GB2493742A - A pulse stretcher for nuclear pulse measurements, using photo-coupler response time - Google Patents

Abstract

An LED D1 is biased near its knee voltage so that a negative input voltage pulse Vin causes radiant energy to be emitted to a Silicon Carbide photodiode D2. A following positive overshoot pulse (4, figure 1) has no optical effect and so does not cancel the effect of the negative pulse. The relatively long fall time of the photodiodes response to the radiant energy provides a current output pulse which is longer than the input voltage pulse Vin. Alternatively, the photodiode may be operated in photovoltaic mode (figure 5), or a phototransistor or photomultiplier may be used. The photodiode output pulse may be further extended by a charge amplifier 10 comprising an operational amplifier with resistor-capacitor feedback (figure 3). The stretched pulses may be processed using slower and simpler components. Amplitude information is not lost. The input pulses may be from proton or neutron detectors.

Description

PULSE PROCESSING

FIELD OF THE INVENTION

The present invention relates to the processing of electrical pulses. The present invention is particularly suitable for, but is not limited to, processing pulses of very narrow width, for example pulses of pulse width/decay time less than 10 nanoseconds.

BACKGROUND

Relatively narrow pulses, for example pulses of pulse width/decay time less than 10 nanoseconds, and even more so pulses of pulse width/decay time less than 5 nanoseconds, are difficult to process, for example detect or measure, at least with conventional routine electronic components.

Such pulses that it is desired to process may be produced in various applications, for example when attempting to detect radiation, for example high energy neutrons.

Such pulses may have an overshoot characteristic. Figure 1 is a schematic illustration in simplified torm of a pulse with such an overshoot characteristic. In the example of Figure 1, the pulse to be processed 2 is a negative pulse of amplitude -lBOmv and has a decay time/pulse width of 2 nanoseconds, but immediately thereafter has a positive overshoot 4.

It is known to provide arrangements for "stretching" such pulses, i.e. extending the decay time/widening the pulse width of the pulse, so that the "stretched" pulse can then be processed using lower speed, simpler and more routine components.

Various types of conventional pulse stretchers are known. One example is passive diode pulse stretchers based on a diode and capacitor arrangement.

Disadvantages of this type are that the diode characteristics are non-linear and a voltage drop across the diode junction leads to a lack of response at low signal levels. Another disadvantage is that the charging time of the capacitor means the pulse amplitude is not retained. Another example is active diode pulse stretchers in which an operational amplifier is used to improve the performance of the above described passive diode pulse stretcher type. The diode is incorporated in a feedback loop of the operational amplifier, thereby reducing the effect of the voltage drop. Nevertheless, the output remains non-linear due to the characteristics of the diode, and the operational amplifier is required to be wideband with a high slew rate. Also, electromagnetic interference tends to be generated due to the fast switching of the operational amplifier. It is noted that all these conventional types of pulse stretcher are provided entirely by electronic components.

SUMMARY OF THE INVENTION

The present inventors have invented apparatus and methods for stretching very narrow width electronic pulses, which apparatus and methods are surprisingly based on opto-electronic components. In view of the limitations of conventional apparatus and methods for stretching very narrow pulse width pulses, the newly invented opto-electronic based apparatus and methods are particularly useful when applied to pulses of pulse width less than or equal to nanoseconds, and even more so when applied to pulses of pulse width less than or equal to 5 nanoseconds.

In a first aspect, the present invention provides an apparatus for stretching a narrow pulse width electrical pulse; the apparatus comprising: a photo-emitter; and a photo-detector; the photo-emitter arranged for being driven by an input electrical pulse to emit an electromagnetic radiation pulse; the photo-detector optically coupled to the photo-emitter for receiving the electromagnetic radiation pulse and to produce a corresponding output electrical pulse.

The apparatus may stretch the input electrical pulse by virtue of the output response time of the photo-detector being greater than the pulse width of the input electrical pulse such that the output electrical pulse is correspondingly of greater pulse width than the input electrical pulse.

The apparatus may further comprise a charge sensitive amplifier arranged to receive the output electrical pulse in the form of a current pulse, and the apparatus may stretch, or further stretch, the input electrical pulse by virtue of the charge sensitive amplifier stretching the input current pulse, the apparatus thereby providing an output electrical pulse of greater pulse width than the input electrical pulse.

The input electrical pulse may be of pulse width less than or equal to 10 nanoseconds.

The input electrical pulse may be of pulse width less than or equal to 5 nanoseconds.

The apparatus may be arranged to receive the input electrical pulse form a radiation detector.

The photo-emitter may be biased to be at its point of photo-emission.

The photo-detector may be biased to be at its point of photo-response.

The photo-emitter may be a light emitting diode.

The photo-detector may be a photodiode.

In a further aspect, the present invention provides a method of stretching a narrow pulse width electrical pulse; the method comprising: driving a photo-emitter with an input electrical pulse; the photo-emitter emitting an electromagnetic radiation pulse; and an optically coupled photo-detector receiving the electromagnetic radiation pulse and providing a corresponding output electrical pulse.

The method may be applied to an input electrical pulse that has an overshoot, the method thereby removing the overshoot.

The method may be applied such as to preserve the amplitude of the pulse.

The input electrical pulse may be of pulse width less than or equal to 10 nanoseconds.

The input electrical pulse may be of pulse width less than or equal to 5 nanoseconds.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration in simplified form of a pulse with an overshoot characteristic; Figure 2 is a circuit diagram of a first embodiment of a pulse stretcher; Figure 3 is a circuit diagram showing further details of a charge sensitive amplifier of the circuit of Figure 2; Figure 4 is a circuit diagram of a further embodiment of a pulse stretcher; and Figure 5 is a circuit diagram of a further embodiment of a pulse stretcher.

DETAILED DESCRIPTION

Figure 2 is a circuit diagram of a first embodiment of a pulse stretcher 6.

The pulse stretcher 6 is a negative pulse stretcher, i.e. it is suitable for stretching a negative pulse such as the pulse 2 shown by way of example in Figure 1.

The pulse stretcher 6 comprises an optoelectronic module 8. The optoelectronic module 8 comprises a light emitting diode (LED) Dl that is optically coupled to a photodiode D2, and a housing in which the LED Dl and the photodiode D2 are housed. The housing comprises a case that protects the LED Dl and the photodiode D2 from external radiation over an appropriate wavelength range compared to the wavelength range of operation of the LED Dl and the photodiode D2. In this embodiment the casing provides protection across the wavelength range of 10 nm to 10 pm.

In this embodiment ground screening is provided to the LED Dl, however this is optional, and in other embodiments, for example, ground screening can be applied to the entire opto-electronic module 8.

The LED Dl and the photodiode D2 are selected such that the spectral output of the LED Dl is matched to the spectral response of the photodiode D2.

The pulse stretcher 6 further comprises connection points for application of a first bias voltage Vbiasl and a second bias voltage Vbias2.

The first bias voltage Vbiasl is appUed to the LED Dl to bias the LED Dl to the point of photo-emission in the absence of any input pulse voltage. In this embodiment the first bias voltage Vbiasl is approximately +1.5 V. The second bias voltage Vbias2 is applied to the photodiode D2 to set the photodiode D2 up in reverse bias mode. This provides firstly that the photodiode is not conducting unless incident radiation from the photodiode Dl is falling on the photodiode, and, secondly, that when incident radiation from the photodiode Dl does fall on the photodiode, the output current from the photodiode D2 is substantially or approximately directly proportional to the incident radiation from the photodiode Dl falling on the photodiode. In this embodiment the second bias voltage Vbias2 is approximately +10 V. In this embodiment, the pulse stretcher 6 further comprises decoupling capacitors in relation to the provision of the bias voltages, namely capacitors Cl and 02 in relation to the first bias voltage Vbiasl, and capacitors C3 and C4 in relation to the second bias voltage Vbias2. In this embodiment the capacitors have the following capacitance values: Cl = 1 nE; C2 = 100 nF; C3 = 100 nE; and C4 = 1 nF. These capacitors operate to reduce noise at the bias points.

The pulse stretcher 6 further comprises an input connection point for input of the electronic pulse to be stretched. This is in the form of an input voltage yin.

In this embodiment the pulse stretcher 6 further comprises a resistor Ri connected between Vin and the LED Dl. The resistor Ri provides impedance matching between the LED Dl and whatever is providing the incoming pulse. In this embodiment the pulse source is a 50 ohms source, and hence the resistance value of the resistor Ri is 50 ohms. In other embodiments other impedance matching arrangements may be provided. In other embodiments, any errors from lack of impedance matching may be acceptable, and/or depending on the nature of the pulse source, there may be no need for or benefit from an impedance matching arrangement, and accordingly the resistor Ri or other impedance matching arrangement may be omitted.

The pulse stretcher 6 further comprises a charge sensitive amplifier (CSA) 10, which is connected to the output of the photodiode D2. Figure 3 is a circuit diagram showing further details of the CSA 10.

The CSA 10 comprises an operational amplifier Ui, which in this embodiment is arranged with DC supply voltages of ±12V. The output of the photodiode D2 is connected to the inverting input of the operational amplifier Ui.

The CSA 10 further comprises a resistor-capacitor arrangement connected between ground and the non-inverting input of the operational amplifier Ui. In this embodiment this resistor-capacitor arrangement comprises a resistor R2 (of resistance value 10 mega ohms) in parallel with a capacitor C5 (of capacitance value 10 nF).

The GSA 10 further comprises a further resistor-capacitor arrangement connected between the inverting input of the operational amplifier Ui and the output of the operational amplifier Ui, thereby providing a feedback loop. In this embodiment this resistor-capacitor arrangement comprises a resistor R3 (of resistance value 10 mega ohms) in parallel with a capacitor C6 (of capacitance value 2.2 pF).

The GSA 10 further comprises a resistor R4 (of resistance value 20 ohms) connected between the output of the operational amplifier Ui and an output Vout of the GSA 10.

In operation of the pulse stretcher 6, when a pulse such as the pulse 2 is input at yin, the pulse 2 drives the LED Di, and the LED Di emits a corresponding pulse of emitted electromagnetic radiation. This falls on the photodiode D2, and the photodiode D2 accordingly outputs a resulting current pulse (i.e. charge pulse).

The output current pulse is stretched, i.e. has an extended decay time/wider pulse width than the original input pulse, by virtue of the physical nature of the photodiode D2. In particular, the rise time of the photodiode D2 is very fast to approximately match the input pulse, whereas the fall time of the photodiode D2 is relatively slow compared to that of the input pulse, thus stretching the pulse. In this embodiment, the LED Dl is a gallium arsenide LED with a sub nanosecond, e.g. 100 picoseconds, rate of rise time (another possibility, for example, is a gallium phosphide LED). In this embodiment, the photodiode D2 is a silicon carbide photodiode with an approximate fall time of 20 nanoseconds.

The amplitude is approximately retained since, firstly, the LED Dl is biased (in this embodiment) to the point of photo-emission, and secondly, there are only relatively small or negligible energy losses between the LED Dl and the photodiode D2. (In other embodiments, the LED Dl is only partially or not at all biased to the point of photo-emission, which will tend to lose the amplitude retention, but which embodiments will nevertheless still tend to detect the presence of the pulse (or aid detection of the pulse), for example, which will be sufficient in certain applications.) Any overshoot (for example the overshoot 4 shown in Figure 1) present in the original signal will tend to be removed by the LED Dl.

Additionally, in this embodiment, by virtue of the current pulse output by the photodiode D2 being fed in to the CSA 10, the pulse is further stretched, as follows. If a negative voltage is applied to the GSA 10 (i.e. to the inverting input of the operational amplifier Ui), the operational amplifier Ui produces a positive output in an attempt to produce the changing voltage across the capacitor C6 that is necessary to maintain the current established by the voltage difference across the resistor R3. The capacitor G6 then discharges over time to further stretch the pulse.

In this embodiment the output of the operational amplifier is provided as an output of the GSA 10 (and hence of the pulse stretcher 6) via impedance matching resistor R4 that is between the output of the operational amplifier Ui and the Vout output i.e. the resistor R4 provides impedance matching between the output of the operational amplifier Ui and whatever the output is being input to. In this embodiment the resistance value of the resistor R4 is 20 ohms. In other embodiments other impedance matching arrangements may be provided.

In other embodiments, any errors from lack of impedance matching may be acceptable, and/or depending on the nature of the output destination, there may be no need for or benefit from an impedance matching arrangement, and accordingly the resistor R4 or other impedance matching arrangement may be omitted.

As mentioned above, any overshoot (for example the overshoot 4 shown in Figure 1) present in the original signal tends to be removed by the LED Dl.

Therefore the integrator arrangement of the CSA 10 will tend not to fall down due to overshoot/ringing.

Indeed, although a CSA 10 is known in the art as such, conventionally it Is is not considered usable with pulses with overshoot, since conventionally the overshoot of the incoming pulse will cancel out the effect of the negative signal, thereby severely limiting the response. Thus it will also be appreciated that one advantage of the pulse stretcher 6 is that it allows a conventional CSA to be used for further stretching the pulse, by virtue of the operation of the pulse stretcher 6 removing entirely, or at least substantially, e.g. the majority of, the overshoot. Indeed, it will be further appreciated that accordingly embodiments of the overall pulse stretcher 6 using a photodiode D2 with a relatively fast fall time may be provided primarily for the purpose of allowing the output of the photodiode D2 to be in a form able to be processed by the GSA part of the pulse stretcher 6, even when or if the fall time of the photodiode D2 is so fast that at the output of the photodiode the pulse has not yet been stretched as such (i.e. all the stretching will then take place in the GSA 10). As such, it will be appreciated that the pulse processing apparatus provided by the components of the pulse stretcher 6 but excluding the CSA 10 represents in itself an advantageous embodiment of a pulse processor/pulse processing apparatus.

Figure 4 is a circuit diagram of a further embodiment of a pulse stretcher 6. In this embodiment the pulse stretcher 6 is a positive pulse stretcher, i.e. it is suitable for stretching a positive pulse, i.e. a pulse that has positive voltage value, and for which any overshoot would have negative voltage value. Except where stated otherwise below, the negative pulse stretcher 6 of this embodiment has the same components arranged in the same way, and operates the same, as the positive pulse stretcher 6 of the above described embodiments as described with reference to Figures 2 and 3. Also, the same reference numerals are used for the same items.

The difference is that the LED Dl is reversed and the Vbiasl polarity is changed.

The above embodiments include the charge sensitive amplifier.

However, this need not be the case, and in other embodiments, other types of circuit may be provided for being fed the current/charge output from the photodiode D2.

In yet further embodiments, the CSA 10 may be omitted and instead the remainder of the circuit arrangement of Figure 2 (and equally of Figure 4) may be modified so as to provide a voltage output rather than a current/charge output. One such embodiment relating to the circuit arrangement of Figure 2 (i.e. the negative pulse version) will now be described with reference to Figure 5, however it is to be appreciated that the same modification can be made to the circuit arrangement of Figure 4 to provide a corresponding positive pulse version.

Accordingly, Figure 5 is a circuit diagram of a further embodiment of a pulse stretcher 6 and, except where stated otherwise below, the pulse stretcher 6 of this embodiment has the same components arranged in the same way, and operates the same, as the pulse stretchers 6 of the above described embodiments as described with reference to Figures 2, and 4. Also, the same reference numerals are used for the same items.

In this embodiment, the CSA 10 is omitted, and the photodiode D2 is operated in photovoltaic mode, i.e. in an unbiased mode. Accordingly, the second bias voltage Vbias2 is also omitted in this embodiment, and therefore also the decoupling capacitors D3 and 04 are omitted. The anode side of the photodiode D2 is grounded, and the cathode side provides the output Vout of the pulse stretcher 6. This embodiment has the advantage of requiring a simpler circuit by omitting the CSA; however it does suffer the disadvantage that since the photodiode D2 is operated in an unbiased mode, it is not "primed" to activate as quickly as in the earlier embodiments.

The values of the specific circuit components in the above described embodiments are not essential, and in other embodiments any one or more of these values may be other than those described above.

In the above described embodiments, the photo-emitter is an LED, and the photo-detector is a photodiode. However, neither of these two specific choices, although preferred, is essential, and in other embodiments either or both the photo-emitter and the photo-detector may be implemented by items other than an LED and a photodiode respectively. For example, in some embodiments, the photo-emitter may be a semiconductor laser diode instead of Is an LED. Because a semiconductor laser diode tends not to be as linear as an LED, measurement or replication of the amplitude of the pulse may not be so accurate, however the stretched pulse will still be detected. Further for example, in some embodiments, the photo-detector may be a phototransistor or a photomultiplier tube, instead of a photodiode.

Also, apart from the inclusion of some form of a photo-emitter and some form of a photo-detector, none of the specific circuit components are essential, and in other embodiments one or more may be omitted, with their functionality thereby omitted, or replaced by other components performing the same or similar or alternative functionality.

Also, in further embodiments, different or additional circuit components and arrangements may be used in addition to or instead of the components used in the above described embodiments. For example, different biasing arrangements may be implemented. Another example is that components for temperature compensation, e.g. temperature compensation of the bias levels, may be included.

As mentioned previously, the above embodiments may be used for the purpose of increasing the time available to detect the presence of a pulse, even when the embodiments are such that measurement of the amplitude is rendered less accurate or even unavailable.

The above described embodiments of pulse stretchers may be used in many applications. One possibility is as an interlace between a detector and monitoring equipment. The detector may, for example, be a radiation detector, for example a neutron detector or a proton detector. Another possibility is for the pulse stretcher to be used as a sample and hold device for analogue to digital conversion.

Another possibility is that the output of the above described embodiments may itself then be fed in to a conventional pulse detector/stretcher such as an active diode pulse stretcher as mentioned in the background section

of this specification.

The above described embodiments tend to provide the following advantages.

The embodiments tend to allow relatively fast, narrow pulses to be stretched.

The embodiments tend to be able to stretch pulses of relatively low amplitude.

The stretching of the pulse tends to allow the pulse stretcher to be used as a form of peak detector.

The embodiments tend to provide reduced electromagnetic interference compared to conventional electronic pulse stretchers, due to the optical isolation introduced by the photo-emitter/photo-detector arrangement.

Different embodiments may be provided, e.g. with different choices of photo-emitter/photo-detector pairs, so as to give potential operation over a very wide wavelength, for example at wavelengths ranging from 100 nm to 10 pm, and most readily from 200 nm to 800 nm.

The embodiments tend to allow overshoot to be accommodated, allowing stretching even when opposing pulses are present e.g. in ringing or transient oscillations.

Claims (1)

1. <claim-text>CLAIMS1. Apparatus for stretching a narrow pulse width electrical pulse; the apparatus comprising: a photo-emitter (Dl); and a photo-detector (D2); the photo-emitter (Dl) arranged for being driven by an input electrical pulse to emit an electromagnetic radiation pulse; the photo-detector (D2) optically coupled to the photo-emitter (Dl) for receiving the electromagnetic radiation pulse and to produce a corresponding output electrical pulse.</claim-text> <claim-text>2. Apparatus according to claim 1, wherein the apparatus stretches the input electrical pulse by virtue of the output response time of the photo-detector (D2) being greater than the pulse width of the input electrical pulse such that the output electrical pulse is correspondingly of greater pulse width than the input electrical pulse.</claim-text> <claim-text>3. Apparatus according to claim 1 or claim 2, further comprising a charge sensitive amplifier (10) arranged to receive the output electrical pulse in the form of a current pulse, and wherein the apparatus stretches, or further stretches, the input electrical pulse by virtue of the charge sensitive amplifier (10) stretching the input current pulse, the apparatus thereby providing an output electrical pulse of greater pulse width than the input electrical pulse.</claim-text> <claim-text>4. Apparatus according to any of claims 1 to 3, wherein the input electrical pulse is of pulse width less than or equal to 10 nanoseconds.</claim-text> <claim-text>5. Apparatus according to claim 4, wherein the input electrical pulse is of pulse width less than or equal to 5 nanoseconds.</claim-text> <claim-text>6. Apparatus according to any of claims 1 to 5, wherein the apparatus is arranged to receive the input electrical pulse form a radiation detector.</claim-text> <claim-text>7. Apparatus according to any of claims 1 to 6, wherein the photo-emitter (Dl) is biased to be at its point of photo-emission.</claim-text> <claim-text>8. Apparatus according to any of claims 1 to 7, wherein the photo-detector (D2) is biased to be at its point of photo-response.</claim-text> <claim-text>9. Apparatus according to any of claims 1 to 8, wherein the photo-emitter (Dl) is a light emitting diode (Dl).</claim-text> <claim-text>10. Apparatus according to any of claims 1 to 9, wherein the photo-detector (D2) is a photodiode (D2).</claim-text> <claim-text>11. A method of stretching a narrow pulse width electrical pulse; the method comprising: driving a photo-emitter (Dl) with an input electrical pulse; the photo-emitter (Dl) emitting an electromagnetic radiation pulse; and an optically coupled photo-detector (D2) receiving the electromagnetic radiation pulse and providing a corresponding output electrical pulse.</claim-text> <claim-text>12. A method according to claim 11, wherein the method is applied to an input electrical pulse that has an overshoot (4), the method thereby removing the overshoot (4).</claim-text> <claim-text>13. A method according to claim 11 or claim 12, wherein the method is applied such as to preserve the amplitude of the pulse.</claim-text> <claim-text>14. A method according to any of claims 11 to 13, wherein the input electrical pulse is of pulse width less than or equal to 10 nanoseconds.</claim-text> <claim-text>15. A method according to claim 14, wherein the input electrical pulse is of pulse width less than or equal to 5 nanoseconds,</claim-text>