Abstract:
Design techniques are describes in which a telephone interface circuit or DAA can be realized using two silicon integrated circuits which include integrated opto couplers. The LED is formed on the chips using technologies which can be easily integrated onto a silicon integrated circuit such as porous silicon, avalanching PN junction, forward biased PN junction, deposited silicon carbide PN junction, deposited organic LED material such as conjugated polymers, or deposited GaAs LEDs. The light detector is realized using either a PN junction based detector or a Schottky diode detector depending on the wavelength of the light transmitted by the LED. The two integrated circuits can be placed in a single package with suitable optical links within the package. This technology thus eliminates the need for discrete opto couplers and transformers.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Provisional Application No. 60096405 Filing Date Aug. 13, 1998. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     REFERENCE TO A MICROFICHE APPENDIX 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is directed to circuits that interface a telephone line to a MODEM and to AC powered telephones and are fabricated using integrated circuit techniques and silicon based LEDs. 
     2. Prior Art 
     Isolation between the telephone line and AC house powered telephone circuits require an isolation barrier. Specifically, answering machines, cordless telephones, speaker phones, FAX machines, and MODEMs require the use of an electrical supply powered by a 60/50 Hz AC source which cannot be electrically connected to the telephone line through a hard wired connection. The ground of the AC powered circuit must be allowed to float with respect to the telephone line. To couple signals between the telephone line and an AC powered circuit a number of isolation devices can employed including transformers, capacitors, and opto-couplers. In addition, relays may also be employed to act as electrically isolated control switches such as an off hook switch in a MODEM. The interface circuit between the telephone line and an AC line powered circuit is referred to as a Data Access Arrangement or DAA. 
     Traditionally, signal isolation has been accomplished by using a 600 Ohm transformer. The biggest drawbacks in using transformers, however, are size and cost. Capacitors can be used as isolation devices but add undesirable capacitance between the isolated circuits unless the capacitance is very small, i.e. on the order of a few pico-farads. Also, capacitively isolated circuits are susceptible to transient disturb if the ground bus of one of the isolated circuits sees a rapid voltage change with respect to the ground bus of the second isolated circuit. This effect is due to the fact that a capacitor must pass charge from the output of one circuit to the input of the other circuit if the ground bus voltage difference between the two circuits changes. Finally, capacitively coupled telephone isolation circuit complexity increases over that using a transformer. However, with integrated circuit technology this disadvantage is greatly reduced. 
     The disturb problem associated with capacitively coupled signal isolation is not present in optically coupled signal isolation. Like the capacitively coupled isolator, the optically coupled isolator requires more circuitry than does a transformer coupled isolator. 
     In the traditional MODEM DAA which uses a transformer for signal isolation an opto coupler is employed for ring detection and a relay for the off hook condition. A Darlington connected bipolar transistor pair is typically used to sink the off hook current while providing a high signal impedance. 
     With the ability to provide complex circuits at a low cost, it is feasible to eliminate the bulky transformer and replace it with either an isolation capacitor or an opto coupler. This was illustrated in U.S. Pat. No. 5,438,210 which showed how SOI combined with silicon based LEDs could be use to fabricate a monolithic DAA. One difficulty with this approach is the lack of general acceptance of SOI technology and the high cost of the substrate material. 
     Other inventions show how opto couplers can be used in a discrete circuit format to perform the isolating function. The opto couplers in these inventions use commercially available opto-couplers which are based on GaAs technology for the LED and silicon for the detector technology. The problem with these approaches is the cost of making a circuit with several discrete integrated circuits. 
     SUMMARY OF THE INSTANT INVENTION 
     This invention relates to an optically isolated DAA which eliminates the traditional signal isolation transformer. Applications include MODEMs, FAX machines, and AC line powered telephones such as but not limited to speaker phones, cordless telephones, and telephone answering machines. 
     It is the objective to show how even low efficiency LEDs can be used eliminate the signal transformer and how to integrate components that have traditionally been discrete. In particular, traditional discrete LEDs such as GaAs diodes can be replaced either with low efficiency silicon based diodes such as porous silicon, avalanche diodes, deposited silicon carbide diodes, etc. or with deposited polymer light sources or even deposited GaAs LEDs. LEDs that are placed directly onto the silicon integrated circuit will lower the cost over discrete LEDs. Furthermore, unlike that of U.S. Pat. No. 5,438,210, this invention seeks to unitize bulk silicon technology which is much more available and lower in cost than SOI technology. The key to using low efficiency LEDs is to digitally encode the signal and transmit it across the optically coupled barrier as a series of digital pulses rather than as an analog signal. This removes the stringent requirement of linearity and phase integrity needed for the analog signal, especially as it applies to MODEMs. 
     It is another object of this invention to reduce the number of LED-light detector pairs to two for off hook and ring signaling in addition to signal coupling. It is another objective to show how a MODEM DAA can be realized using only two silicon chips in a single package. And another objective is to show how a speaker phone, a voice messaging phone, and a cordless phone can be realized using two silicon chip circuits. 
     PRIOR ART STATEMENT 
     U.S. Pat. No. 5,369,687. This patent describes an opto coupler based DAA which uses a D to A and an A to D converter on the telephone line side to drive the transmit and receive opto couplers. Two additional couplers are shown for the ring signal and the off hook signal. 
     U.S. Pat. No. 5,465,298. This patent uses opto couplers to transmit and receive the telephone signal. Transmission through the opto couplers is direct with an additional light detector use as a feedback sensor to improve linearity of the opto coupler. 
     U.S. Pat. No. 5,555,293. This patent show a “transhybrid that incorporates optically-coupled isolation stages”. Optical coupling is by analog means with analog feedback using a second detector. 
     U.S. Pat. No. 5,528,686. This patent shows another example of a linear opto coupler based telephone line interface. 
     U.S. Pat. No. 5,751,803. This patent shows a method of AC coupling both the tip and ring signals into a transmit optocoupler 
     U.S. Pat. No. 5,500,895. This patent is an example of a capacitive type telephone line isolation DAA. 
     U.S. Pat. No. 5,086,454. This patent shows the building blocks of and analog type opto-coupler based DAA for a MODEM. 
     U.S. Pat. No. 5,224,154. This patent shows a micro-controller based DAA which uses opto-couplers to transmit the data signals and the ring signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a diagram of an MODEM interface circuits comprised of two silicon integrated circuits which are mounted in a single package and use LEDs which are fabricated on the silicon chips. 
     FIG. 2 is similar to that of FIG. 1 except that the ring signal and off hook signal have been integrated into the signal path thereby eliminating two LEDs-detector pairs associated with the ring signal and the off hook signal. 
     FIG. 3 shows a diagram of a telephone circuit with a speaker phone, voice message recorder, and a cordless phone capability using the two silicon integrated circuits mounted in a single package. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a diagram of the preferred embodiment of a MODEM DAA which is comprised of  2  silicon integrated circuits; one  1  which interfaces to a host computer or MODEM controller and a second  2  which interfaces to the telephone line. The two chips can be housed in a single package which provides an internal optical path between the two chips. Referring to FIG. 1, the Tip  4  and Ring  3  terminals of the telephone line connect to a bridge rectifier circuit  5 . The + terminal  17  of the bridge rectifier  5  connects to several terminals including a low current regulator  28 , an off hook shunt/regulator  6 , a receive signal coupling capacitor  20 , a transmit coupling capacitor  19  and a ring signal coupling capacitor  45 . The shunt  6  is used to both pull down the telephone line from 48 V to signal an off hook condition to the central office and to provide a regulated DC voltage such as 5 V to power the various circuits interacting with the telephone line. A power switch  7  is used to connect the regulated telephone line power to the circuit once the voltage is at a safe value. The power switch is enabled by the off hook control signal  9 . To guarantee that the signal circuits such as the A to D converter  22  do not see high voltages such as the ring signal the power switch  7  must include a high voltage disable function. 
     To present a high impedance to the signal from the telephone line the shunt device  6  operates as a current source. For the telephone line power provided to the circuits of chip  2  a low pass filter  11  is used so that the telephone line signal present on node  17  is not attenuated. A filter capacitor  47  is connected from the circuit power line  8  to ground  43  and may be external to chip  2  if power supply bypassing requires a large capacitance. The power line  8  supplies a number of chip  2  circuits including digital logic, analog to digital and digital to analog converters, and analog amplifiers/buffers. 
     The signal transmit path for Telephone Interface Chip  2  starts with an optical connection from the Computer Interface Chip  1  to chip  2 . The Computer Interface Chip  1  transmits the data signal via modulated light using an “on chip” LED  37  while the Telephone Interface Chip  2  receives the light using an “on chip” light detector  13 . The on chip LED can be a standard silicon junction diode operated in the forward bias mode which emits light with a photon energy corresponding to the energy of the silicon bandgap, a standard silicon junction diode operated in the avalanche mode which emits light in the visible region, a deposited silicon carbide diode which emits light in the visible region, a porous silicon LED which emits light in the visible region, a deposited GaAs LED which emits light near the visible region, or a light emitter which uses plastic and emits light in the visible region. The detector  13  on Telephone Line Chip  2  can be a standard PN junction silicon diode or a floating base bipolar transistor if the light source is visible or a Schottky barrier diode if the light photon energy is at the bandgap of silicon. Of the LED-detector pairs mentioned, the standard PN junction silicon diode operated in the avalanche mode and a standard PN junction diode used as the detector are the easiest to implement in that no special or non standard silicon processing is required. The next easiest combination is the forward biased silicon diode as the LED and the Schottky barrier diode as the detector. After the latter combination the next easiest is the porous silicon LED and the standard PN junction diode as the detector. The last two LEDs require the deposition of materials onto the silicon substrate. 
     The detector  13  outputs a weak signal which is amplified by  14  to a logic level signal which is fed to the Data Formatter  15 . The Data Formatter  15  takes the serial data stream and converts it to an appropriate format for the D to A converter  16 . This format is typically an 8 bit word or byte at a data rate of 8,000 bytes per second. The D to A converter  16  takes the data and converts it to an analog signal. The D to A converter  16  uses a companding algorithm appropriate for a given telephone system standard. The output from the D to A converter  16  is then fed to an analog driver  18  which drives the node  17  via a coupling capacitor  19 . Node  17  connects the signal to the telephone line via the bridge rectifier  5 . 
     For the receive path the telephone line signal is picked up off of node  17  via capacitor  20  which blocks the DC voltage on node  17 . The signal is then magnified by amplifier  21  and then passed to the A to D converter  22 . The A to D converter  22  will typically output a byte at a data rate of 8,000 bytes per second. The A to D converter  22  uses a companding algorithm appropriate for a given telephone system. The output of the A to D converter  22  is then passed to the Data Formatter  23  which formats the data into a serial format. The output of the Data Formatter  23  is fed to the LED buffer/driver  24  which feeds the serial data signal to the LED for transmission across the isolation barrier  46 . 
     The circuit controlling the off hook status includes a light detector  26  which receives its signal from an LED  30  which is controlled by the computer via an input  36 . The detector  26  signal is then amplified by  27  and fed to the control line  9  which turns the shunt  6  on or off control and to the on/off control of the power supply switch  7 . When enabled by the control line  9 , the shunt will pull the node  17  down which in turn signals the central office by drawing current through the Tip  4  and Ring  3  terminals. Also, when the shunt  6  is turned on the power supply switch  7  is also turned on providing power to various circuits including amplifiers, converters, and drivers. 
     The off hook amplifier  27  is a low power amplifier which receives its power from a special low current regulator  28 . Regulator  28  does not pull down the DC telephone line voltage since it is designed to draw only a small current on the order of a few micro-amps for the amplifier  27 . Thus, amplifier  27  is powered in the on hook state so that it can respond to the command to go off hook. Given the fact that the response of the off hook circuit can be slow allows very low power circuits to be used. 
     The ring signal is detected by noting that it produces a large pulsating DC voltage on node  17 . This voltage is fed into a capacitor  45  which filters out the DC component of this signal. A threshold device  10  such as a Zener diode is used detect the large amplitude of the ring signal by allowing current to flow into the ring LED  12 . The ring signal is then transmitted across the isolation barrier  46  via light emitted from LED  12 . 
     Not shown in the diagram of FIG. 1 is an oscillator in chip  2  which provides timing for the logic circuits. 
     The Computer Interface Chip  1  deals with four principle signal components including the transmit data signal, the receive data signal, the ring signal, and the off hook signal. 
     For data transmission, a bi-direction port  29  is used to receive data from the host computer. This port can either be a parallel port or a serial port. The data is then sent via a tristate I/O buffer to the transmit data formatter  39  which puts the data in a serial format. An LED buffer  38  drives the transmit LED  37  which sends the data across the isolation barrier  46  via light pulses. These pulses are then changed to electrical pulses, formatted, and then converted to an analog signal for transmission to the telephone line. 
     For receiving data, detector  32  is used to receive the light pulses containing the data being received from the telephone line. The pulses are amplified by  33  and are then sent to the data formatter  34  which converts the serial stream of data into a format appropriate for the computer. For the case shown in FIG. 1 the format is assumed to be parallel data which is fed to a tristate I/O buffer  44 . The data is then ported  29  to the computer bus for processing. 
     The off hook command signal  36  is input from the computer and then sent to a buffer/driver  31  which drives the LED  30 . LED  30  then sends a light signal across the isolation barrier  46  so that the Telephone Line Chip  2  can initiate an off hook condition on the telephone line  3 , 4 . 
     To detect a telephone ring condition a light detector  40  on the Computer Interface Chip  1  receives the light generated from LED  12  on the Telephone Line Chip  2  in response to the large amplitude AC ring signal present on the telephone line  3 , 4 . The signal from the detector is amplified by  41  and then sent out to the host computer via chip terminal  42 . 
     Control signals from the host computer are input on port  35 . This port supplies operational codes such as setting the mode of the tri-state buffer  44 . Also, an op code via this port can be used in lieu of the off hook port  36  to set the off hook condition. 
     FIG. 2 is a block diagram of a MODEM DAA circuit similar to FIG. 1 except that two LED-detector pairs are used instead of four. The reduction in LED-detector pairs facilitates packaging and perhaps lower power at the expense of more circuitry. In the diagram shown in FIG. 2 the ring LED-detector pair,  12  and  40  of FIG. 1, is shared with the signal receive path LED-detector pair,  201  and  232 . Also, FIG. 2 shows that the off hook LED-detector pair,  30  and  26  of FIG. 1, is shared with the signal transmit path LED-detector pair,  200  and  237 . 
     The ring signal which appears as a large amplitude pulsating DC voltage is applied to the signal LED through an analog “and” gate  202 . As before, capacitor  45  removes the DC loop voltage of the central office and the threshold device  10  is used to pass only a large amplitude signal. The low frequency ring signal is passed to detector  232  via LED  201  and then the received signal is amplified by  233 . The low frequency ring pulsing is detected by the Ring Detector  211  which is basically a low pass filter. Upon identifying a ring signal the Ring Detector  211  will notify the MODEM processor or host processor of the ring condition via terminal  206 . 
     The off hook activity begins with the MODEM or host processor initiating a command via the off hook terminal  203 . The Data Formatter  204  then sends an off hook signal to the buffer  238  which drives the transmit LED  237 . Next, the light signal from LED  237  is detected by  200 . A “no op” or off hook code can be transmitted which does not cause the Data Formatter  215  to output data to the D to A converter  16 . Amplifier  27  which is powered by the low current regulator  28  amplifies the signal and then outputs a logic enabling signal  210 . Regulator  228  is similar in operation to regulator  28  of FIG. 1 except that it also powers a low power Off Hook Latch  206 . Once received, the logic enabling signal  210  sets the Off Hook Latch  206  in the “off hook state” which then turns “on” the shunt regulator  6  and the Power Switch  7 . 
     The Off Hook Latch  206  is reset or set in the “on hook” state by a code sent from the Data Formatter  204  in response from an on hook command from the MODEM processor or host processor via terminal  203 . The code is deciphered by the Data Formatter  215  and a reset command is sent to the reset terminal  209  of the Off Hook Latch  206  which shuts off the Shunt Regulator  6  and the Power Switch  7 . 
     Thus, with some additional logic, two LED detector pairs are eliminated in the circuit described in FIG. 2 over that of FIG.  1 . Also, of the 4 LED-detector pairs of FIG. 4 it is possible to eliminate only one of the LED-detector pairs if desired using the aforementioned techniques. Thus, using methods described herein it is possible to have three LED-detector pairs with either the ring signal combined with the data receive signal or the off hook signal combined with the data transmit signal. 
     FIG. 3 shows a block diagram of a house AC powered telephone circuit which can interface to the telephone line. The diagram shows three types of telephone operations requiring house AC power: a powered speaker phone, a cordless phone, and a voice message recording phone. Two bulk silicon chips are used, one to interface to the telephone line and the other to humans and the AC house line power. The telephone line interface circuit is the same as that used for the MODEM. In the case of FIG. 3 the two LED-detector pair telephone interface circuit  208  of FIG. 2 is used. Although not shown, the 4 LED detector circuit can also be used or even a three LED detector pair version as mentioned earlier. 
     The transmit and receive audio signals, the ring signal, and off hook signal are all controlled by a central mircocontroller  300 . Thus, the audio signals are controlled or manipulated in digital format. The transmit and receive audio signals are sent and received, as before, in a serial data stream format via the LED-detector pairs,  201 ,  232  and  237 ,  200 . The receive data stream is received by the microcontroller via the amplifier  233  and the transmit data stream is sent out to the LED driver  238  by the microcontroller  300 . 
     For the speaker phone or head set monitor for the voice recording phone an audio interface module  302  is added including an A to D converter  307  and a D to A converter  311  which are used to convert the audio signals into digital data. An interface audio amplifier  312  is used between the D to A converter  311  and the speaker or headset earphone which is hooked up via terminal  314 . An audio amplifier  308  is also used between the A to D converter  307  and the microphone via terminal  310 . The off hook switch terminal  309  feeds directly into the microcontroller  322 . The ring signal  313  connects to a piezoelectric buzzer or the equivalent. A control button interface  305  is provided for such functions as volume, squelching, etc. The microcontoller then controls the data moving back and forth between the telephone line and the acoustic transducers. Also, if a math coprocessor is added to the microcontroller, filtering functions can be added such as echo cancellation. 
     For the cordless phone applications, another audio module  303  is used. This module includes an A to D converter  317 , a D to A converter  316 , and a control interface bus  315  going from the external R.F. module  318  to the microcontroller. Thus, the “microphone”terminal  321  of the external R.F. module and the earphone terminal  321  of the R.F. module connect to the on chip converters  316  and  317 . If the R.F. module  318  transmits data in digital format, then the A to D converter  317  and the D to A converter  316  are not necessary. The output of the R.F. module is an antenna  319 . 
     Another provision is a data/address bus  306  for interfacing the microcontroller  300  to an external flash memory or solid state non volatile memory chip  304  so that an answering machine can be created. Again, a math coprocessor may have to be included in the microcontroller  322  for handling data compression required for efficient digital voice storage. Also, additional lines on the control button bus  305  are required to handle the buttons associated with the human-answering machine interface. 
     For powered telephone applications where excellent linearity is not required it is possible to simplify the DAA design as shown in FIG.  4 . In this design analog signals rather than digital pulses are propagated through the LEDs. Note that most of the functional elements of the Telephone Line Chip  402  are the same as those found in the Telephone Line Chip  2  of FIG.  1 . The D to A and A to D converters have been replaced, however, with analog amplifiers such as  421  and  414 . 
     Because of the dynamic range advantages of compressing an analog signal such as logarithmic amplification, amplifier  421  which is in the receive path can be designed, as an option, to compress the signal before its optical transmission across the transparent insulation barrier  446 . Correspondingly, amplifier  433  decompress the signal if the signal is compressed. In the case of logarithmic compression, amplifier  433  would have an exponential gain characteristic. Amplifier  24  is a buffer amplifier and is used to drive LED  25 . Amplifier  434  is a buffer amplifier and is used to drive an earphone, speaker, etc. For a speakerphone application, two buffer amplifiers may be required; one to drive an earphone and one to drive a loudspeaker. 
     The transmit path of FIG. 4 starts with amplifier  439  which receives a signal from microphone. This amplifier may also incorporate a compression means to improve the dynamic range of the transmit path.  438  is a buffer used to drive LED  37  which transmits the analog signal across the transparent insulating barrier  446 . Light detector  13  receives the light signal and then feeds the corresponding electrical signal into amplifier  414 . If the signal is compressed, then  414  can decompress the signal as well as amplify it. Buffer  18  is used to take the signal and drive the telephone line via coupling capacitor  19 . 
     It should be noted that some linear opto couplers use a feedback diode to obtain a highly linear transfer characteristic. Most potential candidates for LEDs which can be made on silicon chips, however, have poor efficiencies and therefore make this approach more difficult to implement given that the light beam must fed into two rather than one detector; i.e. one detector to receive the signal which is on a chip different from the chip containing the LED and a second detector to provide feedback which is on the chip containing the LED. Also, the two detectors should be well matched but must reside on two different chips. Hence, the difficulty in using this approach. 
     Fortunately, one promising approach for implementing an LED on silicon is the avalanche silicon PN junction diode. Although its quantum efficiency is low its output is very linear with drive current and, therefore, makes a good candidate for implementing the telephone circuit depicted in FIG.  4 . 
     It is worth noting that the two silicon chip DAA concepts shown herein can also be extended to video phones and FAX machines. In fact, the MODEM DAAs described in FIG.  1  and FIG. 2 can also directly apply to FAX machines. 
     In conclusion, it is possible to create a two silicon chip DAA without the need for transformers, discrete opto-couplers, and capacitors. It is shown that “On” silicon chip LEDs and light detectors can be used to realize a DAA with a reduced number of external components.