Full Duplex Digital Fiber Optic Voice Link

Posted on Feb 6, 2014

The goal of this project is to design and build a full duplex digital fiber optic voice link over a length of 50m at minimum data rate of 64 kbps. The cores of this project are the designs of LED driver and photo-detector circuits. From the preceding circuit diagram, it only shows one side of the circuit. The other side of the circuit is just a du

Full Duplex Digital Fiber Optic Voice Link
Click here to download the full size of the above Circuit.

plicate from this circuit. The only part that you need to notice is the output of the LED driver is connected to the photo-detector circuit on the other side, and vice versa. The condenser microphone is biased with a 9V supply and a 4. 7k ohm load resistor. Recall that electret condenser microphone need biasing because of the built-in FET amplifier inside the microphone capsule. The AC-coupling capacitor is used as a high pass filter to block the DC offset, so the microphone signal is AC coupling. Since this audio amplifier is built specifically for audio signal, so this amplifier can not be used as any other general-purpose op-amps. This circuit here is used from the datasheet, which gives a gain of 50. In case of signal distortion, a 10K potentiometer is used to control the amplitude of the input signal. Again, the 220uF AC-coupling capacitor is used to block the DC offset. This second-order sallen-key low pass filter is designed to have a 3. 5kHz cut-off frequency, which bandwidth is sufficient for human speech. We used this specific filter over the typical RC filter because we obtain a sharper roll off and much less noise. The 256kHz clock signal is generated by a signal generator, and it is mainly used as a master clock and bit clock for the audio encoder. The 8kHz clock signal obtained by using the SN54HC393 dual 4-bit binary counter, which divided the 256kHz signal by 32. This 8kHz clock signal is used to synchronize...

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