive voltages longer than less positive
voltages. Whether this is so, depends on the number of inverting
amplifiers that have been inserted into the path, and can therefore never
be predicted with certainty. If the premise is wrong, the equalizer
reinforces the existing equalization, instead of opposing it.
To solve the described problem, a second variable capacitance diode C.sub.b
is provided, as shown in FIG. 1, and is connected in antiparallel with the
first variable capacitance diode C.sub.a. While C.sub.a, is connected to
inductance L by its cathode, C.sub.b is connected to L by its anode. A
voltage source U.sub.b is provided, and, together with resistor R and a
resistor R.sub.b, forms a DC circuit for variable capacitance diode
C.sub.b, so that it can also be biased in the high-resistance direction.
The positive pole of voltage source U.sub.b is connected to the cathode of
variable capacitance diode C.sub.b, so that it is biased in the
high-resistance direction. The nodes of both variable capacitance diodes
not connected to inductance L are grounded through capacitors C.sub.K in
alternating voltage fashion.
As with the known equalizer, the capacitance of a normal capacitor may also
be present, in addition to the capacitance of the variable capacitance
diodes.
The new equalizer in FIG. 1 has the property that, regardless of which
polarity distorts the input signal, one of the two variable capacitance
diodes opposes the equalization and the other reinforces the equalization.
To what extent this takes place depends on the amount of existing bias
voltage polarized in the high-resistance direction.
It will now be explained by means of FIG. 2, that by adjusting the amounts
of both bias voltages, one of the two variable capacitance diodes can be
made to perform the desired equalization function, while the other is
nearly inactive with respect to distortion or equalization. The curve in
FIG. 2 shows schematically and only qualitatively, how the capacitance of
a variable capacitance diode depends on the existing blocking voltage in
the high-resistance direction. It can be seen that the capacitance
decreases with increasing blocking voltage, and that the steepness, at
which the capacitance decreases, also decreases as the blocking voltage
increases. Consequently, a diode must operate at a low blocking voltage,
if it is intended to strongly change its capacitance by means of the
existing voltage, and it must operate at a high blocking voltage, if it is
intended to change its capacitance very little with the voltage. Thus, if
the bias voltage -U.sub.a of variable capacitance diode C.sub.a is chosen
for the circuit in FIG. 1, so that its amount is small, a working point is
thereby selected for this variable capacitance diode, at which the
capacitance is highly voltage-dependent, as shown in FIG. 2. If bias
voltage +U.sub.b of variable capacitance diode C.sub.b is simultaneously
chosen, so that its amount is high, a working point is thereby selected at
which the capacitance depends only slightly on the voltage, as shown in
FIG. 2. In that case, depending on how the distortion of the input signals
depends on their polarity, C.sub.a has a strong equalizing effect or a
strong distorting effect, and C.sub.b has a weak distorting or a weak
equalizing effect. Inversely, a high amount of bias voltage in C.sub.b and
a small amount of bias voltage in C.sub.a can place the former in a
condition of strong voltage-dependence, and the latter in a condition of
only weak voltage-dependence.
In the practical application of the new equalizer, it can be detected as
follows, whether the desired equalization function results from a high
amount of bias voltage in C.sub.a and a low amount of bias voltage in
C.sub.b, or vice versa.
The criterion for control of the equalization by adjusting bias voltages
U.sub.a and U.sub.b, is the nonlinear signal distortion experienced by the
signal to be transmitted due to "laser chirp", and the chromatic
dispersion of the optical fiber that forms the transmission path to the
optical receiver. This nonlinear distortion is expressed by a contribution
to undesirable signal portions in the optical receiver's output signal,
and in a possible electrical amplifier connected downstream. Of the
undesirable signal portions, composite second order distortion products
are particularly disturbing. These are oscillations at frequencies such as
do not occur in the transmitted signal, namely oscillations at frequencies
which are composed of the sum and difference frequencies of the
oscillations that constitute the input signal to the transmission path
laser.
To establish a dimension of the occurring distortion products, a so-called
pilot signal of a determined frequency, i.e. an unmodulated oscillation,
may be added to the laser input signal, and the resulting second order
distortion products, namely oscillations with double their frequency
level, can be measured on the receiving side. However, two such pilot
signals may also be added and the resulting oscillation can be measured on
the receiving side by the sum of their frequency levels. In each instance,
second order distortion products are a measure of the nonlinear
distortions of the transmitted signals.
It is the goal of the equalization to suppress the second order distortion
products as much as possible. The relative measure of the suppression of
second order distortion products is therefore an indication of the
effectiveness of the equalization.
The bias voltages of both variable capacitance diodes C.sub.a and C.sub.b
are now adjusted as follows, as a function of the second order nonlinear
distortion factor of the pilot signal, measured at the equalizer output.
First, both bias voltages are adjusted to their maximum value. According
to FIG. 2, at maximum bias voltage they have no significant effect on the
distortion of the signal. Then the amount of one of the two bias voltages
is reduced and the change in the measured nonlinear distortion factor is
observed. For example, if the amount of U.sub.a is reduced and causes the
distortion factor to increase, it is clear that C.sub.a is not the
variable capacitance diode that can produce the desired equalization at
low blocking voltage. U.sub.a is then reset to the maximum value of e.g.
30 V. After that, an optimum equalization is adjusted by means of bias
voltage U.sub.b, which is achieved by decreasing the amount of U.sub.b,
thereby also reducing the measured distortion factor, until the distortion
factor begins to increase again, i.e. has reached its minimum.
The described adjustment of the bias voltages of both variable capacitance
diodes may be performed manually, as a function of an indication from an
instrument measuring the distortion products, or these voltages may be
adjusted by an automatic control, consisting of such a measuring
instrument and a control, which adjusts the bias voltage as a function of
the measured value. An automatic control is preferable, since the
equalizer of the invention is then automatically adjusted for any optical
transmission path, and can also be adjusted for any changes that may occur
in a transmission path due to maintenance, repair or any other changes in
the transmission network, which invert the polarity of the receiving
signals at the end of the transmission path. An adjustment after the
optimization may be necessary during operation, for example with
temperature fluctuations, or due to the aging of components or other
changes.
The above portion of the description considered the case in which one of
the two variable capacitance diodes, e.g. C.sub.a is so biased, that it
performs the required equalization, i.e. functions at a working point at
which its C-U curve (FIG. 2) has the necessary steepness, while the other
variable capacitance diode functions at a working point at which its C-U
curve has practically no steepness. Now, it may happen during the
operation of such an equalizer, that the required equalization decreases,
either because the laser primarily responsible for the distortion in the
transmission path has been replaced with a better one, or that the
equalizer is connected to an optical fiber or a optical fiber with less
chromatic dispersion, or to a optical fiber of a smaller length. The
blocking voltage must then be increased accordingly, so that the equalizer
can function at a low steepness working point in the C-U curve, thereby
adapting the equalization to the change in circumstances.
As shown in FIG. 2, such an adaptation unavoidably reduces the capacitance
of the variable capacitance diode that performs the equalization, which in
turn can change the wave impedance of the line and thereby the attenuation
of the frequency dependence of the signals to be transmitted. It may
therefore occur that, although the equalization is optimized with the
cited adaptation, the frequency dependence of the equalizer attenuation,
the so-called frequency response, is thereby simultaneously degraded.
It would therefore be desirable to be able to balance any reduction of the
LC component's capacitance, undertaken for equalization purposes.
A further development of the invention described so far will be explained
by means of FIG. 3. The left portion of the circuit in FIG. 3 contains
precisely the LC component shown in FIG. 1, which requires no further
explanation. The right portion shows an antiseries circuit of two variable
capacitance diodes C.sub.8 and C.sub.9, which is connected in parallel
with the capacitance of the LC component explained so far. An adjustable
voltage source U.sub.s, whose one pole is connected to ground and its
other pole, positive with respect to ground, is connected through a
resistor R.sub.s to the cathode connection points of both variable
capacitance diodes, serves to bias both variable capacitance diodes
C.sub.8 and C.sub.9 in the high-resistance direction. A capacitor C.sub.L
is grounded parallel to the voltage source, to block high frequency
oscillations from voltage source U.sub.s.
As will be explained later, the equalizer according to FIG. 3 has the
advantage that the total capacitance of the antiseries circuit of both
variable capacitance diodes C.sub.8 and C.sub.9 depends on the bias
voltage U.sub.s, and can be controlled thereby; however, on the other
hand, and at least in the first approximation, it does not depend on
signal voltage U.sub.i, and therefore has no effect on the distortion or
equalization of the signal. In this way, an adjustable capacitance is
located in parallel with the parallel circuit of the C.sub.a and C.sub.b
capacitances, to compensate for an unavoidable change in the capacitance
of C.sub.a or C.sub.b, occurring in conjunction with the required
equalization of the signal.
Since the adjustable capacitance of the antiseries circuit of C.sub.8 and
C.sub.9 offers the possibility to add the capacitance of variable
capacitance diode C.sub.a, adjusted for optimum equalization, to the total
capacitance of the LC component, which guarantees the desired frequency
response of the LC component, it is also possible to first adjust C.sub.a
for medium equalization, as the starting point, and to adjust the total
capacitance of the LC component for an optimum frequency response, by
appropriately selecting the control voltage U.sub.s. If, when starting
from such a working point, it is necessary to increase the equalization,
therefore to increase the capacitance of C.sub.a, this may be balanced by
a corresponding reduction of the capacitance of the antiseries circuit of
C.sub.8 and C.sub.9, by changing the control voltage U.sub.s.
In general, the series circuit of C.sub.8 and C.sub.9 is an adjustable
capacitance, which serves to balance, with respect to the total
capacitance of the LC component, any change of C.sub.a or C.sub.b needed
to optimize the equalization, so that the LC component's frequency
response can be kept constant, in spite of changes in C.sub.a or C.sub.b.
The following explains why the total capacitance of the series circuit
depends essentially only on control voltage U.sub.s, and not on signal
voltage U.sub.i. As long as signal voltage U.sub.i is zero, the control
voltage U.sub.s lies between the cathode and anode of C.sub.8, and also
between the cathode and anode of C.sub.9 (no DC current flows through
R.sub.s). The variable capacitance diodes C.sub.8 and C.sub.9 are equal,
and their capacitance, adjusted by the control voltage U.sub.s, is always
described by C.sub.O. A signal voltage U.sub.i produces a voltage U.sub.C8
=U.sub.s -1/2U.sub.i between the anode and the cathode of C.sub.8, and a
voltage U.sub.C9 =U.sub.s 1/2U.sub.i between the cathode and the anode of
C.sub.9, since half of the voltage U.sub.i drops off in C.sub.8 and half
in C.sub.9. In other words, a change from 0 to U.sub.i in the signal
voltage lowers the blocking voltage U.sub.s in c.sub.8 by 1/2 U.sub.i, and
raises the blocking voltage U.sub.s in C.sub.9 by 1/2 U.sub.i. This
increases the capacitance of C.sub.8 to C.sub.8 =C.sub.0 +.DELTA.C, and
reduces the capacitance of C.sub.9 to C.sub.9 =C.sub.0 -.DELTA.C, where
.DELTA.C represents a low value. The total capacitance of the series
circuit is therefore:
##EQU1##
because .DELTA.C.sup.2 can be neglected due to the small value of
.DELTA.C. Since C.sub.0, as stated above, is only determined by U.sub.s,
the total capacitance, at least in the first approximation, therefore only
depends on U.sub.s and not on signal voltage U.sub.i.
With the above indicated explanation of the antiseries circuit function of
C.sub.8 and C.sub.9, and once it has been determined which of the two
variable capacitance diodes C.sub.a and C.sub.b has the appropriate
polarity for the equalization, it is assumed that only the bias voltage of
this variable capacitance diode is adjusted for the purpose of optimizing
the equalization, and that the bias voltage of the other is kept at its
maximum value. This latter variable capacitance diode plays practically no
role in optimizing the equalization, and could therefore be omitted, if
the appropriate polarity of the equalizing variable capacitance diode is
found by other means than the second antiparallel connected variable
capacitance diode described in FIG. 1, or if there is no uncertainty about
the polarity of the input signal in the first place. In such instances as
well, the antiseries circuit of the additional variable capacitance diodes
C.sub.8 and C.sub.9, described in FIG. 3, can be used to balance changes
in the bias voltage of the equalizing variable capacitance diode, thereby
balancing its capacitance, so that the wave resistance (impedance or
reactance) of the LC component, and thereby its frequency response,
remains unaffected by the change.
On the other hand, it seems possible to omit the antiseries circuit of
C.sub.8 and C.sub.9, and to balance the change in the capacitance of the
one variable capacitance diode C.sub.a, by changing the capacitance of
C.sub.b, i.e. not leaving at the maximum value, but by readjusting it, so
that the wave resistance of the LC component in FIG. 1, and thereby its
frequency response, stays unaffected by a change in the bias voltage
U.sub.a, made to optimize the equalization performed by C.sub.a.
From the above can be seen that the circuit according to the invention
offers several possibilities for adjusting the bias voltages U.sub.a and
U.sub.b, and possibly the control voltage U.sub.8.
The criterion for adjusting the frequency response of the equalizer is the
difference in attenuation, which can be measured at the equalizer outlet
when different frequencies are transmitted over the entire transmission
path. It is preferably measured by adding, on the sending side, two pilot
signals with different frequencies and constant levels, to the electrical
signal mix being transmitted, and measuring the difference of the levels
of both pilot signals at the outlet of the receiving side equalizer,
which, in the ideal case, will be a specified value.
The criterion for adjusting the equalization of the equalizer is the above
explained second order distortion factor. If this distortion factor is
measured with a nonlinear distortion detector, and the difference in the
levels of two pilot signals of different frequencies is measured by a
so-called frequency response measuring instrument, the equalization, and
possibly the equalizer's frequency response, can be adjusted manually, as
explained earlier.
On the other hand, the equalizer can be expanded by adding a control
circuit to an automatic equalization and frequency response controller as
shown in FIG. 6
The following possibilities arise on the basis of the above explained
adjustment possibilities and measurable criteria:
a) The circuit contains an equalizer according to FIG. 1. A nonlinear
distortion detector is attached to its outlet, to measure second order
distortion products created by one or more pilot signals; its output
signal is supplied to a control, which adjusts the bias voltages U.sub.a
and U.sub.b in accordance with the measured nonlinear distortion factor in
such a way, that one of the two diodes performs the equalization, and the
other functions at a maximum amount of bias voltage and does not
practically contribute to the distortion or equalization.
b) The equalizer is also according to FIG. 1. A nonlinear distortion
detector and an instrument for measuring frequency response are attached
to its outlet, and measure the second order distortion products created by
one or more pilot signals. Both output signals are supplied to a control,
which, as described above, adjusts both the required equalization
essentially through one of the two diodes, as well as keeps the
equalizer's frequency response constant, through the other of the two
diodes. The frequency-response-measuring device measures at the output of
the LC ladder network the level difference of two pilot signals as a
measure of the frequency dependence of the attenuation of the LC ladder
network and the controller adjusts the bias of the essentially one of the
diodes to obtain a maximum reduction of the measured distortion products
as described above and, if necessary, maintains the measured level
difference of the two pilot signals constant during any adjustment to the
essentially one diode by varying the bias of the other varactor diode.
c) The circuit arrangement contains an equalizer according to FIG. 3, and
the control circuit consists of a nonlinear distortion detector attached
to the equalizer's outlet, to measure second order distortion products
created by one or more pilot signals, and of a frequency response
measuring instrument, as described above. Both output signals are supplied
to a control, which therewith establishes the magnitude of the adjustment
of bias voltages U.sub.a, U.sub.b and U.sub.s, thereby controlling the
equalization and keeping the frequency response constant. In this case, as
shown in FIG. 6, for example, the controller adjusts the bias of the
essentially one varactor diode to obtain a maximum reduction of the
measured distortion products while maintaining the bias of the other
varactor diode at a constant maximum value, and adjusts the bias (U.sub.s)
determining the total capacitance of the two varactor diodes (C8, C9) in
series opposition such that the level difference of the two pilot signals
remains.
d) The circuit arrangement contains an equalizer according to FIG. 3, but
without a second variable capacitance diode, therefore only with one
variable capacitance diode C.sub.a and the pertinent bias voltage circuit,
or with only one variable capacitance diode C.sub.b and the pertinent bias
voltage circuit. A nonlinear distortion detector, which measures second
order distortion products created by one or more pilot signals, as well as
a frequency response measuring instrument, are connected to the equalizer
outlet, and the output signals are supplied to a control, which
establishes the magnitude for adjusting the bias voltage of the variable
capacitance diode serving as equalizer, and to keep constant the frequency
response of the equalization. Since not only the laser chirp creates
second order distortions, in conjunction with the optical fiber's
chromatic dispersion, but also the nonlinearity that takes place in the
laser due to intensity modulations, it appears useful for the above
described receiving side equalization, to use a preequalizer with the
laser, to compensate for the laser's nonlinearity, preferably a controlled
preequalizer, e.g. as known from DE-A1 33 07 309.
In the above described configuration examples of the invention, the
required signal equalization is always performed by the voltage dependence
of a single variable capacitance diode, namely the LC component's variable
capacitance diode operating at the lower bias voltage (C.sub.a in the
example of FIG. 2).
Tests have shown that with the above described LC component, sufficient
equalization, which does not attenuate the signals excessively, is only
possible if, on the one hand, the distortions to be equalized are not too
large, and on the other, the bandwidth of the signals to be transmitted is
not too large. The nonlinear distortions experienced by a signal that is
optically transmitted through a optical fiber path, and which are to be
compensated by the equalization, are caused to a considerable degree by
the so-called "laser chirp", i.e. an undesirable wave length oscillation
that is a function of the signal amplitude of the optically transmitted
electrical signal, in conjunction with the chromatic dispersion of the
optical fiber. The larger the "laser chirp", the chromatic dispersion of
the optical fiber and its length, the larger are the nonlinear distortions
experienced by the signal.
It was shown that a satisfactory signal transmission is possible with a
1550 nm wavelength laser, which has a relatively small "laser chirp", with
a standard single-mode optical fiber and an equalizer of the known kind,
or the kind described in FIG. 1 or in FIG. 3, if the length of the optical
fiber is not longer than 12 km and the bandwidth of the signals to be
transmitted is not greater than 450 MHz. If signals, which have traversed
more than a 12 km optical fiber length are to be equalized, the known or
the above described equalizer only offers the possibility of adjusting the
bias voltage of the equalizing variable capacitance diode correspondingly
low, which, however, lowers the limit frequency of the equalizer, i.e. the
maximum frequency of signals transmitted without attenuation. In other
words: if the known or the above described equalizer is able to equalize
considerable signal distortions, its limit frequency is so low, that it is
too small to transmit broad band signals, such as e.g. the signals of the
cable TV frequency band, which extend to 450 MHz.
For certain applications, an equalizer is therefore required which, on the
one hand, is appropriate for strong distortions, and on the other has a
limit frequency that is high enough for broad bandwidth transmission, e.g.
one with a frequency band up to 600
This goal is attained by a further development of the invention, as shown
in FIG. 4 or 5, namely by a chain circuit of LC components as shown in
FIG. 1 or 3, and as explained above.
FIG. 4 shows a chain circuit of three LC components according to FIG. 1,
each of which is designed somewhat differently than the one shown in FIG.
1. The same references as for the elements in FIG. 1 are used for each of
the LC components, except for a second index, which indicates that the
element belongs to the first, second or third LC component.
FIG. 5 shows an LC chain circuit with three LC components according to FIG.
3, therefore the same LC chain circuit as in FIG. 3, but with the
additional circuit belonging to each LC component, with the adjustable
capacitance of the antiseries circuit of C.sub.8 and C.sub.9, for keeping
constant the frequency response of each LC component in the chain, and
thereby the frequency response of the entire chain.
Of course, chain circuits with any number of LC components of the indicated
type are suitable in principle, and also LC chain circuits, whose
capacitive elements are not only formed by variable capacitance diodes.
For example, the chain circuit may contain one or more LC components,
which additionally contain a normal capacitor, like the known equalizer.
Everything explained above about the controllability or adjustability of
the equalization and the frequency response of an individual LC component
in FIG. 1 or 3, applies also to the LC components of the LC chain circuit.
In a chain circuit as shown in FIG. 4 or 5, the inductances and
capacitances C.sub.ai and C.sub.bi (i=1, 2, 3,,,,) can be chosen, in
principle, as required by the wave resistance of the line into which the
circuit is integrated, and the bandwidth of the signals to be transmitted
through the line. It is particularly possible to select the variable
capacitance diodes C.sub.ai and C.sub.bi, and/or to operate them at
suitable bias voltages, so that they have low capacitance, even in the
condition where they must contribute to the equalization.
However, the equalization produced by each individual LC component itself
is then relatively small. For example, it is the kind of equalization that
results when the capacitance of the diode C.sub.ai or C.sub.bi, serving as
the equalizer, is determined by a blocking voltage that lies between the
voltage values indicated in FIG. 2. Still, the total LC chain circuit has
considerable equalization capability, since the equalization produced by
the individual components themselves add to each other, therefore the
totality of the LC components creates a sufficient voltage-dependent delay
of the input signals, thus sufficient equalization of the nonlinear
distorted signals.
Since the total equalization of the LC chain circuit according to the
invention is composed of the contributions of the individual LC
components, the individual LC components may be operated, so that their
capacitance is relatively small, and changes the other circuit parameters
very little, even during adjustment of the equalization. This causes their
effect on the frequency dependence of the attenuation, the so-called
frequency response of the entire LC chain, to be relatively small.
Another advantage of the LC chain circuit according to the invention is
that the bias voltages of the individual variable capacitance diodes are
chosen unequal, and can be changed independently of each other. This
provides as many degrees of freedom as there are variable capacitance
diodes, to adjust the equalizer for any existing distortion. For example,
it is possible to operate one or more of the LC components at such a high
blocking voltage, that it practically contributes nothing to the
equalization, and is only permitted to contribute to the equalization when
its blocking voltage is reduced as needed.
Of course, it is also possible for the connections of equally polarized
variable capacitance diode to be connected to a single voltage source,
either through different resistors R.sub.a1, to R.sub.a3, or R.sub.b1, to
R.sub.b3, or through a single resistor, e.g. resistor R.sub.a1, with the,
in this instance, common voltage source U.sub.a1 or U.sub.b1, thereby
making all equally polarized variable capacitance diodes simultaneously
adjustable or controllable.
The same applies respectively with regard to the bias voltage adjustment of
the antiseries circuit (L.sub.8 and L.sub.9) belonging to the LC
components, and their adjustment.
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