The Infrared Photodetector Module is intended for use as the front end of laser detection Systems which are capable of discriminating the laser pulses from background solar noise. With careful design and layout, such a system will even detect the laser from distances of several kilometres

The module incorporates a low-noise photodiode of area 1 cm2, plus a sunlight filter and fish-eye lens. Operating wavelength is from 830nm to llOOnm - i.e. perfectly matched to 905nm GaAs lasers, but effectively cutting out most of the unwanted sunlight for improved range performance outdoors. The fish-eye lens increases the effective area to 3 cm2 and restricts the Field-of-View to around 40 deg giving a further improvement in range performance.

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In addition to operation with laser diode emitters, the module is very useful for any remote infrared detection applications requiring long range in daylight using low-power infrared LED's - e.g. garage door openers, vehicle collision detectors, alarm activators, voice or data communicators, etc.

The datasheet for the module lists the properties of the device, and includes an illustration of the equivalent circuit diagram. Although the module will operate in photovoltaic mode, better performance is achieved if the diode is presented with a reverse bias voltage (Anode to -ve, Cathode to +ve) via a suitable load. This is known as photocurrent mode. The device may be considered as a current source, where I, is the signal current given by:

I. [Optical Power] E Responsitively

This Signal current is normally converted into a voltage either by a resistive load, or by coupling to a transimpedance amplifier. The current I shown in parallel with the signal current is the "dark current" or leakage current of the photodiode. This represents a noise source as it is not generated by the optical signal. Silicon planar photodiodes are low-leakage devices, making them suitable for low signal applications.

The capacitance C~ has two effects on signal reception. Firstly it tends to limit the speed of response to narrow laser pulses, and secondly it can increase the noise bandwidth of the detection system, resulting in poor sensitivity. Note that the junction capacitance increases as the reverse bias is decreased, which implies that the module should not be biased at too low a voltage for a high-speed high-sensitivity system. These points apply equally to Stray capacitance's in the receiver circuit, so careful construction and layout is required if the full potential of the module is to be realised.

The presence of the fish-eye lens increases the effective area (end hence the sensitivity) of the photodiode, without resorting to a larger physical device with associated higher capacitance.

The following sections give practical examples of receiver circuits. The first two are basic applications for short-range operation, which nevertheless demonstrate the principle of operation. The third is a low-noise receiver circuit capable of stretching the module to its operational limits.

Figure 1 shows how the module may be biased using a resistive load to give a particular sensitivity. No amplification is used, and the voltage output is simply fed into a schmitt inverter to give a TTL logic pulse out for every optical pulse received. The sensitivity calculation assumes that the Schmitt inverter has a threshold of 3.3 volts. Note that there is a limit to how sensitive this circuit can be made by increasing the load resistor. Eventually the voltage drop due to dark current plus background Solar current will increase to a point where the inverter is easily triggered by noise.

The TTL pulses may be observed on an oscilloscope, or fed to a suitable decoder circuit matched to the laser (Or infrared LED) transmitter’s encoder circuit.

Figure 2 is a useful device which gives a visible indication of the presence of infrared pulses. As above, the module is biased using a simple resistive load, suitable for short-range operation. The 74LS122 is a retriggerable monostable, configured to give a 7mS pulse to the LED indicator every time a single optical pulse is detected. So long as the received optical pulses are separated by less than 7,55, the monostable will be retriggered and the LED will remain on continuously. If a string of pulses separated by more than 7mS is detected, the LED will flicker. By AC coupling to the monostable via Cl, the resistance to sunlight is improved since the DC voltage drop across Rl is blocked. Note that this technique is used in Figure 3, end could equally be applied to Figure 1

Refer to Figure 3 for the circuit diagram of the Long Range Receiver. So long as care is taken in its construction, this circuit is capable of detecting laser pulses at ranges of 2 to 3 km, depending on the transmitted peak laser power (sensitivity batter than 10uW/cm2 can readily be achieved). Here the Photodetector Nodule is biased by the active load comprising 01, Rl, R2 and Cl. This is AC coupled into the low-noise transimpedance amplifier, Ul, which converts the signal current into a voltage. Finally, the signal voltage is compared with a threshold voltage by the action of the fast comparator, U2. The variable threshold level is derived from the precision reference diode, D2, by the potential divider action of R8 R10 & R11.

The optimum load for a photodiode operating in sunlight will have a low impedance to DC and slowly varying signals, while presenting a relatively high impedance at the expected signal frequency. A simple resistive load could cause problems under conditions of bright sunlight, as the voltage drop across the resistor results in a reduction in the photodiode bias voltage. This, in turn, causes an increase in the photodiode capacitance, giving a reduction in signal bandwidth.

The active load comprising Q1 R1, R2 and Cl provides a low impedance to sunlight-induced DC and low frequency signals. However, due to the time constant of R2 and Cl the transistor cannot react to rapidly varying signal currents. As a result the high frequency impedance of the load is much higher than its low frequency impedance. This action is described in some detail in reference (1] by Zetex (the manufacturer of the low-noise ZTX214 transistor 01).

The lOV power supply must be clean and free from any switching transients. On no account should a switchmode regulator be used (i.e. use a linear regulator) .

To avoid interference from external sources, the PCB should be housed in a grounded metal box, with the photodetector module protruding through a small aperture. Failure to do this will probably lead to difficulty in setting low threshold levels for good sensitivity, as interference signals will add to the noise background.

Take care not to over-tighten the mounting screws of the module, as this will crack the lens flange. Ideally, a rubber or plastic ring should be used for stress relief.

Although the in-built filter of the module removes most of the unwanted solar radiation, there may be some benefit in mounting the module within a short length of tubing to act as a lens hood. Ensure that the tube is not so long or narrow that the module field of view is restricted.

Alternative components may be used, but this may lead to instability or otherwise affect the overall performance. The important parameters are as follows:

1. Ul must be a low noise pnp type, with Hfe(min) of about 120.

2. Ul must be a low noise, wide bandwidth op-amp. i.e. noise around 10 nanovolts per root hertz or lower at 1KHz, end with a bandwidth greater than 10MHz.

3. U2 most be a fast comparator with propagation delay of the order of tens of nanoseconds.

[1] BPW4lD Application Note - Zetex plc (formerly Ferranti).

1. Infrared Photodetector Module Datasheet.

2. Figure 1 - Short Range Receiver.

3. Figure 2 - Laser Indicator.

4. Figure 3 - Long Range Receiver.

Thus, for low frequency signals, most of the photodiode current is supplied adequately by Q1, while as signal frequency increases more and more of the photodiode current must be drawn from C2 and be subsequently converted to a voltage by Ul.

The values of Cl and 52 may be adjusted to optimise for any particular laser pulse width. Most laser transmitters operate with extremely narrow pulse widths in the range from 20nS to 200nS However values of Cl - l5OnF and R2

= 120K will give good results for both narrow laser pulses and wider LED pulses while working outdoors in bright sunlight. To optimise for narrower laser pulses only, choose values to give a shorter time constant.

Ul is a wide-bandwidth, low-noise amplifier suitable for detection of narrow laser pulses in the presence of noise. It is configured here as a transimpedance amplifier (i.e. a current to voltage converter) . With the transimpedance value of 10K (55), every microamp of signal current is converted to 10 millivolts at the output of U2. While it is possible to increase the sensitivity by increasing the value of 55, this can lead to problems with an associated increase in noise level. 10k is a good value for most systems - if higher values are attempted do not push much beyond 20k.

Any voltage pulses at pin 2 of U2 exceeding the threshold voltage level at pin 3 of U2 will cause the output of the comparator to trip. In order to sat the threshold level, first adjust RH to maximise the voltage at pin 3. Then, with the photodetector covered, slowly adjust 58 until random TTL pulses are observed at pin 7 of U2. This is the result of noise tripping the comparator, and this Setting represents the noise floor of the detection system Now adjust 58 slowly back until no trace of the random TTL pulses remains. Uncover the photodetector and subject it to the expected level of sunlight. If pulses are once more observed at pin 7, the sensitivity must be reduced further using 58. The system is now ready to receive laser pulses.

Note that the above method may result in a higher sensitivity than is absolutely necessary. If false alarms are to be avoided it is better to reduce the sensitivity if possible, once the receiver is functioning at the desired range.

The performance of the long-range receiver is heavily dependent on good PCB layout. The following is a guide to achieving good performance:

1. Keep all tracks as short as possible. Veroboard is ok, but remove

excess copper after construction, to minimise stray capacitance.

2. All resistors should be metal oxide type.

3. The best position for 55 is soldered directly between pins 2 and 6 of Ul on the underside of the board.

4. The Photodetector Module should be positioned as close as practical to the board. If it has to be physically located off the board then use twisted wires and do not exceed a length of 3 inches or so.