Photodiode Circuit

Photodiode circuit for this project is quite simple and easy to implement since I'm using SFH551/1-1V. According to the data sheet the SFH511 is a silicon based and directly connected to a transimpedance amplifier that works as a pre amplifier. And also a differential amplifier is connected in series and works as a post amplifier. Then its output is passed to the internal Schmitt- trigger that drives bipolar NPN transistor. So the data out signal is from the collector of the bipolar transistor. Since SFH551 is a transimpedance amplifier with TTL open collector output stage and therefore a pull up resistor of 330ohm is used as shown in figure. And the capacitor is to reduce interference and it is recommended that it must place close to SFH551.

LED Driver Circuit

Since we know that LED requires specialized devices called LED drivers to operate. So the LED driver provide LED with electricity they require to function and perform at their best. It also protect LED from voltage or current fluctuation. Changing in voltage could cause changing in current being supplied to the LEDs. So the LED light output is proportional to its current supply, and LED are rated to operate within a certain current range. Therefore, too much or to little current can cause light output to vary or degrade faster due to higher temperatures within the LED. For our LED transmitter circuit we used the circuit that recommended from the SFH757V data sheet as shown in Figure. In this circuit as shown in the figure we used a Texas Instruments peripheral driver for high current switching at high speeds which is good for applications like optical transmission especially for LED driver circuit. As in the circuit the two capacitors are for filtering and a 1k resistor is a pull up resistor which is for a stable high level on one of the two inputs of the used AND gate (chip). And then the 50ohm resistor is to reduce reflections between data source and input port of the used AND gate. The R_ILED is very important in this circuit which limits the current through the LED. When we connect the circuit it is very important to take extra care with the LED connection. If we connect with the wrong pin then we could easily blow up the LED. And also the supply voltage as well it is necessary to read the LED data sheet and supply the voltage which is recommended. 

Testing Both ADC/DAC Circuit

When each segment of the circuit is tested and working, now it's time to test both ADC and DAC  circuit without the fiber optic transmitter and receiver. So we can test this by using a sine wave that feed into the ADC and then using the wire to connect to DAC and then observe the output. Bear in mind this it using the same clock for both chip just to check if there is a problem with the chip. First we need to supply master clock, bit clock and frame clock to the ADC and then feed a sine wave to the ADC and then digital data from the ADC output will then feed to the DAC input, therefore the output of the DAC should output a since wave. If we will get this then we can be sure that both chip are working fine. Most of the problems I encountered, were related with the clock. If the clock is not stable, the chip will result in error. Since master clock is main clock in this whole circuit we got to make sure that the master clock is stable. Otherwise, the frequency divider circuit will also get a wrong clock frequency value so they are all linked.

Negative Voltage circuit

Since this chip require negative voltage so this it another problem to solve because if we use power supply then it is fine we can use dual power supply to supply negative voltage without a problem. In order to supply the negative voltage without a power supply is challenging therefore more research needed to be done.  Based on my research and found out a chip which can be supplied a negative voltage. This problem can be solved by implementing this chip into our PCM encoder circuit. As shown in figure is the circuit that will supply the negative voltage and in that circuit the capacitor are very important factor for generating the negative voltage.

PCM Chip Circuit

At this state we got all the clock ready and it's time to set up the PCM encoder. First we need to calculate the resistor value for the amplifier which are R1 and R2. Then using the capacitor value 10uF and 0.01uF capacitor between +5V and GND to decouple the interference from the digital siganls to analog signals. And also it is recommended that these two capacitor should place very close to the chip. And then connect all clock such as master clock, bit clock and frame clock to its dedicated pin to get ready for testing.

Frequency Divider Circuit

 Frequency Divider Circuit

When the master clock is worked and everything is as expected. Then the next step is to build the frequency divider circuit. Since we don't have a lower crystal value the best way to get the low clock frequency value is to divide the master clock by using 2 counters and flip flop. The first counter will the divide the master clock by 32 and then feed into the D flip flop so that will give us 64kHz for bit clock. That 64kHz will divide by second counter by 8 will result 8kHz for FSYN. Since this circuit involved a lot of connection it is a good way to use a colour coding wire. Otherwise it can easily mess up the connection therefore create more problems. And also the chip must be probably grounded otherwise we are not getting the right frequency oscillating clock.