Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to equa1izer arrangernents.
It is known to provide an equalizer for equali7ing signals
received via a transrnission cable to cornpensate for loss-frequency
characteristics of the cable. It is generally desired, for reasons of
cost and simplicity, to use an equalizer which has a fixed equalization
characteristic. However, such an equalizer enables proper equalization to
be achieved only for a specific type and size of cable, because the
loss-frequency characteristics of the cable change for different sizes and
types of cable.
For the transmission of data it is desirable to be able to
use existing telecommunications cables. However, the nature of any
particular cable transmission link established for such transmission is
arbitrary; it may -typically comprise an undetermined mixture of 22, 24,
and/or 26 gauge PIC (plastic insulated conductor) cable with arbitrary
lengths. The use of a fixed-characteristic equalizer for equalizing
signals received via such cables is insufficient for proper equalization
regardless of the cable mixture, and -the use of a complicated variable
equalizer in place of the fixed-characteristic equalizer is undesired in
view of the cost and complexity of such an equalizer. Furthermore,
individual manual adjustmen-t of equalizers to suit particular transmission
11nks is desirably avoided.
Accordingly, an object of this invention is to provide an
improved equalization arrangement for equalizing signals received via
arbitrary cable rnixtures.
According to this invention there is provided an equalizer
arrangement comprising a first equalizer having a fixed, predetermined
equalization characteristic for equalizing signals transmitted via cable
of a predetermined size and type, a second variable amplitude notch
equalizer coupled in series with the first equalizer for complementing the
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equalization of signals by t~e first equalizer -to equalize signals
transmitted via cable of a different size and/or type, and control means
responsive to the equalized signals for automatically controlling the
equalization by the second equalizer~
Thus in accordance with this invention a conventional
fixed-charac-teristic equalizer can be supplemented by a variable notch
equalizer to enable proper equalization of siynals transmitted via various
cable mixtures. As each of the equalizers can have a simple form, and
automatic control of the variable amplitude notch can be provided easily
and simply as described below, this provides a particularly convenient way
of ef-Fecting equalization.
Conveniently the second equalizer comprises a variable
resistance which is controlled by the control means to vary the arnplicude
of the notch. The variable resistance can be constituted by a diode and
means for establishing a controlled current through the diode. In this
case preferably the control means comprises means for detecting a
predetermined sequence of the equalized signals, means for comparing the
level of the equalized signals with at least one reference level, and
rneans responsive to the detection and the cornparison for controlling the
current through the diode~
The invention also extends to a method of equalizing signals
comprising the steps of: passing the signals through a first equalizer
having a fixed, predetermined, equalization characteristic for equalizing
signals transmitted via cable of a predetermined size and type; passing
the signals through a second variable amplitude notch equalizer, coupled
in series with the first equalizer, for complemen-ting the equalization by
the first equalizer to equalize signals transmitted via cable of a
different size and/or type; and controlling the equaliza-tion by the second
equalizer automatically in response to the equalized signals.
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The invention will be further understood frorn -the following
description with reference to the accompanying drawings, in which:
Fig. 1 is a block diagram illustrating a conventional
equalizing arrangement;
Fig. 2 is a graph illustrating responses of the arrangement
of Fig. l;
Fig. 3 is a block diagram illustrating an equalizing
arrangement according to an embodiment of this invention;
Fig. 4 is a graph illustrating responses of a variable
equalizer in the arrangement of Fig. 3;
Fig. 5 is a schematic circuit diagram illustrating one form
of the variable equalizer;
Fig. 6 schematically illustrates an automatic control
arrangement for the variable equalizer; and
Fig. 7 is a schematic circuit diagram illustrating a form oF
the fixed equalizer in the arrangements of Figs. 1 and 3.
Referring to Fig. 1, a conventional arrangement is shown for
transmitting data, for example using duobinary or bipolar signals at a bit
rate of 640 kbps, via a cable 1, and for equalizing the received signals
to compensate for characteristics of the cable. To this end, the cable 1
is cGnnected at one end to a data signal transmitter 2 and at the other
end, via an automatic line build-out (ALB0) circuit 3, to an equalizer 4
from which the received data signals are derived. The ALB0 circuit 3,
which is of known form, serves to equalize the amplitude, at a
predetermined frequency, of the data signals to accommodate cables of
various lengths and hence attenuations. For example the ALB0 circuit
equalizes the amplitude to provide a total loss of 51dB via the cable 1
and circuit 3 for signals at the Nyquist frequency of 320kHz.
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The equalizer 4 serves to compensate for the loss-frequency
characteristics of the cable 1. For example the equalizer 4 may be
designed to compensate for the loss-frequency characteris-tics of 26 PIC
(26 yauge plastic insulated conductor) cable, so that if the cable 1 is
all 26 PIC the resultant gain-frequency characteristic of the equalized
data signals is flat as shown by the line 6 in FigO 2. However, in
practice the make-up of the particular cable 1 which is used is unknown;
it may typically comprise a mixture of arbitrary lengths of 26 PIC,
24 PIC, and 22 PIC, with an overall loss-frequency characteristic which
differs considerably from that of all-26 PIC cable. For example Fig. 2
also shows by lines 7, 8, and 9 the gain-frequency characteristics of the
equalized data signals, assuming that the equalizer 4 is designed as
mentioned above9 for situations where the cable 1 consists of respectively
1 mile of 22 PIC and the remainder 26 PIC, 2 miles of 22 PIC and the
remainder 26 PIC, and all (3.5 miles) 22 PIC.
In order to compensate for these variable gain-frequency
characteristics resulting from arbitrary mixtures of cable gauges, the
present invention provides as shown in Fig. 3 an additional variable
equalizer 10, which is connected in this case in series between the cable
1 and the ALB0 circuit 3. The equalizer 10 could alternatively be
connected following either the circuit 3 or the equalizer 4. To
compensate for the situations discussed above and represented by the
gain-frequency lines 6 to 9 in Fig. 2, the equalizer 10 is arranged to
have gain-frequency responses as shown by the lines 11 to 14 respectively
in Fig. 4, whereby the equalized data signals have a substantially flat
gain-frequency characteristic regardless of the make-up of the cable 1.
To produce the responses shown in Fig. 4, the equalizer 10
is a variable notch equalizer of the form shown in Fig. 5, comprising an
input buffer amplifier 15, a series circuit cornprising a
parallel-connected capacitor 16 and resistor 17, a subsequent shunt
circuit comprising a series-connected induc-tor 18, capacitor 19, and
variable resistance 20 connected -to circuit ground, and an output buffer
amplifier 21~ The component values indicated in Fig. 5 are for the bit
rate men-tioned above. To produce the response lines 11 to 14 the
resistance 20 has a value of respectively ~, 760Q, 392~ and 231~o
As illustrated in Fig~ 6, the variable resistance 20 can be
conveniently constituted by a diode 22 which is supplied with a controlled
forward current. This arrangement enables the effective resistance to be
varied automatically to compensate for diFferent cable loss-frequency
characteristics and hence different cable mixtures, using a suitable
automatic control arrangement such as that illustrated in Fig. 6 and
described below. Fig. 6 also shows the inductor 18 and -the capacitor 19
of the variable notch equalizer of Fig. 5. Instead oF a diode as
described here a varistor or other variable resistance element may be used
and suitably controlled to constitute the resistance 20~
The control arrangement illustrated in Fig. 6 comprises a
pattern detector 23~ two comparators 2~ and 25~ two AND gates 26 and 27
an up-down counter 28~ a weighted resistance summing ne-twork 29~ a
transistor 30~ and a diode 31~ The pattern detector 23 is supplied with
the received bipolar data signal on a line 32~ and with a recovered clock
signal on a line 33~ and serves to detect each 1, 0 sequence in the
received data to produce an output signal on a line 3~, enabling the AND
gates 26 and 27, during each data 0 which follows a data ~1 signal. The
comparators 24 and 25 compare the actual data signal level on the line 32
with the correct, zero (circuit ground), level which should occur during
each data 0. If, during a data O following a data +1, the actual data
signal level on the line 32 is positive or negative, the comparator 24 or
25 respectively produces an output signal which is passed by the enabled
gate 26 or 27 respectively and applied to an up input U or down input D
respectively of the counter 28 to cause the counter to coun-t a pulse of
the clock signal on the line 33 respectively up or down. The most
significant bit outputs of the counter 28 are summed by the network 29 to
control the current conducted by the transistor 30, and hence by the
diodes 31 and 22, according to the count of -the counter, thereby
controlling the characteristic of the variable notch equalizer 10 to
reduce the data O error in the received and equalized data to a low level.
The use oF only the most significant bit outputs of the
counter 28 to control the transistor 30, and not all of the bit outputs,
provides an averaging effect in the control loop for the equalizer 10 so
that this is not continuously adjusted. Obviously adjustment of the
equalizer can be effected in dependence upon other data signal patterns
and/or other da-ta signal levels, as desired.
For completeness, Fig. 7 illustrates a possible form of the
fixed equalizer 4, the component values shown being for the data rate
mentioned above, for 26 PIC cable. No further description of this
equalizer is believed to be necessary here.
Numerous modifications, variations, and adaptations to -the
particular embodiments of the invention described above may be made
without departing from the scope of this invention, which is defined in
the claims.
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