GB2154834A

GB2154834A - A two wire DC power/signal transmission system

Info

Publication number: GB2154834A
Application number: GB08404772A
Authority: GB
Inventors: Roy Martin Duncombe
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1984-02-23
Filing date: 1984-02-23
Publication date: 1985-09-11
Also published as: GB2154834B

Abstract

A two wire DC power/signal transmission system comprises a sensing circuit, located at a sensing station (10), connected to one end of a pair of dc supply lines (12 and 13) and a base circuit, located at a base station (11), connected to the other end of the supply lines. The sensing circuit comprises a series connection of a first resistor (R1), a first and a second voltage reference (Z1 and Z2) and a second resistor (R1), between the pair of supply lines (12 and 13), and a transistor (TR1) having its collector electrode coupled with the supply line (12), the emitter electrode coupled with the supply line (13) and its base electrode coupled with an input for a signal to be transmitted via the supply lines, and the base circuit comprises a pair of resistors (R2) each of which is connected in series between a different one of the dc supply lines and a different one of a pair of inputs for a dc power source. <IMAGE>

Description

SPECIFICATION A two wire DC power/signal transmission system This invention relates to a two wire dc power/signal transmission system.

There are many applications where sensing devices are required to be located remotely and the signals provided thereby have to be transferred over cables to a base station for processing. Such signals are subject to interference during transmission over the cables and in order to reduce the effect of interference, pre-amplification is sometimes carried out at the remote position. Such systems require power to be supplied to the remote points. Whilst the power can be supplied by separate cables it is particularly convenient if the power can be supplied over the same cables as are empioyed for receiving signals from the remote station. This is particularly advantageous in the case of long distances or where a large number of remote stations are to be interrogated. Some examples of situations where these requirements can be applied are as follows: 1.The transferring of pre-amplified hydrophone signals from transducers outside a ship to a base station inside the ship; 2. Telephones; 3. Vibration sensors e.g. geophones; 4. Microphones; 5. Radio receivers; 6. Temperature sensors; 7. Magnetic sensors; 8. Pressure sensors; 9. Optical sensors; 10. Infra-red transmitter/receiver, remotely sited from local receiver.

The present invention seeks to provide a system which permits a wide dynamic range of analogue signals including dc to be transmitted, between a base stationa and a sensing station, along two wires whilst the sensing station simultaneously receives dc power via the same two wires.

According to the invention there is provided a two wire dc power/signal transmission system comprising a sensing circuit, located at a sensing station, connected to one end of a pair of dc supply lines and a base circuit, located at a base station, connected to the other end of the supply lines, wherein the sensing circuit comprises a series connection of a first resistor, a first and a second voltage reference and a second resistor, of resistance value substantially equal to the first resistor, between the pair of dc supply lines, and a transistor having its collector electrode coupled with one of the supply lines, the emitter electrode coupled with the other one of the supply lines and its base electrode coupled with an input for a signal to be transmitted via the supply lines, and the base circuit comprises a pair of substantially equal resistors each of which is connected in series between a different one of the dc supply lines and a different one of a pair of inputs for a dc power source.

The system is advantageous in that a balanced output is provided on the two supply lines each of which has a common low impedance. Each of a second pair of substantially equal resistors may respectively be connected between the first and second resitors of the sensing circuit and respective supply lines at said one end. The values of the first and second pairs of resistors are preferably chosen so that the output impedance of the sensing circuit matches the input impedance of the base circuit. Such impedance matching eliminates or at least reduces reflection of signals between ends of the supply lines. The first pair of substantially equal resistors in the base circuit provides a means whereby the signal transmitted down each of the supply lines can be recovered.

The use of the first and second voltage references, which may be, for example first and second zener diodes, establishes supply voltages for subsequent circuitry and results in a reference point at their junction so that the signal applied can be developed between this reference and the base electrode. The first and second zener diodes may be of the same voltage value. The emitter electrode may be coupled with said other one of the supply lines via a third voltage reference, for example a third zener diode, and this third zener diode may be of the same voltage value as the first and/or second zener diodes.

An operational amplifier may be powered by being coupled across the first and second zener diodes. The operational amplifier may be coupled to the sensing circuit by having its output coupled with the base electrode of the transistor and the amplifier has an input for receiving a signal to be transmitted via the supply lines. In the case where a third zener diode is employed the inverting input of the operational amplifier may be coupled with the emitter of the transistor via a resistor.

The operational amplifier may have a first input coupled to a reference voltage and a second input, for the signal to be transmitted, which is coupled with a signal input. The first input may be coupled with the junction between the first and second zener diodes to provide the reference voltage.

In the base circuit, where two substantially equal value resistors are connected one to each side of the source of power so as to couple the source to each supply line, a differential amplifier having its differential inputs coupled one to each of the two cables may be provided.

In order that the invention and its various other preferred features may be understood more easily, embodiments thereof will now be described, by way of example only, with reference to the drawing in which: Figure 1 is a simplified circuit of a current drive transmitter circuit of a type known to us; Figure 2 is a simplified circuit of a voltage drive transmitter circuit of a type known to us; Figure 3 is a simplified circuit of a possible modification of the circuit of Fig. 2 to provide a balanced output arrangement; Figure 4 is a simplified circuit of a two wire dc power/signai transmission system constructed in accordance with the invention; Figure 5 is a detailed circuit diagram of one implementation of the circuit of Fig. 4; and Figure 6 is a load line graph for the circuits of Fig. 5.

In the current drive circuit of Fig. 1 a remote sensing station 10 is supplied with power from a base station 11 via two cables 1 2 and 13. The power supplied is used to drive a constant current generator 1c which supplies zener diodes Z1 and Z2 and provides balanced + V, - V and zero output lines for powering for example amplifying circuitry of a sensor. The sensor provides a controlling signal for the variable current source i5 which thereby provides a varying signal current which is fed over lines 1 2 and 1 3 and develops a voltage across the load R, at the base station which voltage can be detected and/or processed.

The voltage drive system of Fig. 2 is similar to Fig. 1 except that the variable current generator i, is replaced by a variable voltage generator vs having a source impedence Rs. In this case the voltage of v5 is variable in dependence upon the sensed signal and the voltage developed across RL is detected and/or processed.

In both of the circuits of Figs. 1 and 2 the constant current generator lc isolates the circuit consumption variations from the signal variations and reduces their effect across the load R,. The arrangement of Fig. 2 has been used in process control. Here, a small output signal from a remote sensor e.g. a temperature sensor, which would otherwise be corrupted by electrical interference and attenuated by cable length may be amplified and transmitted along the cable as a current in the range 4mA to 20mA (an industrial standard).

The current mode is more suitable on account that the signal accuracy is not a function of cable resistance, i.e. it is independent of the cable type and length (within limits) between the remote sensor and local receiver. The typical requirements in this application are dc to low frequency operation up to 1OHz and moderate dynamic range of 40-50 db. A limitation of two wire voltage drive circuits is that the received signal amplitude can be somewhat dependent on cable resistance.

However if the cable details are known, then the attenuation can be compensated for.

To reduce the susceptibility to electrical pick-up and thereby improve dynamic range, the voltage drive configuration of Fig. 2 can be modified to a balanced output arrangement as shown in Fig. 3 and can then be operated with a differential receiver. In practice, difficulties arise in matching two dc coupled low impedence drivers v5. Also two constant current generators lc are required if the circuit is to have symmetry of supplies (+ v) to the transmitter circuitry. Again difficulties arise in matching of component characteristics.

Fig. 4 shows a simplified circuit embodying the present invention which seeks to overcome the previously mentioned difficulties.

The constant current generators are replaced by two equal value resistors R 1. A single low impedance signal source is provided by a transistor TOR 1 coupled between the lines 1 2 and 1 3 at the sensing station and this operates into both the resistors R1 and resistors R2 of equal value provided at the base station in series with a power supply V5. The base of TRI is driven by a signal from the sensor and amplifier 14.

The transistor TRi is arranged to have a quiescent voltage Vq such that a quiescent current is drawn through the transistor. By inspection, the same net current flows in the bottom resistor R2 as in the top R2 and therefore, the collector voltage is - Vq. By further symmetry, the same current in the top resistor R1 flows in the lower resistor R1 and the mid-point of the sensor is located at zero volts (for equal first and second zener voltages). By modulating the emitter voltage Vq the net current through both resistors R2 is modulated and the collector voltage exhibits an equal amplitude opposite sign voltage to that of the emitter voltage. The symmetry of current in resistors Ri is maintained although the current varies in magnitude as the output is modulated.At the collector, since the same signal voltage and current flow at this node as at the emitter node, the impedence of each node is the same and low in value. This low source impedence balanced operation, is suitable for interfacing to a conventional differential receiver RX in the base station so that the effects of unwanted pick-up on the cables 1 2 and 1 3 which would normally be in phase are cancelled or at least significantly reduced.

For high frequency use where it is desirable to terminate the cables 1 2 and 1 3 by their characteristic impedance this can be chieved by connecting each of a pair of substantially equal resistors (not shown) respectively between each of the resistors R1 and their respective supply cables (12 and 13) at the sensing station end of the supply cables 12 and 1 3. Alternatively, this can be achieved by splitting R2 into two parts, one part (for example half R2) at the transmitting end and the other part (for example the other half of R2) at the receiving end of the cables 12 and 13.

Fig. 6 indicates the load line graph for the upper half of the circuit in Fig. 5 (this applies to the lower half circuit also, but with the sign changed on the voltages).

Referring now to Fig. 5 a more detailed circuit indicating specific components and values also provides details of an operational amplifier 1 5 for amplifying the signal of the sensor. The operational amplifier 1 5 has + v and - v supply lines connected to opposite ends of the series connection of zener diodes Z1 and Z2. The non-inverting input of the operation amplifier is connected via a potential divider R5 and R6 between the zero voltage line and + V. The inverting input is coupled via a resistor R3 to the emitter electrode of transistor TR 1. If the emitter of the transistor TR1 is connected directly to the line 12, then the base of the transistor TR 1 is controlled by approximately the voltage of the line 1 2. In the case where the voltage of the line 12 exceeds that which can be provided by the output of the operational amplifier 15, then a third zener diode Z3, connected as shown in Fig. 5, enables the base of the transistor TR1 to be controlled by a voltage which is within the range of the voltage of the output of the amplifier 1 5. In this case the emitter is connected to the line 1 2 via the zener diode Z3 which is of the same type as Z1 and Z2. The inverting input is connected via a resistor R4 and capacitor C to a sensor S which provides the signal to be transmitted.

Claims

1. A two wire d.c. power/signal transmission system comprising a sensing circuit, located at a sensing station, connected to one end of a pair of dc supply lines and a base circuit, located at a base station, connected to the other end of the supply lines, wherein the sensing circuit comprises a series connection of a first resistor, a first and a second voltage reference and a second resistor, of resistance value substantially equal to the first resistor, between the pair of dc supply lines, and a transistor having its collector electrode coupled with one of the supply lines the emitter electrode coupled with the other one of the supply lines and its base electrode coupled with an input for a signal to be transmitted via the supply lines, and the base circuit comprises a pair of substantially equal resistors each of which is connected in series between a different one of the dc supply lines and a different one of a pair of inputs for a dc power source.

2. A circuit as claimed in claim 1, wherein said first and said second voltage references are of the same voltage value.

3. A circuit as claimed in claim 1 or 2, wherein the emitter electrode is coupled with said other one of the supply lines via a third voltage reference.

4. A circuit as claimed in claim 3, wherein the third voltage reference is of the same voltage value as said first and second voltage references.

5. A circuit as claimed in any one of the preceding claims, including an operational amplifier coupled across said first and second voltage references, having an output coupled with the base electrode of the transistor and an input for receiving a signal to be transmitted via the supply lines.

6. A circuit as claimed in claim 3 or 4, including an operational amplifier coupled across said first and second voltage references, having an output coupled with the base electrode of the transistor and an input for a signal to be transmitted via the supply lines which is also coupled with the emitter of the transistor via a resistor.

7. A circuit as claimed in claim 5 or 6, wherein the operational amplifier has a first input line coupled to a reference voltage and a second input line, for the signal to be transmitted, which is coupled with a signal source.

8. A circuit as claimed in claim 7, wherein said first input line is coupled with the junction between said first and second voltage references to provide said reference voltage.

9. A two wire d.c. power/signal transmission system as claimed in any one of the preceding claims, wherein each of a second pair of substantially equal resistors is respectively connected between the first and the second resistors of the sensing circuit and respective supply lines at said one end.

1 0. A two wire d.c. power/signal transmission system as claimed in any one of the preceding claims, wherein the first voltage reference and the second voltage reference are zener diodes.

11. A two wire d.c. power/signal transmission system as claimed in claims 3 and 4, wherein the third voltage reference is a zener diode.

1 2. A system as claimed in any one of the preceding claims, wherein the base circuit comprises a differential amplifier having its differential inputs coupled one to each of the two cables.

1 3. A two wire d.c. power/signal transmission system substantially as described herein with reference to Figs. 4, 5 and 6 of the drawings.