Note: Descriptions are shown in the official language in which they were submitted.
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Background of_the Invention
1. Technical Field
This invention relates to electrical isolation devices
and particularly to an improved optocoupler device having
an output gain dependent upon loop length.
2. Background Art
It has become known that it is possible to design an
isolation device for passing~signals from an input to an
output using optocoupling at the barrier between the input
and output terminals. This provides a solution to the
problems of linearity of the output signal with respect to
the input signal. A pair of light dete~ting diodes and a
LED (light emitting diode) are used in a feedback
arrangement such that -the light output characteristics of
the photodetector in the output circuit linearly track
input signal. Thus, an optocoupler can be built that
corrects for light output differences between the LED and
the photodiodes as well as corrects for temperature
coefficient differences between devices.
One usage of the optocoupled device is as a barrier
between the input and output sides of a telephone line for
isolation purposes. The use of such a device in such a
manner solves many of the poblems inherent with such
isolation devices. However, a problem that exists in all
transmission lines is that as the loop length increases
the quality of the transmission deteriorates. This
deterioration is related to the resistance of the line
itself and is manifested in a drop in the DC voltage level
from one end of the line to the other. In addition, as
the length of the transmission line increases there is a
fall off in signal gain due to the frequency of the
transmission.
Accordingly, a need exists in the art for a circuit
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capable of automatically correcting for changes in loop
length. A furtler need exists for a circuit capable of
correcting both the AC and DC transmission signals passin
over variable length lines.
Summary of the Invention
In accordance with an aspect of the inventlon there is
provided an isolation device having an input terminal and
an output terminal said terminals separated by an
interface, the input side of said interface containing
transmitting means operable for providing transmission
across said interface in accordance with input AC and DC
transmission signals, and the output side of said interface
containing detecting means operable under control of
received transmission for providing an output transmission
signal representative of said input transmission signal,
said device including means on said input side of said
interface for generating a bias signal proportional to
said input DC signal, means for applying said generated
bias signal to said transmitting means so that one com-
ponent of said transmission across said lnterface is afunction of said input DC transmission signal as well as a
function of the physical characteristics of said interface,
means for comparing a reference DC voltage level to a DC
voltage level generated by said detecting means as a result
of transmission received across said interface, and means
on said output side of said interface for modifying the AC
gain of said output transmission signal in accordance with
said compared voltage levels.
In one embodiment of our invention the DC line
voltage, which is attentuated by the transmission line
resistance, is applied to the input side of an optocoupled
circuit. The DC line voltge is used to generate DC bias
current in the input photodiode, such that the output side
photodetector contains a bias current dependent upon the
line length. The output side of the optocoupled barrier
is arranged such that a DC reference voltage is used to
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adjust the AC gain based upon the difference between the
reference voltage and the DC generated bias voltage.
Thus, the circuit is automatically, and continuously,
corrected for changes in transmission line lengths.
In a seconA embodiment of our invention a compound DC
bias voltage dependent upon loop length is generated on
thè input side of the barrier. This compound voltage is
used to control the effective value of a capacitor thereby
compensating for changes in frequency such that the
frequency response of the circuit is approximately constant
over a wide range of frequencies and line lengthsO The
compound voltage is applied to the input side photodiode
and communicated to the output circuit in order to cancel
the frequency component of the output AC signal, making
the output circuit frequency insensitive.
Accordingly, it is a feature of our invention to
construct an optocoupled circuit for automatically
compensating for transmission line length changes.
It is a further feature of our invention to arrange
such an optocoupled circuit with a DC reference voltage in
the output circuit for correcting the output AC signal for
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signal losses due to the resistance of the input
transmission line.
It is still a further feature of our invention to
arrange such a circuit with a compound voltage, derived
from the input signal to correct the output signal for
losses due to the frequency dependent characteristic of
the transmission line.
Description of the Drawings
These features and objects of our invention will be
more fully appreciated from a review of the drawings in
which:
FIG. 1 shows a loop length corrected optocoupled
device,
FIGS. 2 and 3 show equations pertaining to FIG. 1,
FIG. 4 shows a loop length corrected 2-way device,
FIG. 5 shows the equivalent circuit for a DC corrected
optocoupled device,
FIG. 6 shows the equations pertaining to FIG. 5,
FIG. 7 is a graph showing the gain fall off as a
function of frequency,
FIG 8 is an analog signal processor,
FIG. 9 shows the equations pertaining to FIG. 8,
FIG. 10 is an equivalent circuit showing both AC and
DC loop correction,
FIG. 11 are the equations pertaining to FIG. 10,
FIGS. 12 through 15 are block diagrams showing the use
of a loop length compensated optocoupled device in a
telephone system.
Detailed Description
Prior to discussing the operation of our invention it
may be helpful to understand the operation of prior art
linear optocoupled devices. With reference to FIG. 1
hereof, the output side of the light barrier is designed
to take advantage of the characteristic of light coupled
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devices where the DC gain of the light detecting diode
lL~D2 tracks almost identically to the AC gain of that
light detecting diode over a substantial range of
operating currents. Thus, the output of the device
operates by comparing the DC signal of light detecting
diode lLDD2 with a known DC reference signal, thereby
making it possible to control t:he ~C output gain of the
circuit.
In a known prior art the DC bias voltage on the input
side of the barrier was fixed at a certain level. The DC
level was transmitted across the barrier as a steady state
DC value passing through light detecting diode. The only
change in DC current on the output side of the barrier
resulted from the actual physical light transfer char-
acteristics of the optocoupled device itself. Thus by
comparing the DC output voltage to the reference voltage
and by adjusting the AC gain of the output circuit
accordingly, the circuit is compensatable for changes in
light transfer characteristics of the optocoupled device.
We have recognized that if the input DC bias becomes a
function of loop lengthr then as the loop leng~h increases(decreasing loop current) the DC bias voltage would
decrease. This decrease in DC bias voltage is then
transmitted across the light barrier and becomes a factor
in the DC output voltage. This DC output voltage is
compared to the DC reference voltage and the AC signal
output is gain compensated both for the light barrier
transfer characteristics (which is one factor in the DC
output current) and for the loop length change (which is a
second factor in the DC output current).
Thus as shown in FIG. 1 the DC voltage of the tip and
ring leads, T and R, of the line loop, which voltage is
reduced as the loop from the central office or other
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switching machine increases, determines the DC voltage
drop across resistor lRIN. The voltage drop across
resistor lRIN determines the ~C bias which in turn
controls the DC current which flows through photodiode
lLDD-l. This DC bias is then transmitted across the light
barrier causing voltage V~(DC) to have a component
dependent on the loop length.
FI~. 2 shows the DC voltages of FIG. 1. Amplifier lA3
is an open loop comparator and is arranged in conjunction
with multiplier lMl in a closed loop feedback circuit such
that VO(DC) equals the DC reference voltage VREF(DC).
Multiplier lMl can be constructed in any one of the well
known circuit configurations. Amplifier lA2 is a voltage
follower. Of course, it is known that the multiplier can
be substituted for by a divider with proper inversion of
the signal.
The relationships of the AC voltages of FIG. 1 are
shown in FIG. 3 where VO(AC) is shown in equation (1) to
be a function of the AC output detector surrent divided by
the DC output detector current times the reference DC
voltage or, by substitution, the output qoltage VO(AC)
equals the AC input voltage across the tip and ring leads
of the loop multiplied by the DC reference voltage divided
by the DC voltage of the tip and ring loopO Since both
the AC voltage and DC voltage across the tip and ring
attenuate as the loop becomes longer the output AC voltage
VOAC has a tendency to remain at a constant magnitude
thereby equalizing gain losses due tG increased loop
length.
For short loop length, attenuation of the loop is
rather insensitive to frequency. However, as the loop
becomes longer, signal attenuation varies sharply with
frequency. This is shown in FIG. 7. In FI~. 7 we can see
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that at zero frequency (~C) for a loop of 7.5 kilofeet the
loss would be, for example, uncler 5 dBo The loss for a
loop of 15 kilofeet would be slightly over S dB. However,
for a loop of 7.5 kilofeet and a frequency of, for
example, 8 KHz the loss approaches 10 dB, while for 15
kilofeet at 8 KHz the loss is over 20 dB. Thus, as
described by equation (2) of FIG. 3, it is seen that the
AC voltage VTR(AC3 of the loop drops faster than loop DC
voltage VTR(DC). Accordingly, since the AC voltage is in
the numerator of the equation, the net result is that the
AC gain of the circuit will not be completely compensated
at higher frequencies.
For example, using equation (2) of FIG.3 in
combination with FIG. 7 and assuming a loop length of 15
kilofeet and a frequency of 3 KHz the circuit of FIG. 1
would only compensate for approximately 6.5 dB of AC
signal loss while the loss due to the frequency and loop
length would be approximately 13 dB. Thus the circuit of
FIG. 1 only partially compensates for the line loss.
If the frequency of the circuit is known to be
relatively constant, then the DC reference voltage may be
increased to an amount where the 13 dB loss is reinserted
at the output. However, in situations where the signal
frequency varies it is impractical to add 13 dB gain to
the output circuit since if the frequency is decreased-
from 3 KHz to, for example, 2 KHz the gain would then be
overpowering.
The circuit shown in FIG. 10 is arranged to correct
for the increased loss due to frequency attenuation by
introducing into the DC control signal, via muiltipliers
lOM2 and lOM3, a complex DC voltage which is dependent on
loop length in a manner so that the output circuit is
compensated in accordance with the frequency received.
Prior to discussing FIG. 10 it will be helpful to
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study FIG. 5 where the input parameters to the circuit are
shown in a theoretical manner.
FIG 5 shows a more detailed representation of FIG. 1
where the input parameters are shown in a theoretical
lumped parameter configuration. This lumped parameter
model is good for voice frequency transmission (300 Hz-
3000 Hz) up to lS kilofeet loop. As shown in FIGo 5, r is
representative of the resistance of unit length of the
transmission line, c is the capacitance per unit length, Q
is the actual loop length and s is the Laplace
transformation parameter. Thus the lumped capacitive
value is shown to be frequency dependent. The circuit of
FIG. 5 operates in exactly the same manner as discussed
for FIG. 1 such that input DC current is provided via
amplifier 5Al to light emitting diode 5LEDl . The AC input
signal is also provided to light emittng diode 5LEDl.
These signals are in turn passed across the light barrier
and received by photodetector diode 5LDD2.
As discussed previously, the DC component of the
output voltage VX contains the AC input signal as well as
a first DC component voltage based on the transfer
characteristics of the output coupled device and a second
component based upon the input DC voltage level.
FIG. 6 shows the equations of FIG S and it can be seen
by equation (1) of FIG~ 6 that the AC output voltage
VO(AC) is a function of the input voltage VS times several
terms which are loop length and frequency dependent.
Since the fequency component is in the denominator it is
clear that as the frequency increases the AC output signal
will decrease as previously discussed and thus the circuit
of FIG. 3 does not provide frequency equalization.
- FIG. 8 is an analog signal processor 801 which
operates such that from three DC inputs, such as 1.07 V,
lV and Vl(DC~ there will be generated two outputs VOl and
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V02. The input voltages are arranged such that V01 equals
rQ and V02 equals r2Q2 when the input voltage Vl (DC)
is of the form shown in equation (1) of FIG. 9.
Turning now to FIG. 10 there is shown a circuit which
compensates both for the attenuation due to loop length
resistance as well as the attenuaton due to the frequency
component. As shown in FIG. 10 the lumped parameters are
again defined as before for FIG. 5. Note that certain
signals are provided to analog signal processor 1001.
These signals are in accordance with the discussion
pertaining to FIG. 8. As shown in FIG. 10, there are
three components of the current which is provided to the
photodiode lOLDDl. The first component is provided via
resistor capacitor combination lOC3 and lORll, which
component is the same as discussed for FIGS. 1 and 5 and
is based upon the DC and AC components of the input
signal. The second component of the signal applied to
photodiode lOLDDl via capacitor CA which is a function of
the loop resistance and loop length and the third
component of the signal is a function of the square of the
above described signal and is applied via capacitor CB.
The second and third components of the signal carry the
variable loop length capacitance information and are used
to compensate for frequency attenuation. These values,
which depend on the compound voltage V01 and V02, can be
adjusted to provide any desired frequency response. As
discussed, these three signals are passed through the
optocoupled device 101 across the barrier to provide an
output signal VO(AC) controlled by the reference voltage
30 VREF (DC~ . The equations pertaining to FIG. 10 are shown
in FIG. 11 where as is shown in equation (1) thereof both
the numerator and the denominator contain frequency com-
~ ponents in the same form. Therefore the output signal
VO(AC) is no longer frequency dependent and thus sub-
stantially compensated for by the circuit shown in FIG. 10.
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For different line parameters r and c, we can select
K2, Cl and C2 so that
K2 + SClrQ ~ SC2r Q2 equals or approaches
+ 1 scQR.
2 Thus, since the frequency components of equation ~1)
FIG. 11 in both the numerator and denominator are of the
same form, frequency compensation is achieved in good
approximation.
For the purpose of showing a mode comtemplated by us
at this time for practicing our invention, FIGS. 12-15
have been provided. FIG. 12 is an overall block diagram
showing how our invention may be used in a telephone
system to connect a central office loop with a station
1208 through a local crosspoint network. As shown, the
central office line side of our invention is shown in
Block 1202 and the output side of the barrier as shown in
Block 1203 connected to any type of well known local
switching network shown in Block 1204, which network may
advantageously be a mechanical crosspoint network or an
electronic network.
On the station side of the local crosspoint network an
identical circuit configuration is shown in Blocks 1205
and 1206. Using the block diagram of FIG. 12 it can be
seen that an optocoupled barrier is placed both before a
switching network and after a switching network for
maximum protection of the telephone system.
In FIG. 13, which is equivalent to the input side oE
the circuit shown in FIG. 4, there is shown an extended
block diagram of Block 1202 showing several of the
circuits necessary for operating our device. Current
generator and shunt regulator 1302 is arranged to supply
current to tlle LED driver. High accuracy voltage
regulation is derived from the band gap voltage and
current reference and serves as a floating voltage source.
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Voltage and current reference 1303 is arranged to
generate highly accurate, temperature insensitive, voltage
and current references for use by the other circuits.
This circuit would typically use the well-known band gap
reference technique.
Circuit 1303, in combination with circuit 1302, is the
equivalent of voltage regulator ~ shown in the circuit of
FIG. 4.
Signal inverter and voltage to current converter 1304
is arranged to provide the reciprocal of the incoming
signal. This circuit operates to convert the input signal
voltage to the current form for use by divider 1305.
Divider 1305 is arranged to accept the inverted signal
current and to perform analog division thereon. This
circuit operates to automatically adjust AC transmission
gain and, in conjunction with signal inverter 1304, is the
equivalent of elements 4M2 and 4A5 of the circuit shown in
FIG. 4.
Modulator 1306 operates to modulate the AC signal
current transmitted from the network to the central office
thereby driving the loop, and includes element 401.
Hybrid and clamp circuit 1307 serves the normal hybrid
cancellation function and includes elements 4Rl and 4R2 of
the circuit shown in FIG. 4. The clamp circuit limits the
AC gain for very long loops.
LED driver 130~ is arranged to drive the LEDS and
includes elements 4Al and 4LEDl of the circuit shown in
FIG. 4.
The network line 1401 shown in FIG. 14 is the
equivalent of the circuit~r~D ~ own in the circuit oE FIG. 4
where elements 4A4 and 4~e~ represent LED driver 1404
- in FIG . 14. Hybrid 1405 in FIG . 14 includes elements 4Rll
and 4R21 in FIG. 4. Modulator 1406 in FIG. 14 includes
element 4Q2 in FIG. 4. Divider 1407 in FIG. 14 includes
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elements 4A3 and 4Ml in FIG. 4.
The station side of the optocoupled station circuit
shown in FIG. 15 is substantially a mirror image of the
circuitry described with respect to FIG. 13.
The circuit of FIG. 4 performs the hybrid furlction.
Conclusion
Only a few of the possible uses of our device have
been discussed herein. There is no doubt that one skilled
in the art will be able to utilize our invention
advantageously in a variety of circuit configurations both
in conjunction with a telephone system and for other
purposes where isolation is necessary between an input and
an output or in situations where the gain of a circuit
must be automaticaly controlled dependent upon loop length
or frequency considerations.
We have shown a circuit for loop length and frequency
compensation where the gain is held to a constant level.
- However, it may be desirable to increase gain with
frequency or loop length or to provide a special frequency
response. To do so would only require a different control
function for the compound voltage discussed herein.
