GB2261727A - Event marker circuit - Google Patents

Abstract

In a circuit for marking events in an electrical signal (107), the signal is connected to an adder (120) where it is added to an amplified, delayed and inverted replica (108) of itself. The output of the adder (120) is connected to a zero crossing detector (106) to provide a timing marker which is substantially independent of the slope and spectral content of the signal (107). The delay is such that the replica commences when the electrical signal rise is substantially linear (1010). Zener diodes may provide constant reference levels against which the signal crossing events are marked. A modulator may frequency or phase modulate the electrical signal. The zero crossing detector output may be connected to a Schmitt trigger circuit or square wave generator. The event marker circuit may be used in a remote gas detector to detect a shift in the visibility function of interference fringes produced by band-pass filtering light from the gas, splitting it into two paths, and varying the length of one path before recombining the light. <IMAGE>

Description

Electrical Signal Event Marker Circuit The invention relates to circuitry for the detection or marking of electrical signals and in particular, though not exclusively to the detection of gases by remote optical means and in particular to the detection of target gases by measurement of changes in the coherence of the illuminating radiation on interaction with such gases.

Various techniques are used for marking or recording "events" in electrical signals. A commonly used method is the threshold or zero crossing detector. Other methods involve marking the beginning of a pulse, the position of maximum slope or the point of inflection in a received electrical signal. All of these methods are dependent on the harmonic content of the received signal and its amplitude and hence are susceptible to error.

GB Patent Application Number 8406690 describes apparatus and method for remote sensing of gases, vapours or aerosols. The presence of a target gas is detected by measurement of the change in temporal coherence of light which has passed through the gas. Careful selection of wavelength for a particular target gas can maximise the change in temporal coherence. The invention described makes use of transforms in the optical domain prior to detection.This type of detection system differs from Fourier transform (FT) spectroscopy in that: a) Only a small part of the interferogram is measured (or processed) during the process of detection; b) In an example where the absorption spectral profile is rectangular the measurement is made at the region near the first zero of the sinc function only (the visibility/path difference curve reaches zero only if the spectrum is perfectly symmetrical; if it is asymmetric, the function will be complex); c) No Fourier transform has to be calculated (as with FT spectroscopy); d) The signal used to detect the presence of a gas and to discriminate it against a cluttered background has the advantage of being in the complex optical domain; e) The optical system retains high through-put (as it does not contain slits); f) The gas to be detected does not have to fill the field-of-view (as with many other interferometric arrangements); g) The system has a high rejection to background fluctuations; and h) It has a high immunity to both fluctuations of the source and the atmospheric propagation (provided the modulation frequencies employed in the receiver are kept away from the natural background and propagation fluctuations; usually centred at about 400 Hz).

In order to gain the maximum advantage of such optical transform detector systems it is necessary to be able to measure changes due to transmission of radiation through target gases with high accuracy.

Optical processing can be used to sensitise a detector system to the measurements to be made, however, there is also a need for signal processors to accurately characterise or mark a signal in the absence of a target gas and then to mark the signal in the presence of the gas.

Thus there is a need to be able to provide a signal marker circuit whose operation is substantially independent of pulse shape and of the spectral content of the pulse.

The object of the present invention is to provide an electrical signal processing means to provide an event marker characteristic of an electrical signal thereby to enable changes in the signal to be accurately measured.

A secondary object of the invention is to provide signal processing means to determine the change in temporal coherence of an optical signal received by a detector.

The invention provides: an event marker circuit for marking reception of a signal in an electrical circuit comprising: an input to which the electrical signal is connected; an adder having a first input connected to the circuit input to receive the electrical signal, and a second input connected to the circuit input so as to receive an amplified, delayed and inverted replica of the electrical signal; a zero crossing detector connected to the output of the adder so as to provide an output timing marker signal which is substantially independent of the slope and spectral content on the electrical signal.

The order of the processes: amplify, delay and invert may be selected to suit the signal being processed. Throughout the specification references to zero crossing are to be construed to include signal crossings at any preselected level.

In many practical circuits the zero level is not easily defined. In such circumstances it is often easier to maintain constant reference levels, eg voltage as defined by zener diode, than to maintain a zero level from the signal itself. In some arrangements of this circuitry it may be peferable to have a plurality of measurement levels as defined by zener diodes to mark the signal crossings against each of these levels to thereby provide more information on the signal.

Preferably, where the form of the expected electrical signal is known, the replica signal delay is selected such that the delayed replica commences at a time when the electrical signal rise is substantially linear.

In a particular application the invention provides a remote gas detector comprising: a) an optical receiver to receive optical radiation from a field of view; b) means to spectrally pre-process the received optical radiation as for example in a simple case by means of a spectral bandpass filter; c) means to separate the received radiation into two beams and to combine the beams with a path difference therebetween to form a single beam; d) a detector to detect the interference pattern of the combined beams; e) means to vary the optical radiation within the optical receiver to cause the interference pattern to scan past the detector; and f) means to measure the fringes in the interference pattern as the visibility function is scanned and to determine at least one feature of the visibility function which is characteristic of the received radiation; the arrangement being such that the measuring means is provided with electronic signal processing means to determine the presence of a gas from a shift in the said characteristic feature of the visibility function.

Preferably the event marker circuit is used to mark the zero crossing points of the intensity signal measured by the detector as the visibility function is scanned. The zero crossing points may then be used to generate a square wave.

In one arrangement the scanning limits are arranged such that the fringe visibility function is repeatedly scanned around the position of the null between the first and second lobes of the fringe visibility function.

The output from the square wave generator may be connected to a counter circuit arranged such that the respective numbers of counts from the two extremities of scanning are used to determine the position of the null within the scan cycle. Advantageously reference signals are available to indicate the limits of scanning.

In the preferred arrangement the path difference between the two beams in the inteferometer is scanned cyclically between two predetermined limits to thereby cause the fringe visibility function to scan past the detector.

The path difference scanning may be done by cyclically moving a mirror in the interferometer. The mirror may be moved by means of a piezo-electric transducer.

The electrical circuit may conveniently be provided with an input limited to reduce sensitivity of the circuit to signal amplitude variation.

The invention will now be described by way of example only with particular reference to a remote gas detector arrangement including an electrical signal processor and with reference to the accompanying Drawings of which: Figure 1 illustrates a passive remote gas detector arrangement; Figure 2 illustrates the effect of bandlimiting at the detector; Figure 3 illustrates the effect of transmission through an absorbing gas on the spectrum illuminating the detector and the spectrum entering the detector; Figures 4a and b illustrates the effects on the received spectrum after transmission respectively through a gas adding emission light to the source and through a gas having absorption and emission effects on the source spectrum;; Figure 5 illustrates the effect on the temporal coherence profile of the light within the detector of varying the spectral overlap of a gas absorption feature on the detector passband; Figures 6 a-d graphically illustrate the optical field and the detected electrical signal within the detector; Figure 7 shows graphically the change of phase of the electrical signal in passing through the nulls in the sinc function shown in Figure 5b; Figure 8 is a graph of experimental results showing the detector output signal against optical path difference in the arms of an interferometer through which the light passes before detection; Figure 9 is a schematic diagram of the interferometer used in the detection system which produced the Figure 8 results; Figure 10a shows the electrical signal processing circuitry used to mark the zero crossing points of the signal;; Figure lOb illustrates the signal waveforms before and after processing by the event marker circuit; Figure lOc graphically illustrates operation of the Figure lOa circuit Figure 11 illustrates a first order proof of the independence of the Figure lOa zero point on the slope of the signal being processed; Figure 12 shows a block diagram of a circuit for determining the position of the null in the temporal coherence profile; Figure 13 shows the output signal from the square wave generator of Figure 12; Figure 14 graphically illustrates the operation of the square wave counter for determining the null position; Figure 15 illustrates a modification of the detector system for simultaneous determination of the first and second nulls of the temporal coherence profile sinc function; and Figure 16 is a functinal block diagram illustrating the application of an event marker circuit to signals from a remote gas detector.

GB Patent Application No 8406690 describes a remote gas detection system which operates as indicated in Figure 1. Light from a source, which may range from a broadband source S such as the sun 10 (passive system) to a narrowband source such as a laser (active system), illuminates a target gas G and radiation from the cloud G is then detected by the optical interferometer receiver R.

The presence of the target gas is detected by observation of the change in temporal coherence in the received light resulting from the presence of the gas G. In an example where the source S has a broadband spectrum 20 (Figure 2a) a narrowband spectral filter is provided in the receiver R with the passband 21 of the filter being selected to optimise the gas detection (Figure 2b).

The effect of bandlimiting is illustrated in Figure 3: if the gas G has an absorption line 30 (Figure 3a) centred at wavelength x3, then the spectral profile seen at the receiver will be as shown in Figure 3b when bandlimited to the range A1 - A2 where > 1 < A3 < X,. The change in spectral profile between Figure 2b and Figure 3b is measured as a function of the change in temporal coherence. In addition, temporal coherence changes can be detected where an emission line 40 characteristic of the gas G is added (Figure 4a) or where changes are due to a more complex combination of gas emission 41 and absorption (Figure 4b).

The effect of changes in the input spectrum to an interferometer are illustrated in Figure 5. In the interferometer received light is split into two beams and then recombined with a path difference L between the beams. In the present arrangement the path difference L can be scanned.

In the ideal case where the intensity 50 is constant over the spectral bandwidth > 2 (Figure 5a) then the degree of coherence plotted against path difference L (the temporal coherence profile) in the interferometer is a sinc function 51 (Figure 5b). This arises because of the Fourier transform relationship between the temporal coherence profile and the rectangular spectrum. Within the sinc function envelope is a cosine function. If the light passes through a gas having an absorption feature 52 which is symmetrical and centred midway between A1 and /\2 at %a' then the temporal coherence profile will shift to the position of the dotted curve 53.If the pass band is now shifted in wavelength such that the absorption feature is no longer centred within the passband (curve 54 - symmetrical about Ab) then the resulting temporal coherence profile 55 is again shifted with respect to the no gas profile 51 but is now complex and has a first minimum 56 which does not reach zero. Thus by detecting the change in the temporal coherence profile a target gas can be detected. A detection technique is required which is: a) simple b) sensitive c) free from interference and distortion.

Thus the aim is to obtain from the temporal coherence profile a fiducial event marker, the shift of which is caused exclusively by the effect of absorption (or emission) by the target gas on the Fourier transform of the input spectrum.

Figure 6 illustrates the electronic signal which is derived from an interferometer as the path difference L between the two interfering beams is varied. Where mirrors are included in the respective paths of the two beams then the path difference is conveniently effected by moving or scanning one of the mirrors. As the moving mirror is scanned, a sinusoidal output function 60 is produced by alternate cancellation and reinforcement of the interfering beams at the output observation plane. The normalised amplitude 61 of this envelope traces the visibility function (which is a measure of the temporal coherence of the optical signal) against path difference within the interferometer.

Figure 6a shows the complex optical signal 60 which on detection (rectification) gives the signal 62 shown in Figure 6b. After the process of detection, the inversion of the negative-going half-cycle 63 of the sinc function causes an abrupt phase change in the sinusoidal signal. This is illustrated by inversion of the negative cycle in Figure 6c which produces a phase change of it at the zero point 64 in Figure 6d. Any small change in the Fourier components of the - input optical spectrum, for example as shown in Figure 5a, will cause the signal envelope 65 to shift its position relative to the path difference axis 66 and so an electronic phase measurement can be used to detect this change. Processing need not depend on small phase changes which might be concentration dependent.In null processing the measurement will ultimately depend upon employing an electronic technique to define the cross-over event point 64, taking into consideration the whole phase curve together with an integration of a very large number of scans of the curve. For example, if the interferometer mirror is scanned at 1KHz then the curve would be traced at 2000 lines per second of observation time.

In most practical cases the absorption due to a target gas (or even the band-limited solar emission, when used) will not have perfect symmetry and as a consequence the temporal coherence or visibility function will not be solely real, but will be complex. Then the temporal coherence profile will not reach zero, as shown by the curve 55 in Figure 5b.. The phase change against path difference will then be as illustrated by Figure 7. The full line 70 is for the case where the visibility curve (51) is real and the envelope reaches zero, leading to an abrupt It change of phase. If the absorption is symmetrically placed within the window filter (as shown by curve 52 of Figure 5a) then the position of the abrupt phase change will move to the path difference position 71 from 72 (with no absorption) as illustrated by the dashed curve 73.In practice the spectral profile is unlikely to be symmetric and in consequence the phase curve will have a smooth transition as shown by curve 74 (corresponding to curve 55).

Figure 8 shows a graph of experimental results plotting the detector output (a function of the visibility of the interferometer fringes) against path difference. Curve 80 shows the normalised detector output without a gas present and curve 81 shows the same normalised output with a target gas present. As can be seen, the null point shifts approximately 3pm along the path difference axis from point 82 to point 83 when the gas is present in the field of view. The change in phase of the signals at the two null points can also be seen in the graph.

Figures 9 and 10 show in simplified form an interferometer arrangement which is used in the present invention wherein an input beam of light 90 is split or divided to form two beams 91 and 92, the two beams then being recombined with a path difference L between them to produce a single output beam 93 incident on detector 94. The path difference L is cyclically scanned as by varying the position of a mirror used to fold the optical path of the beam 92.

The output 95 from the detector 94 is connected to the input 100 of a zero crossing event marker circuit shown in Figure 10a. A signal at the input 100 is connected to one input 101 of an adder 102 where it is added to a replica of the signal after delay (103), amplification (104) and inversion (105). The effect of this addition is shown in Figure lOb where a signal 107 is added to a delayed replica 108 after inversion and amplification. The resultant output signal 109 from the adder 102 has a cross-over point P which can be shown to be substantially independent of the amplitude, slope and general shape of the signal 107. The output from the adder 102 is connected to the input of a circuit 106 which produces an output event marker at the cross-over point P.

The electronic delay 103, as far as possible, should be free from dispersion and the delay setting is selected to fall within a reasonably straight portion of the rise 1010 of the pulse. In this application, the waveform being processed from the interferometer is a simple near-sinusoidal wave and thus event point markers can be created with sufficient accuracy to determine the phase reversal point with an accuracy well within a small percentage of the period of the wave.

The order of the circuit processes: delay, multiply and invert can be changed. In some circumstances the preferred order may depend upon the shape of the signal being processed. Thus, for example, it would be probable be better to delay and invert a signal with small dynamic range before amplification (by factor n) as this would help to limit dispersion in the electronics.

In many applications the "zero" line is difficult to define The running zero can only be found from that part of the signal that has already occurred. If a large positive excursion of the signal occurs then the perceived zero will move upwards. Thus the instrumental signal will undulate and in consequence the crossing event markers will be perturbed. It would be possible to store the signal (eg digitally) and then calculate the true zero line before determining the event markers.

This adds to the circuit complexity and expense. This difficulty can be mitigated by employing event marker circuits operating at different levels defined by zener diodes, for example. Multiple slicing of the signal in this manner can thus be used to increase the accuracy of the event marking process as well as providing a larger number of events for a given signal.

Figure lOc is a graphical representation of the raw signal 1011 connected to the input of the event marker circuit 1012, the ouput signal 1013 from the zero crossing detector 106 marking events 1014 and the output square wave pulse signal 1015 after connecting the signal 1013 to a Scmitt trigger circuit 1016.

Figure 11 illustrates a linear portion y1 of a rising pulse passing through the origin of an x,y coordinate system.

The equation for this portion of the pulse is given by: y1 = mx (1) where m is the slope.

The inverted, amplified replica signal y2 starts from the x axis after a delay D with a slope -nm (where n > 1).

The equation for the replica y2 is given by: y2 = -nmx + c (2) where + c is the intercept on the y axis.

Substituting y2 = 0 when x = D in (2) gives: c = nmD (3) Substituting (3) into (2) then gives y2 = -nmx +nmD (4) The output from adder 102 of Figure 10 is the sum of y1 and y2 thus: output = y1 + y2 = -mx (n-l) + nmD (5) The cross-over point P occurs at x = xO when this sum is zero, thus: 0 = -mxO(n-l) +nmD (6) Hence: xo = n D (7) n-l Thus xO is dependent on the ratio of the slopes n and upon the delay D but is independent of the slope m of the signal y. In practice this applies to all the harmonics of the signal.

The output signal from the zero crossing event marker 106 may be arranged as shown in Figure 12 with the processed signal as described above being connected to the input 120 of a square wave generator 121. The square wave generator is arranged such that its output 122 switches between +V1 and -V1 at respective times P associated with - to + zero crossing points 123 and between +V1 and -V1 for + to - zero crossing points 124. The interferometer is arranged such that the path difference L is scanned around the position of the null 64 between the first and second lobes of the sinc function. A laser may be used within the interferometer for generating a calibration reference signal for the mirror movements, defining the scanned range of path difference L.

Figure 13 illustrates the square wave generator output signal 130 as the sinc fringe visibility function passes through the null. Since in some cases the signal may be very small in the null region, as shown in Figure 8, the square wave signal may be unreliable near the null and thus there would be a region of uncertainty 131. This uncertainty can be resolved by connecting the output from the square wave generator 121 (Figure 12) to a counter 124. The counter 124 counts the number of pulses from the respective limits of scanning of the path difference.

These two counts 140,141 can then be extrapolated on the count-time graph (Figure 14) through the region of uncertainty to give the phase change point (first null in sinc function 142) and hence the path difference can be determined for this fiducial null point.

Presence of a target gas in the field of view of the sensor will result in movement of this fiducial point and this will be measurable by this technique. If the periodicity of the zero crossings of the square waves changes then this technique can still be used to define the fiducial point. Clearly, when it is required to attain the highest sensitivity with the smallest amount of absorption, it will be necessary to employ these interpolation techniques.

Generally, the event point marking technique can be used anywhere within an electronic system; anywhere that requires a decision to be made requiring marking of an event. It can also be used in several places in one system. One effective arrangement is to connect the event marker output to a Schmitt trigger circuit for producing square wave signals corresponding to the event marking pulses. The time position of these pulses can then be determined by counter systems as described above or by known analogue techniques.

The effect of input noise which may cause time jitter in the square waves can be reduced by firstly the input can be provided with a limiter or a ratio detector (or both) so that the amplitude variations of the input signal produce no output; and; secondly, by employing a frequency deviation sufficiently large that for full modulation the frequency deviation is large compared with unity.

If the electronic signals shown in Figures 8 are passed through a high pass filter, the d.c. component can be removed so that the signal output from the interferometer oscillates about a mean of zero volts.

This signal can then be used to modulate an f.m. carrier of considerably higher frequency, processed and demodulated. This frequency modulation can be used within the sensor electronics to give protection against internally generated noise. However, this has the disadvantage that the signal passes through a very small value which can result in there being little protection against noise. It might be satisfactory, however, near zero, but by electronically extrapolating forwards from the first portion of the signal and backwards from the second part of the signal so that an event is reliably marked which does not depend upon a measurement at the smallest values of the actual signal.

A more appropriate method would be to use the output signal directly as described above, so that the deviation of the f.m. signal does not reach zero, but maintains a value of iX f appropriate to the d.c.

off-set at the point of the phase change. The points of average deviation then mark all the zero-crossings, and themselves indicate the position of the change-of-phase. An additional technique suitable for processing this output signal can be to convert this sequence into a square-wave signal from the event point markers, and then the position of the visibility curve can be defined by the major event-point, where the phase-reversal is considered to be centred. This technique employing interpolation from all the zero crossing points can then give an accurate value for the null position which cannot otherwise be measured easily or accurately in most situations because of the small signals near this point.

In one arrangement, a piezo-electric transducer has been used to move a single mirror in order to vary the path-difference within the interferometer. It is also possible to use a single detector with a stepped-mirror arrangement so that a number of features (e.g. minima) from a single absorption (and/or emission) can be combined onto one detector for the electronic processing of the composite waveform. For example, Figure 15 sketches a sinc function visibility curve 150 (equivalent to Figure 6b) where two (or more) zeros 151 and 152 are measurable. Both zero points could be observed simultaneously by employing a stepped mirror with two suitable displacements, as indicated by the line 153.This two-detector arrangement could be used to ensure that the two features used for the identification and discrimination of the target gas are verified as occurring at the correct and expected displacements. Or, if a single detector were used, that the two features produced the expected phase-time profile associated with the combination of the phase features.

As described in the above arrangements the electrical signal from the detector of the remote gas sensor is applied to the zero crossing detector such that event markers in this signal are used to determine the presence of a target gas. In some circumstances there may be advantage in using frequency modulation (FM) or phase modulation (PM) of the signal. This may be done before connection to the event or zero marker circuit or alternatively elements of the phase or frequency modulation circuits may be included within the event marker circuit.

The presence of target gases would again be indicated by movement of such markers. A combination of FM and PM could also be beneficial.

Alternatively or additionally there may be advantage in connecting the output from the event marker circuit to an FM or PM circuit and then to an output event marker circuit. Such arrangements being tailored to optimise operation of the circuit to enhance sensitivity and selectivity to the required gas detection.

Figure 16 shows diagrammatically how raw signal information may be processed to provide information on the change of signal position as required in the remote gas sensor. The raw signal 160 is connected to an event point marker circuit 161 either directly (162) or after processing: by a frequency modulation circuit 163; by a phase modulation circuit 164; or by a combined FM and PM processing circuit 165. The output from the event marker citcuit 161 may then be connected via a Schmitt trigger 166 to a circuit 167 arranged to sense change in position of the raw signal on detection of a target gas and to produce an appropriate output.

Conventional correlation techniques can be incorporated where the reference waveform can be derived from a reference laser integral with the optical system or from some accurate frequency generator.

In many practical circuits the zero level is not easily defined. In such circumstances it is often easier to maintain constant reference levels, eg voltage as defined by zener diode, than to maintain a zero level from the signal itself. Furthermore in such arrangements of this circuitry it by using a plurality of measurement levels as defined by zener diodes to mark the signal crossings against each of these levels it is possible to obtain more information on the signal.

Claims (17)

Claims

1. An event marker circuit for marking events in a signal in an electrical circuit comprising: an input to which the electrical signal is connected; an adder having a first input connected to the circuit input to receive the electrical signal, and a second input connected to the circuit input so as to receive an amplified, delayed and inverted replica of the electrical signal; a zero crossing detector connected to the output of the adder so as to provide an output timing marker signal which is substantially independent of the slope and spectral content on the electrical signal.

2. An event marker circuit as claimed in claim 1 wherein the replica signal delay is selected such that the delayed replica commences at a time when the electrical signal rise is substantially linear.

3. An event marker circuit as claimed in claim 1 or 2 wherein a zener diode is included to provide a constant reference signal level against which the electrical signal crossing events are marked.

4. An event marker circuit as claimed in claim 3 wherein a plurality of reference signal levels is provided by respective zener diodes and sets of event markers are produced with reference to each of the reference signal levels.

5. An event marker circuit as claimed in any one preceding claim wherein a frequency modulator is provided to frequency modulate the electrical signal.

6. An event marker circuit as claimed in any one preceding claim wherein a phase modulator is provided to phase modulate the electrical signal.

7. An event marker circuit as claimed in any one preceding claim wherein the output signal from the zero crossing detector is connected to a Schmitt trigger circuit.

8. An event marker circuit as claimed or used in any preceding claim wherein the zero crossing points are used to generate a square wave.

9. A remote gas detector comprising: a) an optical receiver to receive optical radiation from a field of view; b) means to spectrally pre-process the received optical radiation as for example in a simple case by means of a spectral bandpass filter; c) means to separate the received radiation into two beams and to combine the beams with a path difference therebetween to form a single beam; d) a detector to detect the interference pattern of the combined beams; e) means to vary the optical radiation within the optical receiver to cause the interference pattern to scan past the detector; and f) means to measure the fringes in the interference pattern as the visibility function is scanned and to determine at least one feature of the visibility function which is characteristic of the received radiation; the arrangement being such that the measuring means is provided with electronic signal processing means including an exent marker as claimed in any one of claims 1 to 8 to determine the presence of a gas from a shift in the said characteristic feature of the visibility function.

10. A remote gas detector as claimed in claim 9 wherein the event marker circuit is used to mark the zero crossing points of the intensity signal measured by the detector as the visibility function is scanned.

11. A remote gas detector as claimed in claim 9 or 10 wherein the scanning limits are arranged such that the fringe visibility function is repeatedly scanned around the position of the null between the first and second lobes of the fringe visibility function.

12. A remote gas detector as claimed in claim 9 or 10 when modified as in claim 11 wherein the output from the square wave generator is connected to a counter circuit arranged such that the respective numbers of counts from the two extremities of scanning are used to determine the position of the null within the scan cycle.

13. A remote gas detector as claimed in claim 12 wherein reference signals are available to indicate the limits of scanning.

14. A remote gas detector as claimed in claim 13 wherein the path difference between the two beams in the inteferometer is scanned cyclically between two predetermined limits to thereby cause the fringe visibility function to scan past the detector.

15. A remote gas detector as claimed in claim 14 wherein the path difference scanning is done by cyclically moving a mirror in the interferometer.

16. A remote gas detector as claimed in claim 15 wherein the mirror is moved by means of a piezo-electric transducer.

17. A remote gas detector or an event marker circuit as claimed in any one preceding claim wherein the electrical circuit is provided with an input limiter to reduce sensitivity of the circuit to signal amplitude variation.