Writing Final Report

I started writing my report since the half way through of the project. I started by writing a bit by bit in this way it gives me more time to check and rewrite again if I'm not happy with that. Some of the chapter I've to change it may be two or three time to get it right. Before start writing the report it is necessary to know that what kind of element require in the report and also the structure of  the report is also need to be logical. The basic structure of my report is shown as follow: But one must bear in mind this it not finish yet and could be changed again.

Title page

Abstract

Acknowledgement

Contents page

Chapter 1: Introduction

1.1: Aims and Objectives

1.2: Project Background

1.2.1: Pulse code Modulation (PCM)

1.2.1.1: Sampling

1.2.1.2: Quantization

Chapter: 2 Fibre Optic Technology

2.1: Optical Transmission

2.1.1: Laws of Reflection

2.1.2: Law of Refraction

2.1.3: Critical Angle

2.1.4: Total Internal Reflection

2.1.5: Numerical Aperture

2.2: Types of Fibre

2.2.1: Step Index

2.2.2: Graded Index

2.2.3: Multimode

2.2.4: Single mode

2.2.5: Plastic Optical Fibre

Chapter 3: LED (Light Emitting Diode) and Photo-detector

3.1: LED Driver Circuit

3.2: Photo-detector Circuit

Chapter 4: System's Circuit

4.1: PCM Codec Circuit

4.2: Crystal Master Clock

4.3: Frequency Divider Circuit

Chapter 5: Result and Discussion

5.1: Crystal Master Clock

5.2: Bit Clock

5.3: Frame Synchronization Clock

5.4: Serial Data Output

5.3: Testing Transmitter's Input and Receiver's Output

5.6: System's 3dB Bandwidth

Chapter 6: Future Work

Chapter 7: Investigate Time Division Multiplexing (TDM)

Chapter 8: Conclusion

Reference List

Poster

Before we start making the poster we got to make sure that we have something already in our mind. Therefore we need to decide the type of design or which kind of design of the poster we are going to make. And prepare everything before start making the poster in the power point is essential. In my case I decided to make portrait design poster to suit my ideal and design. Based on my research most of the poster are very simple design without overload the poster with text because a poster is simply just a visual presentation. Planning is very important before we even start. So we can mock up our design by hand and plan our content and layout. And also it is important to design our poster at the actual size that it will be printed. And also the resolution of the poster are also very important. Since most of the posters are printed out using 300dpi resolution. Therefore, extra care must be taken whatever photo we will insert into the poster. All photo must have 300dpi resolution because most of the images download from the internet only have a resolution of 70dpi. This could create a problem when printing out and the poster image will be blurred will not be able to see it clearly. There are number of software can be used to create a poster such as power point and photoshop. In my case I used powerpoint since it is very easy to used. After finich the poster design we can test it out whether the poster has the quality that we want by simply save the design into pdf format. And then open it and then zoom the poster up to 400% to check that the quality is good or not. If the poster is blurred then for sure we can't print it because after all it will look blurry and not clear.

3dB Bandwidth

3dB bandwidth is the frequency at which a input signal is attenuated by 3dB and which is 70.71% of the signal true amplitude. So this means that at its specified bandwidth the device can show as little as 70.71% of the actual amplitude of the signal. This can be measured by simply using the function generator and feed the since wave frequency into the system and then monitor it with oscilloscope then take the output voltage from the oscilloscope using the formula show here to calculate the gain. So we do this by increase the frequency gradually until the response drop to 3dB which is 70.71%. The whole process of measuring this taking a lot time because we need to do different value of frequency until we found the point that drop to 3dB.

Gain = 20 x Log10 (Vout/Vin)

Clock Synchronization Problem

After testing the whole circuit and it is working as expected by using the same clock for both transmitter and receiver. But the biggest problem is that when I separate the two pieces apart and using different separate clock the problem occurred because the two clocks were not in sync both transmitter and receiver. But the system still transmitting and receiving data perfectly fine, when they are in sync I can hear the music when they are not I've got this jitter noise. Found out that the problem was the frame clock since the frame clock trigger the PCM transmitter and receiver. Therefore, the frame clock must be absolutely in sync. Since the PCM receiver need to know when the information is being sent and this can be achieve by using structure called frame. The beginning of the frame consists of repeated copies of a special bit pattern called a frame marker. So the frame markers are repeated several times to allow time for bit synchronization to be achieved before the arrival of the information bits. And then the frame markers are followed by a stare of message marker which is another special bit pattern, to mark the start of the information bits. Therefore, in order to solve this problem I need to do a lot research on how to recovery the frame clock from the transmitter so both transmitter and receiver clock are in sync.

Combining all Circuit and Testing

At this state all individual circuit were working and now it's time to connect them all together and testing them out. When connect all the circuit it is necessary to pay extra attention because if some connection between the pin or chips are not properly grounded will have a problem during operation. Based on my experience it is better to do it step by step and follow the schematic. And also check all connection as it goes. When it's all being connected then used since wave to check each chip's input and output so we know it is behaved as expected. When all the chip working as expected then we can use mp3 music player to send a music into the ADC, for this project we used iphone to play music then feed it into PCM encoder and then the output of the encoder will output digital data which then send into LED driver to transmit through fiber cable. And at the other end the receiver photodiode will receive the data been transmitted and then send it to PCM decoder and it will convert it back to analog signal. And then to play the music I'm going to use my computer to play back so the analog output of the PCM decoder will connect to audio interface and then the interface will connect to my computer and output the music. During this process I've encountered lot of problem such as wrong connection of the clock, LED driver (wrong calculation of resistor), lot of noise from LED transmitter and photodiode receiver and so on. The worse thing is identify the problem when the problem is identified then it can be easily solve but sometime a little bit difficult. 

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.