GB2328816A

GB2328816A - Optical data communications receiver

Info

Publication number: GB2328816A
Application number: GB9718294A
Authority: GB
Inventors: John Hewinson
Original assignee: Siemens PLC; Chiyoda Technol Corp
Current assignee: Siemens PLC; Chiyoda Technol Corp
Priority date: 1997-08-30
Filing date: 1997-08-30
Publication date: 1999-03-03
Anticipated expiration: 2017-08-30
Also published as: FR2767994A1; FR2767994B1; FI981842A; FI981842A0; GB2328816B; FI116925B; GB9718294D0

Abstract

Optical data receiving apparatus comprises a photo detector 1 which generates electrical pulse signals representative of pulses of light received by said photo detector. A pulse stretcher 4 coupled to the photo detector 1 receives the electrical pulses and selectively increases the length of the electrical pulses to a minimum predetermined temporal length. A threshold generator 5 generates a detection threshold equal to the average amplitude of the electrical pulse signals received from the photo detector (1) multiplied by a predetermined scaling factor. Comparator 6 receives on a first input the output of pulse enhancer 4 and on a second input to the output of amplitude scaler 5 and generates an output signal pulse when the stretched pulse signals have an amplitude which is greater than the detection threshold. This facilitates interoperability of systems which use a different pulse length and/or amplitude.

Description

OPTICAL DATA COMMUNICATIONS RECEIVER The present invention relates to optical data communications receivers and in particular but not exclusively to optical data communications receivers which operate to detect pulses of light representative of digital data.

Data may be communicated in a form of pulses of light between an optical transmitter and an optical receiver.

Communication of data in this way provides a means for electrically isolating apparatus associated with a data transmitter and a data receiver.

It is often a requirement that commercial electronic equipment produced by one manufacturer is interchangeable with equipment produced by another manufacturer. However, in a situation where data is communicated in a form of light pulses, there may exist a wide variation in duration and intensity of such light pulses produced by equipment from different manufacturers. Furthermore, there may be other factors which determine that there is a significant variation in the duration and intensity of light pulses which an optical receiver must detect. Arranging for an optical receiver to detect pulses with a significant variation in temporal width and intensity whilst minimising complexity and cost of the optical receiver represents a technical problem.

The technical problem of detecting light pulses which have a significant variation in temporal width and intensity is addressed by an optical receiver according to the present invention.

According to the present invention there is provided an optical data receiver comprising a photo detector which operates to generate electrical pulse signals representative of pulses of light received by said photo detector, a pulse enhancer coupled to said photo detector and arranged to receive said electrical pulse signals and to increase selectively a temporal length of said electrical pulses so that said electrical pulse signals enhanced thereby are provided with a minimum predetermined temporal length, a pulse amplitude scaler which operates to generate a detection threshold in dependence upon the amplitude of a plurality of said electrical pulse signals received from the photo detector and a predetermined scaling threshold, and a comparator coupled on a first input to said pulse enhancer and on a second input to said pulse amplitude scaler and arranged to generate an output signal pulse when said enhanced pulse signals have an amplitude which is greater than said detection threshold.

Some pulses of light detected by a photo detector may be of a significantly shorter duration than other pulses. A pulse enhancer may be used to increase the temporal length of such short duration pulses detected by said photo detector so that electrical pulse signals representative of the detected light pulses are arranged to have a minimum predetermined length. Furthermore, the light pulses may have a significant variation in energy and therefore intensity. As such the amplitude of such pulses may vary. However, this variation will be on a relatively long term basis in that the amplitude of the pulses will not change for a significant number of pulses. Therefore a peak amplitude scaler can be used to generate a threshold in dependence upon the mean amplitude of pulses detected by the photo detector. Scaling this mean amplitude by a scaling factor, in accordance with a predefined level, serves to generate a detection threshold for the pulses. By feeding the stretched pulses and the threshold to first and second inputs of a comparator, the optical receiver is provided with a means for generating data pulses representative of the optical data without a requirement for a relatively fast acting and therefore expensive comparator circuit.

The term light as used herein refers to and includes light in both the visible and invisible spectra.

The optical receiver may further include a fast recovery amplifier connected between said photo detector and said pulse stretcher and said pulse amplitude scaler and arranged to amplify the electrical pulse signals from said photo detector. The fast recovery amplifier may be further arranged to recover quickly from a saturation state caused by comparatively high amplitude pulses received from the photo detector.

According to a first aspect of the present invention there is provided a method of detecting optical data pulses and generating signals representative of these data pulses comprising the steps of generating signal pulses representative of optical data pulses detected by an optical detector, selectively increasing the duration of the signal pulses so that each signal pulse is of a minimum predetermined temporal length, averaging an amplitude of detected signal pulses so as to provide a mean amplitude, scaling the mean amplitude to generate a detection threshold consequent upon signal pulses to be detected and comparing the threshold with the amplitude of the stretched signal pulses, and generating in accordance with the comparison an output signal representative of detected data pulses.

One embodiment of the present invention will now be described by way of example only, with reference to the accompanying drawings wherein, FIGURE 1 shows a schematic block diagram of an optical data receiving circuit and FIGURE 2 is a circuit diagram of an implementation of a first part of the optical receiving circuit shown in Figure 1.

FIGURE 3 is a circuit diagram of an implementation of a second part of the optical receiving circuit shown in Figure 1.

Infra-red Data Association (IrDA) detectors and receiver systems have become a defecto standard for communicating data using optical pulses. Communication in accordance with this standard is arranged by transmitting and receiving infra-red pulses.

The IrDA serial infra-red physical layer link specification version 1.1 October 1995, specifies that a receiver of infra-red pulses must detect such pulses with a width of between 1.4 and 20 micro seconds and an intensity of between 4 and 500,000 micro watts/cm2. This is for data rates up to and including 9.6 kilobits/second. Such optical receivers must also be immune to other spurious radiation which may be received by the detector. A receiver of such infra-red pulses must be arranged to generate well-defined pulses representative of each of the infra-red pulses of radiation received by the optical receiver, without dependence upon a width or intensity of pulses within the aforementioned ranges.

An optical receiver which is arranged to receive infra-red pulses in accordance with the aforementioned specification is shown in Figure 1. The arrangement of the circuit block diagram of Figure 1 provides a low cost and efficient way of detecting such infra-red pulses and will operate with ranges of supply voltage down to and including 3 volts. Such a circuit is also arranged to operate with substantially improved economy of power over known optical receivers.

In Figure 1 a photo detector 1 is arranged to receive light pulses represented by line 2 communicated by a suitable communications means such as a fibre optic. Connected to the photo detector 1 is a fast recovery amplifier 3. To an output of the fast recovery amplifier 3, is connected an input of a pulse enhancer 4, and a pulse amplitude scaler 5. Respective outputs of the pulse enhancer 4 and the pulse amplitude scaler 5, are connected to first and second inputs of a comparator 6. The comparator 6 is arranged to drive an output conductor 8.

In operation pulses of infra-red light are received by the photo detector 1 which operates to generate electrical pulse signals on conductor 10 appertaining to a representation of a temporal length and intensity of the received infra-red pulses. The electrical signals are conveyed on conductor 10 to the fast recovery amplifier 3. In general the fast recovery amplifier 3 operates to amplify the electrical pulse signals representative of the infra-red pulses received from the photo detector 1. However the fast recovery amplifier 3, is further arranged to amplify electrical pulse signals with a significant dynamic range. This is effected by providing the fast recovery amplifier 3 with means to recover after receiving an electrical signal pulse which has an amplitude sufficient to drive the amplifier into saturation. As such the amplifier 3, is arranged to recover quickly from a condition of saturation such that a subsequent pulse may be amplified and communicated to pulse enhancer 3, and pulse amplitude scaler 5.

Amplified electrical pulses representative of the infra-red light pulses are produced by photo detector 1, and fed to pulse enhancer 4 and pulse amplitude scaler 5 on a conductor 10. The pulse enhancer 4, operates to increase selectively the temporal width of each pulse such that all pulses presented at the output of the pulse enhancer are provided with a minimum predetermined temporal width. The pulse enhancer 4, can therefore be seen to operate as a short time constant integrator of the pulses received from the amplifier 3. Pulse amplitude scaler 5, also operates on the electrical pulse signals representative of the infra-red pulses detected by the photo detector 1. The pulse amplitude scaler 5, operates to determine a mean amplitude of a plurality of received electrical pulses. The number of pulses selected to determine the mean amplitude may be set in accordance with operating conditions of the optical receiver and serve to provide a relatively long-tenn average of the amplitude of received pulses. The mean pulse amplitude is thereafter scaled by a predetermined factor so that a reference, or detection threshold is generated which may be used to determine whether a pulse has been detected. This detection threshold is presented at the output of the pulse average scaler S as a threshold fed to a second input of comparator 6. The detection threshold is therefore a function of time on a long-term basis in accordance with a number of pulses received. A threshold fed on the second input of the comparator 6 is therefore representative of a decision point for detecting pulses. The comparator 6, receives on a first input the output of the pulse enhancer 4. Therefore, if the pulse enhancer 4, communicates a pulse with the minimum predetermined temporal width and with an amplitude which is greater than that of the threshold, the comparator 6, is arranged to generate an output pulse on conductor 8 which has a predetermined width and amplitude which is sufficient to drive a subsequent stage of electrical equipment. An arrangement of the pulse enhancer 4 in combination with the pulse amplitude averager 5 operate in co-operation to the effect that fast and low amplitude pulses have a substantially equal probability of detection as that of longer and higher amplitude pulses.

A result of this improved probability of detection is that a less sensitive and therefore less expensive comparator 6, may be used to detect the pulses. Furthermore the optical receiver may be arranged to operate with components which have a substantially slower response time and as such a considerably improved economy of power.

An implementation embodying the present invention is illustrated by the circuit diagrams of Figures 2 and 3 where parts also appearing in Figure 1 bear identical numerical designations. In Figure 2 the photo detector 1, is shown to be comprised of a photo diode D1 and a resistor R. The photo diode D1 is arranged to be operated in photo voltaic mode, wherein it is AC coupled to an amplifier and arranged to have a low value or parallel resistance. This arrangement provides a fast response time and substantial immunity to daylight and other radiation without incurring significant quiescent current drain. Also shown in Figure 2, connected to the photo detector 1 is the fast response amplifier 3.

Fast response amplifier 3, is comprised five transistors T1, T2, T3, T4, T5, which are arranged to effect fast amplification of the pulses detected by the photo detector 1, and feed the electrical pulse signal representation of the optical pulses to a conductor 20. The collectors of transistors T1, T2,T3, T4, T5, are connected to a positive supply voltage VCC via resistors R2, R3, R4, R5, whereas the collector of T5 is connected directly to the supply voltage VCC. Transistors T2, T3 and T5 have emitters connected to ground via resistors R6, R7 and R8. Other resistors R9, R10, R11 and capacitors C1, C2, C3, C4 and Cs are arranged to provide biasing and coupling of signals within the amplifier 3.

In Figure 3 a schematic circuit diagram is shown which of the pulse enhancer 4, pulse amplitude scaler 5, and comparator 6. The pulse enhancer 4, is shown to be comprised transistor T6, a collector of which is connected to the positive supply rail VCC and an emitter of which is connected via resistors R12, R13, and capacitor C9 to a ground rail, GND. The emitter of T6 is coupled by C10 and R14 to a negative input of a comparator 22 which forms the comparator 6.

The pulse amplitude scaler 5, is shown to be comprised transistor T7, a collector of which is connected to the positive supply rail Vcc and an emitter of which is connected through resistor R15 and capacitor C6 to ground. The base of T7 and the base of T6 are connected to terminal 24, which in use is connected to terminal 20, and arranged to receive electrical pulse signals from the fast response amplifier 3. A bias circuit for the pulse enhancer and pulse amplitude scaler 26, is shown to be comprised of transistor T8 an emitter of which is connected to the ground rail GND, via resistor R16 and a collector of which is connected through R17 to terminal 24. A positive input of the comparator 22, is connected to an output of the pulse amplitude scaler 5, at a mid point between resistor R15 and capacitor C6 via resistor Rl 8. Resistors R19 and R20 form in combination with the resistor R2 1 a biasing arrangement for the operational amplifier 22, which forms the comparator 6. Also forming part of the circuit in Figure 3 are coupling capacitors C7, C8, C11. The comparator 22, is arranged to generate an output pulse of defined amplitude and temporal width which appears at output terminal 28.

As will be appreciated by those skilled in the art various modifications may be made to the embodiment hereinbefore described without departing from the scope of the present invention.

For example the invention finds application in detection of any form of light pulses'representative of data to be communicated, and may also find application for detecting data conveyed by other types of electromagnetic radiation, such as microve radio signals.

Claims

CLAIMS:

1. An optical data receiver comprising a photo detector which operates to generate electrical pulse signals representative of pulses of light received by said photo detector, a pulse enhancer coupled to said photo detector and arranged to receive said electrical pulse signals and to increase selectively a temporal length of said electrical pulses so that said electrical pulse signals enhanced thereby are provided with a minimum predetermined temporal length, a pulse amplitude scaler which operates to generate a detection threshold in dependence upon the amplitude of a plurality of said electrical pulse signals received from the photo detector and a predetermined scaling threshold, and a comparator coupled on a first input to said pulse enhancer and on a second input to said pulse amplitude scaler and arranged to generate an output signal pulse when said enhanced pulse signals have an amplitude which is greater than said detection threshold.

2. An optical data receiver as claimed in Claim 1, wherein said optical receiver further includes recovery amplifier connected between said photo detector and said pulse enhancer and said pulse amplitude scaler and arranged to amplify the electrical pulse signals from said photo detector.

3. An optical data receiver as claimed in Claim 2, wherein the recovery amplifier is a fast recovery amplifier arranged to recover quickly from a saturation state caused by comparatively high amplitude pulses received from the photo detector.

4. An optical data receiver as claimed in any preceding claim, wherein said photo detector is operated in photo voltaic mode, thereby providing substantial reduction in quiescent current drain.

5. A method of detecting optical data pulses and generating signals representative of these data pulses comprising the steps of, generating signal pulses representative of optical data pulses detected by an optical detector, selectively increasing the duration of the signal pulses so that each signal pulse is of a minimum predetermined temporal length, averaging an amplitude of detected signal pulses so as to provide a mean amplitude, scaling the mean amplitude to generate a detection threshold consequent upon signal pulses to be detected and comparing the threshold with the amplitude of the stretched signal pulses, and generating in accordance with the comparison an output signal representative of detected data pulses.

6. An optical data receiver as hereinbefore described with reference to the accompanying drawings.