Telephone isolation device

An isolation circuit for a telephone line interface includes a pair of capacitors that differentially couple analog transmit and receive channels in full duplex communication across an isolation barrier. Each transmit or receive signal is processed to generate respective paired complementary signals. A switching network is clock driven to differentially charge the paired capacitors in incremental fashion with the paired complementary signals. On the opposite side of the barrier, a clock driven switching network receives the differential charges on the paired capacitors, generating paired complementary signals that are processed to form an analog information signal. Clock signals, off-hook and ringing signals are transmitted in similar fashion. The invention can use two pairs of capacitors that differentially couple analog transmit and receive channels in a user device across an isolation barrier to a line pair for full duplex communication. The interface circuit includes first and second oppositely directed capacitor isolation circuits. The first is disposed between the transmit channel and a hybrid circuit; the second is connected between the receive channel and the hybrid. The transmit capacitor isolation circuit has an input located on the user device side of the isolation barrier and an output on the line side of the barrier and generates analog signals corresponding to analog signals at its input. A receive capacitor isolation circuit has an input located on the line side of the barrier and an output on the user side of the barrier and generates analog signals corresponding to analog signals at its input.

BACKGROUND OF THE INVENTION 
The present invention relates generally to electronic circuitry and more 
particularly to isolation circuitry such as used in telephone line 
interface equipment. 
Isolation of end user telephone equipment from the telephone network is 
required by the U.S. Federal Communications Commission and other 
counterpart regulatory agencies in other countries. The primary purpose of 
this regulation is to protect the network from faulty end user devices and 
from accidental connections to power lines and the like. 
The specific regulations for such isolation change from country to country. 
The U.S. requires 1000 VRMS isolation while other countries may require 
3500 to 4000 volt isolation. These regulations are usually dependent on 
power line mains voltage and internal country safety requirements. Common 
wisdom among those skilled in the field of Direct Access Arrangements or 
DAA's has shown other techniques than the application of isolation 
transformers between the end user equipment and the network. There has 
been some recent recognition that certain paths can be isolated using 
other techniques. For example, U.S. Pat. No. 4,864,605 to Ramsey et al 
shows the use of a coupling transformer in the main signal path and 
optical isolators in the ring detect and off-hook driving circuitry. In a 
similar way, U.S. Pat. No. 4,417,099 to Pierce discloses the use of 
optoisolators in the digital portion of the data paths of a line-powered 
modem. Moreover, U.S. Pat. No. 4,757,528 to Falater et al discloses an 
electro-thermal isolator coupler in conjunction with an RF capacitive 
coupling mechanism. This system claims to provide a DAA device, but 
requires a substrate having a subscriber portion and a line portion 
electrically isolated from each other. 
Moreover, U.S. Pat. No. 5,245,654 to Wilkison et al discloses an optical 
isolation coupler circuit characterized by wide band width, low noise, and 
high linearity. This circuit has proven expensive due to the tightly 
coupled optical devices required to provide the necessary feedback for 
linearity. 
The present invention eliminates the requirement for thermal, resistive, 
and optical isolation. The thermal and optical isolation requirements are 
complex and expensive to produce. The use of an integrated circuit to 
thermally transfer low frequency signals can be supplanted by other means 
that are more cost effective to produce. The capacitive approach lends 
itself to a more integrated design using less external components. 
SUMMARY OF THE INVENTION 
The present invention provides an isolation circuit that is devoid of a 
coupling transformer, optical isolator elements, and high value resistors, 
and yet is practical for use as a telephone line interface. 
In one aspect, the present invention uses a pair of capacitors that 
differentially couple analog transmit and receive channels in full duplex 
communication across an isolation barrier. The capacitors are connected 
across the isolation barrier, and each transmit or receive signal is 
processed to generate respective paired complementary signals. A switching 
network on one side of the barrier is clock driven to differentially 
charge the paired capacitors in incremental fashion with the paired 
complementary signals. On the opposite side of the barrier, a clock driven 
switching network receives the differential charges on the paired 
capacitors, generating paired complementary signals that are processed to 
form a single analog information signal. Clock signals and off-hook and 
ringing signals are also transmitted across the barrier in similar 
fashion. 
In another aspect, the invention can be configured to engage two pairs of 
capacitors that differentially couple analog transmit and receive channels 
in a user device across an isolation barrier to a telephone line pair for 
full duplex communication on the line side. The interface circuit, which 
also includes a hybrid circuit, includes first and second oppositely 
directed capacitor isolation circuits. The first is disposed between the 
transmit channel and the hybrid; the second is connected between the 
receive channel and the hybrid. 
More specifically, a transmit capacitor isolation circuit has an input 
located on the user device side of the isolation barrier and an output on 
the line side of the barrier and generates analog signals at its output 
corresponding to analog signals at its input. A receive capacitor 
isolation circuit has in input located on the line side of the barrier and 
an output on the user side of the barrier and generates analog signals at 
its output corresponding to analog signals at its input.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention provides a telephone isolation circuit 22, shown in 
FIG. 1, that is used to connect a user device 21 to a telephone line 
circuit 23, symbolized as a RJ-11 connector. The telephone isolation 
circuit 22 is referred to as a Direct Access Arrangement (alternatively, 
Data Access Arrangement), or DAA, and is devoid of coupling transformers, 
optical isolator elements, and high value resistors, while providing 
optimal communication paths and isolation between the user device 21 and 
the line circuit 23. 
With regard to FIG. 2, the DAA 22 of the invention generally includes a 
user side having a transmit coupling device 26 which accepts communication 
signals from the user device, a receive coupling device 27 which delivers 
communication signals to the user device, a ring detector 28, an 
oscillator and clock driver 29, and an off-hook driver 30. A salient 
feature of the DAA 22 is the use of capacitive coupling to electrically 
isolate the user device from the line circuit. Thus, a transmit capacitive 
isolation circuit 31 connects the transmit coupling device 26 to a 
transmit line coupling device 32, and a receive capacitive isolation 
device 33 connects the receive coupling device 27 to a receive line 
coupling device 34. Likewise, a clock capacitive isolation device 36 
connects the oscillator and clock driver 29 to an oscillator detector 37, 
and an off-hook capacitive isolation device connects the off-hook driver 
30 to an off-hook detector 39. The isolation devices 31, 33, 36, and 38 
transmit their respective signals across the user-line interface to permit 
standard telephone operations, while providing high level electrical 
isolation between the user device and telephone line. 
The transmit line coupling device 32 and the receive line coupling device 
34 are connected to a hybrid circuit 41, which in turn is connected 
through a line interface circuit 42 and a relay and detector circuit 43 to 
the telephone line. The oscillator detector 37 is connected to the receive 
line coupling device and to the transmit circuit 32. The off-hook detector 
39 is connected directly to the line switch and detector circuit 43. A 
ring indicator oscillator and driver 44 is connected between the receive 
capacitive isolation device 33 and the line switch and detector circuit 
43. Generally speaking, the devices and circuits 32, 34, 37, 39, and 41-44 
are considered to be on the telephone line side of the DAA 22. 
The circuit configuration may be simplified, as shown in FIG. 3, in which 
similar components are labeled with the same reference numeral and a prime 
(') designation. In the embodiment of FIG. 3, the clock driver circuit and 
off-hook driver device are connected through the same clock capacitive 
isolation device 36', and the transmit and receive functions are conducted 
through a single capacitive isolation device 31' using a time division 
multiplexing technique. This simplification permits the elimination of 
components 30, 33, 38, and 39, with the accompanying savings in parts and 
labor cost. 
With regard to FIG. 4, the transmit coupling device 26 includes an input 
buffer circuit 51. The TXA+ and TXA- signals are received differentially 
by the buffer 51 from the user device, and the buffer 51 translates the 
incoming signals into a single ended signal to be used by the differential 
translator signal driver 52. (The TXA- signal may be grounded for a single 
ended input device without affecting the operation of the circuitry as 
described herein.) The differential signal from driver 52 is fed to a 
transmit switch network 53, which charges the capacitors 54 and 56 of the 
transmit capacitive isolation device 31, as explained below. On the line 
side of the isolation interface, the transmit line coupling device 32 
includes a transmitter signal recovery switch network 57, as shown in FIG. 
7. The network 57 is composed of a quad switch circuit configured 
similarly to the capacitor input network 53 on the user side, and 
synchronized therewith by a common clock signal. The two networks 53 and 
57 operate cooperatively to switch an information signal across the 
interface barrier in a reiterative process. The following explanation 
pertains not only to networks 53 and 57, but to all the differential 
network circuits described herein for transmitting signals across the 
isolation barrier. 
With regard to FIG. 14, the two differential switching networks can assume 
two different configurations, depending upon whether the clock signal is 
high or low. When the clock signal is high, as shown in FIG. 14A, the 
positive differential signal V+ is conducted to capacitor Ca, charging Ca 
and causing the signal to be conducted directly to op amp A, resulting in 
signal Va. Likewise, differential signal V- passes through capacitor Cb to 
op amp B, resulting in signal Vb. When the clock signal shifts low, the 
quad switches flip on both sides of the capacitors, resulting in the 
circuit arrangement of FIG. 14B. Signal V+ charges capacitor Cb, and 
signal V- charges capacitor Ca. The signal from Cb goes to op amp A, and 
the signal from capacitor CA goes to op amp B. Thus, although the charges 
on capacitors Ca and Cb have been reversed, the signal V+ is conducted to 
op amp A and the signal V- is conducted to op amp B, so that Va remains a 
function of V+, and Vb remains a function of V-. This process reiterates 
in synchronism with the clock signal, transmitting the signals across the 
isolation barrier. 
Using this technique of switching the capacitor polarity in an opposite way 
the telephone voice band signal frequencies are able to pass through a 
small value capacitor and still transfer 80% of the signal value at Va and 
Vb. 
This signal transmission technique significantly reduces common mode noise. 
The equivalent circuit for common mode noise, Vn, is depicted in FIG. 15, 
Assuming that the capacitor ratio .sup.Ca /.sub.Cb is less than the 
resistance ratio of the voltage divider, then in the high clock state, 
Va=+1/2 Vn and Vb=-1/2 Vn, and in the low clock state Va=-1/2 Vn and 
Vb=+1/2 Vn. Given a Vn signal input as depicted in FIG. 16A, the 
corresponding signal output is depicted in FIG. 16B. The common mode noise 
has been shifted from the voice band into the clock frequency by the 
signal transmission technique and eliminated by a low pass filter having 
an upper corner cutoff of 10 KHZ to 15 Khz. 
Returning to FIG. 7, the two op amps 58 and 59 serve as a buffer 61 to 
recover the differential signals and restore them to a proper level. The 
buffered differential signals are fed to an op amp converter circuit 62 
that restores the differential signals to a single-ended signal. The value 
of resistors 63 and 63A may be selectively varied to alter the gain of the 
transmit path, in accordance with the requirements of various modem 
integrated circuits. The single-ended signal is fed through an anti-alias 
low pass filter 64 which has an upper corner frequency of 15 KHz to remove 
the residual effects and noise generated by the switching network 57, as 
well as common mode noise. The resulting output is the TXAL signal, a 
smooth, filtered, gain-controlled analog signal equivalent to the user 
generated information signal. 
With regard to FIG. 9, the TXAL signal is connected to drive a low 
distortion transmitter signal current driver 71, which in turn is fed to 
the hybrid circuit 41 or 41' of FIGS. 2 and 3. The hybrid circuit consists 
of an op amp connected to function as a receive path amplifier 73 that 
produces a voltage equal to the superposition of the outgoing and incoming 
currents on the line side, and a return signal cancellation circuit 72 
that removes a selected proportion of the outgoing signal. For voice 
communication, a small amount of the outgoing signal is permitted to pass 
through the receive channel, creating a side tone effect. The speaker thus 
hears a small portion of his/her own voice, giving presence to the 
connection. For modem applications and the like, circuit 72 provides 
almost complete elimination of the outgoing signal to the user device. The 
degree of side tone removal is determined by the accuracy of the impedance 
match between the telephone line and resistor 74. 
Resistor 74 and capacitor 79 are connected in series between L+ and L- 
(common), and the output V.sub.L to power super stable voltage reference 
81. Resistor 74 generates a voltage proportional to the current flowing in 
the line, and capacitor 79 bypasses the AC signal on the line. Circuit 76 
comprises a positive voltage regulator producing a voltage V.sub.1 (+3V), 
which also provides a stable driving voltage for a current reference 
generator 77, which produces a current I.sub.1. The stable current I.sub.1 
is fed to a DC resistance control circuit 78, which sets the DC resistance 
slope for the DAA in accordance with the regulations formulated in the 
EIA/TIA and Bell specifications. Circuit 75 produces a steady -3VDC from 
the V.sub.1 input to power many of the op amp circuits on the line side 
described herein. This unique control circuitry provides AC termination, 
allow drawing current for the line side apparatus without interfering with 
the proper DC termination, provides a low distortion transmit signal 
generation mechanism, and converts the current from the line into a dual 
voltage supply. 
The circuit 72 also produces the receive line signal RCVL, which is fed 
through a differential signal processor 100 (FIG. 8), to convert the 
single-ended RCVL signal to complementary differential signals. These 
signals are fed to a clocked switch network 90 which is driven by the 
complementary clock signals CKL and CKL to differentially charge the 
isolation capacitors of circuit 33. The operation of this network are 
described with reference to FIG. 14 in the preceding description. 
With regard to FIG. 12, the line interface 42 and the line switch and 
detector circuit 43 (of FIG. 3) includes a solid state off-hook line 
switch 82 comprised of a pair of parallel transistors to open and close 
the current path from the telephone line to the L+ output. An off-hook 
switch driver 84 is controlled by an off-hook clock receiver and charge 
pump circuit 85, which is connected to the line sides of off-hook 
isolation capacitors in device 36 of FIG. 2. A secondary transistor 83 is 
used in conjunction with the off-hook switch driver 84 to insure only 
leakage current flows until the energy across capacitor 85A is sufficient 
to forward bias the darlington pair of the off-hook switch driver 84. A 
bridge rectifier 86, connected between the TIP connection and the off-hook 
line switch, is formed of Schottky diodes or the like, to regulate plus 
and minus polarity from the TIP signal. 
With regard to FIG. 6, when the user device signals the DAA to go off-hook 
by placing the OFFHK line in a low state, transistor 87 is forward biased 
and turns conductive. Square wave generator 88 operates continuously to 
produce a constant frequency (e.g., 600 KHz), which is fed to 
divide-by-two counter 89 to achieve a 50% duty signal. Activation of 
transistor 87 permits the counter 89 to operate, passing the square wave 
(at 300 KHz) to a differential off-hook clock driver 91. The clock driver 
91 supplies the signals OHCAP and OH CAP to the user sides of the 
capacitors in circuit 38 (FIG. 2). 
Returning to FIG. 12, bridge rectifier 92 is connected to ring signal AC 
coupling capacitor 93 to develop a DC level greater than the threshold of 
zener diode 94. When ring signals from the telephone line reach peaks 
above approximately 30 volts peak they raise the DC level across the 
parallel resistor-zener network 96. This voltage attains a level no 
greater than approximately 5.1 volts, and comprises the RIG signal that is 
used to power the switching network 97 and ring oscillator of FIG. 11. 
Switching network 97 passes the ring oscillator signal to the line side of 
the receive isolation capacitors of device 33 (FIG. 2). Ring oscillator 98 
(FIG. 11 ) is set to operate at approximately 300 KHz, and feeds the 
differential driver circuit 99. A lockout circuit 101 prevents the ring 
signal information from being generated unless the DAA is on-hook. When 
the DAA is off-hook the transistor of circuit 101 is turned on, preventing 
the RIG signal from actuating the switches of circuit 97. 
On the user side of device 33, differential switched network 102 (FIG. 5) 
receives signals from the isolation capacitors of device 33 and is 
synchronized with them by the clock signals described previously. The 
received signal is recovered in differential analog form and fed to an 
input signal buffer 103. The buffered signal is fed to a converter circuit 
104 which transforms the differential analog signal to a single-ended 
signal. The output of the op amp in circuit 104 sets the gain of the 
receive side. The value of resistor 105 can be adjusted to adapt to 
different modem chips. 
The signal output of the differential to single ended converter 104 
provides input for two different applications. One is the receiver section 
that is active when the DAA is off-hook. The signal from converter 104 is 
fed to an anti-alias low pass filter circuit 106. The filter corner 
frequency is approximately 15 KHz to remove the switch noise and high 
frequency RF noise on the telephone line. The op amp of filter circuit 106 
provides a buffered output signal RCV at the user interface point 
connection. The RCV signal comprises the analog received signal for all 
user devices connected to the interface. The output of buffer circuit 104 
is also fed through switch 107 to a ring indicator circuit 108. The 
received analog signal is representative of the ringing signal. If the DAA 
is on-hook and the OFFHK signal is absent, op amp 109 will turn on switch 
107 and feed the received analog signal to a ring indicator circuit 108. 
The signal is rectified by diode 110 and used to charge capacitor 111. The 
capacitor charge is compared by op amp 112 to a fixed voltage reference, 
and a ring signal output RI is generated when the charge exceeds the 
reference. The output RI is provided to an output connection point of the 
user interface. 
One circuit necessary for the functioning of various portions of the user 
side of the DAA is a reference voltage generator 113, depicted in FIG. 13. 
The circuit 113 operates on the standard V.sub.CC provided on the user 
side, and generates a 2.5 VDC stable output for the op amps on the user 
side. Returning to FIG. 6, the 300 KHz square wave output of the counter 
89 is also fed to a differential clock driver circuit 116, which generates 
the signals CK NCAP and CKCAP used to transfer energy across the isolation 
capacitors. The output of counter 89 is also fed to a phase delay circuit 
117, which is designed to match the delays of the external isolation 
capacitor elements of circuits 31, 33, 36, and 38. The phase delay circuit 
117 generates a correctly timed clock signal for the two phase clock 
circuit 118, which in turn supplies the clock signal pair CK and CK to 
all the user side coupling devices. 
Another circuit necessary for various portions of the line side of the DAA 
is the clock system depicted in FIG. 10. Clock signals transmitted through 
the capacitors of circuit 36 are received by a buffer circuit 121, and 
processed by a squaring circuit 122. The output of circuit 122 is the 
clock signals CKL and CKL which are used to drive the switching networks 
of FIG. 7 on the telephone line side. Thus the clock signals originating 
on the user side are transmitted across the interface to synchronize the 
signal decoding on the line side. 
The circuits described herein may be formed of devices that are discrete 
components, or may be part of an integrated circuit formed using 
conventional ASIC techniques. 
The foregoing description of the preferred embodiment of the invention has 
been presented for purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and many modifications and variations are possible in light of 
the above teaching without deviating from the spirit and the scope of the 
invention. The embodiment described is selected to best explain the 
principles of the invention and its practical application to thereby 
enable others skilled in the art to best utilize the invention in various 
embodiments and with various modifications as suited to the particular 
purpose contemplated. It is intended that the scope of the invention be 
defined by the claims appended hereto.