Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
ATTENUATOR CONTROL ARRANGEMENTS
This invention relates to attenuator control arran~ements, and
is particularly concerned with an attenuator control arrangement which
is suitable -for level control of signals in frequency multiplexed
communications systems.
In a frequency multiplexed communications system, it is
desirable to control the level of transmitted signals rapidly and over
a wide dynamic range, without introducing distortion or nolse to the
signals. In addition, such control is desirably effected -in a simple
manner at a relatively low cost, for signals with a wide range o-f
frequencies, and in a manner which is stable with respect to
temperature changes.
An object of this invention is to provide an attenuator
control arrangement which can facilitate level control of signals,
particularly in a frequency multiplexed communications system.
According to this invention there is provided an attenuator
control arrangement comprising: an attenuator comprising a first
potential divider including a first light-dependent resistance; a
second potential divider including a second light-dependent
resistance; a light source for illuminating the first and second
light-dependent resistances; control means for controlling the light
source thereby to control the magnitudes of the first and second
light-dependent resistances; means f`or producing a first control
signal in response to differences between a po-tential derived from the
second potential divider and a re-ference potential; means for
producing a second control signal in dependence upon a level of a
predetermined signal at an ou-tput of the attenuator; and switching
means responsive to the absence or presence of the predetermined
signal for selecting respectively the first or second control signal
as the control signal for the control means, whereby the attenuator
has an attenuation determined respectively by -the reference poten-tial
of the level of the predetermined signal.
In an embodiment of the invention, conveniently the
predeterrnined signal comprises a pilot tone having a predetermined
frequency, the arrangement including a bandpass filter for passing
the predetermined frequency coupled between the ou-tpu-t of the
attenuator and the means for producing the second control signal.
Such an arrangement is particular'ly suited to attenuation of
frequency multiplexed signals, including a pilot tone, in a
communications system. In normal operation, the pilot tone is
filtered and detected, and the light source is controlled in
dependence upon the pilot tone level so that a desired signal level is
maintained at the output of the attenuator. In the event of failure
-to detect an appropriate level of the pilot tone (e.g. loss of the
pilot tone signal) a stable attenuation (as opposed to a very low
attenuation due to the low or zero level of pi'lot tone) is achieved in
-that the attenuator is controlled by the first-mentioned control
slgnal.
The invention will be further understood from the following
description with reference to the accompanying drawing, which
schematically illustrates an attenuator control arrangement in
accordance with an embodiment of the invention.
Referring to the drawing, the attenuator control arrangement
illustrated therein comprises a cadmium selenide photoconductive cell
which constitu~es a variable resistance 10 which is capaci-tively
coupled in series between the ou-tput of a main signal amp'lifier 12,
which is supplied with a frequency multiple~ed input signal via an
input 14, and an output drive amp'lifier 16 which supplies a constant-
level signal to an output 18. The resistance 10 serves, together with
a resistor 20 connected in series with an inductor 22 between the
output side of -the resistance 10 and ground, to constitute a variable
attenuator controlled optically by light from a light-emitting diode
(led) 24. A series-connected resistor 26 and inductor 28 are
connected in parallel with the resistance 10 to provide a desired
upper limit of attenuation.
Current through the led 24,and hence its light output and the
magnitude of the resistance 10, is controlled by a transis-tor 30,
-the collector of which is connected to the led 24 via a resistor 32
and the emitter of which is maintained at a constant potential by a
zener diode 34 biassed via a resistor 36. Base current to the
transistor 30, limited by a series resistor 38, is derived from
either of two feedback paths 40 and 42, depending on the position of
an analog switch 44.
The -feedback path 40 comprises a pilot tone drive amplifier
46, whose input is connected to -the input of -the amplifier 16 and
hence receives the attenuated signal From the output side of the
resistance 10, a pilot tone bandpass filter 48 coupled to the output
of the amplifier 46, a pilot tone rectifying amplifier 50 coupled to
the output of the filter 48, and a gain control circuit 52 coupled
between the output of the amplifier 50 and one position of the
switch 44. In addition, a pilot tone detector 54 is coupled to the
output of the amplifier 50 -for detecting an appropriate level oF a
pilot tone, which is normally present in the frequency multiplexed
signal supplied to the input 14, and upon such detection supplying a
control signal to a control logic unit 56 which controls the switch
44. In the presence of the pilot tone at an appropriate level the
switch 44 has the position shown, in which the output of the circuit
52 is connected to the resistor 38.
~ or example, -the pilot tone may have a frequency of
1.552MHz, and the pilot tone detector 54 may comprise a window
comparator, the appropriate level of the pilot tone being anywhere
within a ranye corresponding to the comparator window. The gain
control circuit 52 can comprise a delay element and a level
comparator for comparing the delayed pilot tone level with a
predetermined reference level.
The feedback path 42 includes a second cadmium selenide
photoconductive cell cons-tituting a resiskance 58, also responsive
to the light output of the led 24, which resistance 5g is connected
in a potential divider circuit including resistors 60 and 62 to
provide a voltage which is coupled via a resistor 64 to an inverting
input oF a di-f-Feren-tial amplifier 66. A manually variable voltage
divider 6g supplies a reFerence voltage -to a non-inverting input of
-the ampliFier 66, whose output is connec-ted to another position oF
-the switch 44, to be connected to -the resistor 38 in the absence of
-the pilot -tone. A -Feedback resistor 70 and capacitor 74 provide
for stable operation of the ampliFier 66.
In normal operation, with the pilot tone present, at a level
within the range set by the window comparator, in the input signal
supplied to the input 14, this pilot tone is detec-ted by the
l ~S~ 3`~
detector 54 so that the switch 44 is controlled to have the position
shown, whereby the level of the pilot tone is monitored by the gain
control circuit 52 and thereby maintained constant by feedback
control via the switch 44, transistor 30, and led 24 controlling the
resistance lOo As the pilot tone has a level which is fixed
relative to the remainder of the frequency multiplexed input signal,
so the level of this signal at -the output 18 is also maintained
constant.
In the even-t that the pilot tone level is outside the range
set by the window comparator, -for example in the event o-f a loss o-F
signal at the input 14, it is desirable to avoid a situation in
which -the variable resistance 10 provides a very low attenuation,
whereby substantial levels of noise are supplied to the output 180
In this situation3 the failure of the detector 5~ to detect the
pilot tone results in the unit 56 controlling the switch to adopt
its other position~ in which the output of the ampli-Fier 66 controls
-the transistor 30 and hence light output of the led 24, and thereby
the attenuation by the resistance 10. As a result of the feedback
provided by the resistance 58, this attenuation is held at a desired
level established by the setting oF the variable voltage divider
68.
As indicated by a broken line box 74 in the drawing, the led
24 and resistances 10 and 58 are conveniently all part of a single
led/photocell module, such as that available from Silonex Inc. under
part No. PL-5S33 comprising a gallium phosphide led and two
fast-responding cadmium selenide photoconductive cells optically
coupled thereto. The integration of the resistances 10 and 58 in
the same module results in close matching and thermal coupling,
whereby the feedback path 42 automatically compensates for changes
in the characteristics oF the led 24 and resistance 10 with changing
tempera ture.
Al-though a particular embodiment of -the invention has been
described in detail above, it should be apprecia-ted that numerous
modi-fications, variations, and adaptations may be made thereto
without departing from the scope of the invention, which is defined
by the claims~
