IAN LANG ELECTRONICS
than 5 pounds to build. It is a popular system for those indulging in illegal activities, for instance industrial espionage. If you can get the transmitter into the boardroom and run it down an optical cable (perhaps disguised as an electrical socket or light fitting) you could have a listening post several hundred metres away and the system would be indetectable by normal means. If you could attach it to a telephone system your post need not even be on the same continent.
Our practical illustration above is a fairly unrefined example but it works and serves to illustrate the advantage of transmitting by fibre optic rather than co-axial cable or wirelessly. Firstly there are no inductive parts; therefore it is very difficult for any EMI (electro-magnetic interference) to manifest itself in the receiver. In addition they are next door to impossible to tap without highly sophisticated equipment and expertise. Resistive losses are not present and providing the cable is giving total internal reflection throughout, the only real worry of loss is at the design of the receiver end making for a streamlined design process.
There is one more thing that we have not covered and that is repeaters. Eventually the light power will dissipate over a very long run and we will have to have a re-amplification stage. This is merely another receiver passing to a length of cable. For analogue these are known as repeaters, and are linear amplifiers. For digital these are restorers, and take the form of a fast switch. We shall study these further in Q11: Optical Interfacing.
Aspects of Optical Reception.
In prior steps we have taken an electrical signal and turned it into an optical one. Then we have amplified it and sent it down a cable. Now to be any use to us, that signal must be recovered at the other end of the transmission line and returned to the electrical state. Thus it is that whatever we do the electrical input at the transmitter end, we must do in reverse at the receiver end. Let us say that we have taken the output of a microphone and turned it into light pulses which we have subsequently sent down a single mode cable. We could now set up a system such as the one below using a simple 386 amplifier IC:
The light pulses come in and are amplified by a photo-transistor. Between the transistor and the collector resistor exists a varying voltage caused by the varying amplitude of the light hitting the transistor which , via the capacitor at pin 2, provides a signal for the LM386 power amplifier to process. This returns the signal to the electrical state. A gain of 200 can be achieved with the capacitor between pins 1 and 8 and an 8 ohm speaker can be driven directly, thus transducing to sound and hopefully a good copy of the sound that originally came to the microphone. This system is probably the simplest convertor and costs less