TELEPHONE LINE DATA ISOLATOR

Abstract: An isolation device, primarily for isolating telephone terminal elements and data transmission and/or receiving modules from telephone lines, whilst enabling telephone and/or encoded data signals to be transferred between the elements/modules and the lines, consisting of a high frequency transformer (16) serving as an isolation barrier, and circuitry including an oscillator (18) capable of representing line signals as modulated voltages across the transformer to produce a direct current across the element/module load, and to represent fluctuations in said direct current at said line. The circuitry also enables direct current to be provided at the load to power and element or module.

Description: TITLE: TELEPHONE LINE DATA ISOLATOR This invention relates to an active isolator for coupling data, audio frequency, or other signals to and from telephone lines or other systems. Conventional devices for isolating signals from telephone lines utilise high quality audio coupling transformers. A schematic circuit diagram illustrating such an arrangement is shown in Figure 1 of the accompanying drawings. The letter L in Figs. 1 to 4 denotes the connections to a telephone line. Resistor R6 is typically 600 ohms. The transformers have a relatively large laminated iron or ferrite material core, and a large number of turns on primary and secondary windings. The DC line current normally flows through the primary, serving no useful purpose, and necessitates the large core and usually an air gap to prevent core saturation. The resistive isolation (and the high voltage breakdown) can be made high but because of the large mass of the windings in close proximity to each other the capacitive isolation is poor. Because of the response of the transformer to a wide range of audio frequencies, the isolation of transverse AC voltages (AC voltages such as 50 or 60Hz mains voltages impressed across the secondary under fault conditions) , is extremely poor. To overcome this problem it is usually necessary to fit a transverse voltage protection network to the - 2 - secondary of the transformer. A simplified circuit diagram of such a network is shown in Figure 2 of the accompanying drawings. With this network fitted the isolation unit (transformer and protection network) will provide a reasonably safe coupling to the telephone line with the following limitations. (1) The bulk and cost of the transformer and network is high. (2) The system only provides coupling of audio signals to and from the telephone line and is basically unsuitable for transferring power from the line, the line power being simply dissipated in the primary winding of the transformer. Any data encoding, decoding, or signal processing circuitry connected to the isolated side of the system must of necessity be powered by external power sources such as mains AC or batteries. Other conventional isolators include opto isolators, which are usually solid state devices such as light emitting diodes packaged with light sen'sitive diodes or transistors. A simplified circuit for coupling signals from a line is shown in Figure 3 of the accompanying drawings, and a simplified circuit for coupling signals to and from a line is shown in Figure 4. The circuits of Figures 3 and 4 are able to provide high isolation, but still require external power supplies from mains or batteries as they are basically unsuitable for transferring power. In addition, the signal losses across the devices are also quite high. Voltage breakdown devices such as diodes, zener diodes, and gas discharge tubes are sometimes used in conjunction with limiting components such as capacitors or resistors. These systems provide a capability to limit the maximum voltages and currents applied to the line, but - 3 - the degree of isolation is not high and power transfer is generally not possible. When these devices are used, a great deal of care and cost must be put into components such as mains transformers and other circuitry connected to them. The devices are unsuitable for connection of biological signal processing, circuitry which may be connected to human patients. An object of the present invention is to provide a telephone line data isolator which will overcome the disadvantages of prior devices. It may also attempt to provide a telephone line data isolator in which the power required for operation of a data input device may be obtained from the line or other system. According to the invention there is provided an isolation device for the bi-directional transfer of desired signals between a first system and a second system, and for preventing the transfer of undesirable signals there¬ between, characterized in that said systems are connected either side of a high-frequency transformer (16) . Embodiments of the invention will be described in detail hereinafter, with reference to the accompanying drawings, in which:- Fig. 5 is a circuit diagram of a first, simple, embodiment of the invention; Fig. 6 is a circuit diagram of a second embodiment of the invention; Fig. 7 is a circuit diagram of a typical active hybrid circuit; Fig. 8 is a block diagram of a typical application of the circuit of Fig. 6; and Fig. 9 is a circuit diagram of a third embodiment of the invention. Referring firstly to Fig. 5, there is shown the primary side 10 of the isolating device, and the secondary side 12 thereof is separated therefrom by an imaginary isolation barrier 14, represented in broken lines. The isolation transformer 16 has been, in prototypes, constructed from two ferrite aerial slabs such as those used in low cost transistor radios. These ferrite slabs were each wound with a single layer of insulated wire and mounted at either side of a 3mm thick highly insulated plastic isolation barrier. The main criteria for the selection of the transformer 16 are - (1) Good square wave response at the frequencies chosen, for example 50 KHz to 500 KHz; and (2) High insulation and capacitive isolation between the windings. In practice, using modern ferrite components, high frequency transformers have been built for prototype testing using ferrite cores less than 20mm in diameter and containing less than 30 turns of fine gauge insulated wire on each segment of the core bobbin. With such isolation transformers, high voltage breakdown levels of 10 to 20 KV, and capacitance between windings of less than 5 pico farads can be realized. The cost of such high frequency isolation transformers can be as low as 5% of the cost of currently available conventional audio-isolation transformers. The isolation transformer may, of course, be manufactured in other ways, using other materials. In the secondary side 12 of the circuit of Fig. 5, RL may consist of (1) a resistor across which signals may be sent to or received from the line or other system, (2) a telephone instrument which is required to be isolated from the telephone line L for safety or for any other reason, or (3) a secondary circuit such as that described later in this specification. In Fig. 5 diode bridge DBl is present to ensure that the polarity of the voltage supplied to the primary side of the isolator is always constant regardless of actual line polarity. This is important particularly when the device is connected to telephone lines which do not have a guaranteed or defined polarity. ZDl and Rl form a line surge protection network. Under normal operating conditions the breakdown voltage of ZDl is chosen to be higher than the highest voltage across the primary side of the isolator under all possible line resistance conditions. If under surge or fault conditions the line voltage increases, ZDl will conduct and so limit the voltage applied to the primary side of the isolator to within safe limits for the electronic components used. A passive low pass filter consisting of CHl, Cl and C2 is present to prevent the passage of high frequency currents in the primary side of the isolator from reaching the telephone line or other system to which the isolator is connected. Capacitor C2 provides a low impedance path for correct operation of the high frequency transformer driver (QI and Q2) . The low pass filter has a high series and low parallel impedance for the high frequency currents, and a low series and high parallel impedance for the signals within the bandwidth of the line: audio, data or other signals in the approximate range DC to 30KHz. ZD2 is present to provide, via R2 from the positive line voltage, a regulated supply for the high frequency oscillator. Capacitor C3 is a low impedance bypass for all frequencies present in the isolator. Resistor R2 may be replaced by a constant current source or constant current diode, with a consequent advantage of higher dynamic impedance for the signals transferred to and from the isolator. The high frequency oscillator 18 is an astable - 6 - multivibrator built around a HEX C.M.O.S. inverter integrated circuit. The function of the oscillator could be accomplished in other ways including the use of discrete or integrated transistor circuits. The optimum frequency for the oscillator, taking into account economy, efficiency, and size of filter components and the isolatio transformer, has been established by calculation and experiment to be in the order of 150 KHz. The system will operate over a far greater range of oscillator frequencies, and any other frequency may be chosen to suit particular data bandwidths or applications. The output of the oscillator is coupled to a high frequenc drive circuit via R5 and C5. R5 is present to limit the base current of transistor QI, and capacitor C5 is present to speed up the switching time of transistor Q2. Transistor QI is switched on and off at the frequency of the oscillator and an output square wave form with an amplitude close to the DC voltage between A and B on the primary side of the isolator. This square wave form, at the frequency of the oscillator, is connected to the base of transistor Q2 whic drives the primary of high frequency isolation transformer Tl in the emitter follower mode. Transistors Ql, Q2 may b discrete or integrated. Other suitable drive circuit configurations could of course be used. For some applications, where extremely high isolation, such as in connecting patients for biological measurements or communications to telephone lines or systems, a tuned system may be preferable. For the tuned system the values of LI, at the primary of transformer 16, and L6 are chosen to form a series LC resonant circuit with a frequency of resonance a or near the chosen optimum oscillator frequency. In practice the oscillator frequency is set to a higher or lower operating frequency than the L1/C6 series tuned circuit, or alternatively the oscillator is fixed in frequency and the constants of the tuned circuit are set to tune above or below it. The operation of the tuned circuit is as follows:- the primary side of the isolator is coupled to Ll, which is part of a series resonant tuned circuit, by L2 and by mutual induction between the coils an alternating voltage is set up across L2. This alternating voltage is rectified by D2 and filtered by low pass filter CH2, C7, C8. A DC voltage will thus appear across RL. By the choice of the coupling constant between Ll and L2 the Q factor of the primary series resonant circuit may be altered. The Q of . the series resonant tuned circuit L1/L6 is also altered by the value of Rl, or to put it another way, is altered by the amount of power drawn from the tuned circuit. Additionally, a direct or alternating voltage applied across RL will increase or decrease the Q factor of the series resonant circuit depending on whether the polarity boosts the voltage produced across RL by the secondary of the isolation transformer. It is well known that a series LC resonant circuit has a minimum impedance at its resonant frequency. It is also well known that if the Q factor of a series resonant circuit is reduced, its impedance increases, and that if the Q factor is increased its impedance decreases. It will therefore be seen that if the value of RL is changed or modulated, then the loading effect of the series resonant circuit L1/C6 on the line via the high frequency transformer driver will also change or be modulated. In practice, the difference between the oscillator frequency and the series resonant circuit frequency is set to provide an optimum bias point for best linearity of modulation in keeping with supplying sufficient power for the requirements of RL. Signals from the line produce amplitude modulation - 8 - of the voltage between circuit points A and B, and hence produce modulation of the drive level voltages across the series resonant tuned circuit and high frequency transformer. These signals therefore appear after being effectively demodulated and filtered by Dl, CH2, C7, C8, across RL. It may therefore be seen that the isolator described may transfer signals from the line to RL and from RL to the line as well as providing DC power into RL for any requirement that may be necessary within the constraints of the power available. The power transferred across this isolation transformer from the normal telephone line, using the circuit described, was sufficient to operate a specially designed full duplex computer data moden, and data communications with a time share computer via telephone were entirely successful. For an untuned system, C6 is made much larger in value and transformer 16 is made a tightly coupled, closed or near-closed core high frequency transformer. C6 is included in the untuned system to eliminate effective DC current flow due to any possible imbalance in the mark/space ratio of the drive waveform applied to the transformer 16 primary. An AC sine wave drive, or an accurate 1:1 mark/space square wave drive could be used, in which cases C6 could possibly be omitted. Ll and L2 of transformer 16 can be wound on either side of a double insulated bobbin of any one of a number of proprietary ferrite cores such as potcores, or other cores suitable for high frequency operation. In experimental prototypes of the circuit of Fig.5 the following component values were used, by way of example Rl 4 ohm R2 1. 8 Kiloohm R3 68 Kiloohm - 9 - R4 15 Kiloohm R5 2.2 Kiloohm R6 2.7 Kiloohm Cl 0.033 microfarad C2 0.033 microfarad C3 1.000 microfarad C4 68 picofarad C5 68 picofarad C6 to be selected for tuned or untuned circuit Bl to B3 part of C.M.O.S. hex invertor pack QI to Q2 2N3725 CHI 2.5 megahenry radio frequency choke ZDl 24 volt 10 watt ZD2 6.2 volt 400 milliwatt. A major advantage of the tuned system described is that the high frequency isolation transformer may be of open construction and thus provide an extremely high degree of isolation. A major advantage of the untuned system is higher efficiency of power transfer from the line or system to RL, and in addition exact setting of the oscillation frequency is not necessary due to the untuned nature of the system. The embodiment of Figure 5 operates as follows. In the untuned system the same circuitry may be used except for the changes in Tl and C6 previously mentioned. Signals from the line will appear between A and B and thus cause changes or modulation in the voltage level applied to primary of the high frequency transformer driver 16. These signals will cause amplitude modulation of the high frequency carrier drive level to the primary of the transformer. The amplitude modulated carrier will appear across L2 by transformer action and after rectification by D2 and filtering by CH2, C7, C8, a signal , , nn«0 1/00658 - 10 - caused by the original signal applied to the line will appear across RL. Signals applied to RL will either buck or boost the DC level across it causing increase or decrease in the current drawn from the line and hence a replica or near replica of the voltage wave form applied to RL will appear across the line. Similarly, increases o decreases in RL will cause decreases or increases respectively in the line current. It will be seen that the untuned isolator described will transfer signals from the line to RL, from RL to the line and provide DC power to RL within the constraints of the power available, whilst maintaining an extremely high degree of isolation to and from the line or system. Turning now to the embodiment of Figure 6, it wil be seen that the circuitry of the primary side is similar to the circuitry of Fig. 5, with the addition of some components. These components are utilized to provide bridge drive of the high frequency waveform to the primary of transformer 16. In practice, depending on the application, this bridge drive circuit can provide higher efficiency than the simpler circuit of Fig. 5. The primary/secondary ratio of transformer 16 could be altered to produce an increase in efficiency whilst utilizing either single ended or bridge drive. Similarly, the primary could be tapped to allow for a push-pull drive circuit of any suitable configuration. Transistors QI to Q4 may be discrete, but it has been found convenient in practice to utilize an integrated or hybrid quad core switching transistor package such as the DH3725. Alternatively other suitable drive circuits either discrete or integrated may be used. The secondary circuit contains all the previously outlined functions of rectifying and filtering with the addition of another supply output 20 which may be required for any data encoding/decoding circuitry required for particular applications. LED 1 is a proprietary constant current light emitting diode with the function of providing:- (A) A visual indication that the line or system is connected to the isolator; (B) A constant current for voltage regulator ZD3; and (C) A high dynamic impedance because of its constant current characteristics, which greatly reduces the load on the signals both in and out of C, and hence increases the efficiency of signal transfer across the isolator in both directions. In practice, LED 1 could be replaced by a resistor, an audio choke or another form of constant current source such as a constant current diode or active constant current source such as a constant current diode or active constant current source. However, the constant current LED has been found to be the ideal device for the purpose. The negative rail circuitry could be realized around the same components as the positive rail if more " current was required from it. A hybrid circuit either passive or active, may be connected to the secondary side of the isolator to provide separation between input and output signals. The circuit of Fig. 7 is a typical active hybrid circuit. 22,24 are, respectively signal input and signal output, and 26 is an in/out connection to signal. A major advantage of connecting the hybrid circuit to the secondary side of the isolator described is the effect of line resistance and/or impedance changes on the balance of the hybrid circuit is greatly reduced over the effect of these changes when a hybrid is connected directly to the line side of a system. In telephone systems or other audio line systems regulators such as voltage dependent resistors are sometimes used to assist the maintenance of hybrid balance over varying line "BURE A . V - ' ---" resistance conditions. This effect of reduced sensitivity to the line is due in part to the fact that the current to ZD3 is kept constant by LED 1 (or other constant current circuit) and hence the impedance at C is kept relatively constant over a wide range of line resistance. This means that the ' signal voltage impressed upon C via RIO will remain relatively constant and hence the voltage drop across RIO and the balance of the hybrid will remain similarly constant over a wide range of line conditions. If R9 is replaced by a constant•current circuit of suitable type a further decrease in the sensitivity of the hybrid balance to line changes will result. As will be seen from Figure 5, circuitry to perform functions such as the full duplex data modem of Fig. 8 may be powered from the line via the isolator and signals passed to and from the line either via a suitable hybrid circuit or not, whilst maintaining extremely high isolation to and from the line. In Fig. 8, 22 and 24 are connections to the hybrid circuit, such as that of Fig. 7. 28 is an FSK oscillator, and 30 is an FSK decoder. 32 is a common return, and 34, 36 are, respectively, digital data input and digital data output, from and to digital data terminal 38. A further advantage is that data terminals of unknown or untested/approved safety standards may be connected to the data circuits of the isolator described with the knowledge that the line or system is completely protected from any inherent or accidental faults occurring on these terminals. It can be seen that the device of Figs. 5 and 6 provides:- (1) a high degree of electrical isolation between the data or signal processing side of the isolator and the telephone line or system; - 13 - (2) a capacity to utilize the electrical power present on the telephone line or system to supply the data or signal processing circuitry whilst maintaining the isolation described in (1) ; and (3) the necessity for only one isolation coupling element to provide in association with the active circuitry of the device, coupling of signals to and from the telephone line or system and transfer of power from the telephone line or system to operate the circuitry for data encoding and decoding or signal processing. In contrast to prior art devices, the present device will provide all the power necessary for the data encoding, decoding or signal processing circuitry from the telephone line whilst maintaining a far greater degree of isolation than the traditional approach with the added advantages of low cost and much lower size and weight. In addition, the device is able to provide isolation exceeding opto isolators and with a low signal loss comparable to the standard audio transformer system. Fig. 9 is the circuit of a device which may be used for the isolation from telephone lines of mains- powered equipment associated with telephone terminals. Such equipment may be used as an intermediate element in a telephone system, such as radio, i.e. transmit/receive equipment used to provide remote terminal capability. In the circuit, Rl and Z01 are utilized for transient voltage protection to prevent excessive line transients damaging the active circuitry of the isolator. Ll, Cl and C2, form a passive low pass filter which allows the passage to and from the line of DC levels and signals within the useable range of the device, but prevents high frequency carrier components from being impressed onto the line. CMOS inverters II and 12 are connected in an astable oscillator 18 configuration to 00658 - 14 - produce the high frequency square wave drive for the transformer driver stage. RVl allows adjustment of the frequency, in the range 75-175 KHz, of this oscillator to suit the specific requirements. The high frequency transformer driver is of the complimentary collector output type and is realized around high voltage high speed PNP and NPN switching ' transistors Q2 and Q3. Supply current for the transformer driver is sourced via R3 with Ql conducting above a fixed threshold. The presence of the circuit including Ql and R3 enables the power to the high frequency oscillator to be increased with the direct current demand of the driver stage. This enables the idle current of the complete circuit to be reduced to very low levels, via sustaining resistor R2, in the absense of load on the isolator secondary circuit 12. C7 is present to prevent DC flow in the primary of transformer 16. The simplified secondary circuit 12 provides DC power (D3 and filter C8/L1/C9) for the load (RL) and the filter which is similar to that on the primary side and prevents the passage of high frequency carrier to the load whilst passing the required signals in both directions. In the circuit of Fig. 9, the transformer 16 is transparent to the normal signals used in telephone equipment, but creates a barrier 14 to other signals, and thus prevents the transference of dangerous signals to the telephone line L. In addition, the circuit, when the telephone terminal is not in use, draws very little power from line L, insufficient to trigger automatic detection apparatus.

Claims: - 15 - CLAIMS 1. An isolation device for the bi-directional transfer of desired signals between a first system and a second system, and for preventing the transfer of undesirable signals therebetween, characterized in that said systems are connected either side of a high-frequency transformer (16) . 2. An isolation device according to claim 1, characterized in that said first system includes drive means (18) for said transformer (16) , said drive means being powered by said first system, and said second system includes means to enable power from said first system to be represented as a direct current across a load of said second system. 3. An isolation device according to claim 2, characterized in that fluctuations in said direct current may be represented at a signal input/output stage of said first system. 4. An isolation device according to claim 3, characterized in that fluctuations in the signal at said first system input/output stage may be represented at said second system load. 5. An isolation device according to any one of claims 2 to 4, characterized in that said drive means includes an oscillator, which operates to represent voltage fluctuations at said input/output stage as modulations of the drive level voltages across said transformer. 6. An isolation device according .to any preceding claim, characterized in that the circuits of said first system are tuned. 7. An isolation device according to claim 1, characterized in that said first system is connected to a telephone line, and said second system is connected to a telephone terminal element, in that means are provided in said first system to enable telephone signals appearing at the line or the element to be represented at the element and line respectively. 8. An isolation device according to claim 1, characterized in that means are provided in said first system to sense signal changes at said element and to activate said first system to represent those signals at said line.

Inventors: WHITBY D
Applicants: ELECTRO MED ENG PTY LTD, WHITBY D
Application year: 1980
Main classification: H04M19
IPC codes: H04M11