Telecommunication Engineering Skills
CE1.1) Introduction
The episode demonstrates how I practiced my telecommunication engineering skills throughout an individual project titled “FM Receiver Design” at COMSATS Institute of Information Technology (CIIT), Islamabad, Pakistan. I commenced this project in July 2011 and submitted the final report in November of that year.
CE1.2) Background
The most crucial engineering subject for telecommunication engineering student at CIIT was Telecommunication Systems Engineering which revolved around the premise of most important and basic communication systems. This subject was integrated with laboratory experiments and students were obligated to work on the design of a Frequency Modulation (FM) receiver to successfully complete the course.
From the outset of this project, I was determined to understand how to link the theoretical leanings with practical activities and most importantly apply my electronic engineering skills and knowledge to implement my ideas. In this project I aimed for:
- Developing my knowledge in regard with telecommunication engineering principles
- Applying electronic principles to implement my ideas
- Analysis of designed FM receiver performance
For having this project accomplished, I communicated effectively with lab demonstrators when encountered challenges and used their assistance in programming PSpice software and circuit fabrication.
CE1.3) Activities Regarding Personal Engineering
CE1.3.1) Requirement of FM Receiver
An important part of the lecture time was allocated to FM technology and design of Receivers. I used lecture notes and text books to study the nature of FM technique and their applications, Modulators and Demodulators, Amplitude & Phase Modulation methods, Frequency Spectrum Sharing and Filtering principles. The first step of the project was to theoretically analyse the behaviour of an FM receiver. In order to do so I considered a scenario that the FM receiver is a radio which should be able to receive any type of audio sources simultaneously. In Pakistan, the radio stations are designated with a specific carrier frequency and they share the frequency spectrum over the air through AM and FM modulation. Considering the above facts, I used Fourier mathematical rules to design different signals spectrum and applied Frequency Division Multiplexing (FDM) technique to create a carrier which can transmit both FM and AM signals simultaneously. Having the fundamentals of transmission system created, I came to this understanding that I require to amplify the received signal, filter out all noises and interfering signals from other stations, and a demodulator to build my receiver.
CE1.3.2) FM Receiver Design
Using the results of my research and following the instruction of the lab demonstrators, I applied Phase Lock Loop (PLL) technique to design my receiver. My system was comprised of three main blocks:
- Phase Detector (PD): I employed this device to produce a voltage signal to distinguish the difference in phase between two signal inputs
- Filter: I considered the design of a filter to eliminate undesirable frequencies components which pass the phase detector
- Oscillator: By using this device I created a synchronous representation of sinusoidal waveforms
From this stage on I had to come up with novel ideas to link electronic principles to turn my ideas into reality. The first component that I required to have installed for my circuit was a
multiplexer. I used this multiplexer as a representation of PD. I did mathematical analysis on the role of PD by investigating the spectrum of multiple signals which were modelled based on radio signals used in Pakistan with similar waveforms to compute the phase difference and DC offset among them. I used PSpice software and used these signals as input to my designated multiplexer and studied its performance on detecting phase difference and DC shift. The next step of the design was creating a low pass filter which could eliminate unwanted signals from the carrier. Therefore, any signal that is received on the antenna that is not in this range should be rejected. In order to do so, I mathematically modelled the low pass filter using Laplace transfer functions. I calculated zeros and poles of the function and ensured that this filter can allow the desired frequency to be passed. Then, I had to create an electronic circuit for the filter, using resistors and capacitors. I calculated the value of these devices by considering the desired cut-off frequency and required bandwidth values and employing proper equations which linked these parameters to each other. I used PSpice to simulate my circuit and fed different carrier signals through and tested the performance of the filter by generating diagrams which showed the frequency and amplitude response of the filter against different frequencies. Moreover, using this simulation software, I could try different topologies for the filter circuit and changed the value of capacitors and resistors to achieve an optimum performance. Afterwards, I connected the multiplexer to an oscillator to produce frequency signals resembling FM radio frequencies. However, during laboratory practices I faced a technical challenge which led to poor signal recovery. I noticed that the received signal is not strong enough to be processed properly and the outcome quality was unacceptable. I resolve this issue with high frequency amplifiers. I designed my amplifier to operate with 60 Ohm impedance, functional frequency of 50 and 500 MHz, and 14dB of gain at 150MHz. Moreover, I improved the quality of the recovered signal by using an Anti-Aliasing filter to work as a restriction towards the bandwidth of signal to satisfy the sampling theorem. Firstly I applied Nyquist theorem to discover the best sampling rates and then I applied this filter by employing a 3 pole Sallen Key Butterworth active low pass filter. I calculated the nodal equations and the transfer function of the circuit and have the values of resistors and capacitors for a 10Hz cut-off frequency calculated.
CE1.3.3) Circuit Assembly
After having the different parts of the system simulated in PSpsice and ensuring that they operate within the proper range and band width, I gathered the necessary electronic component that I used in my design along with a Printed Circuit Board (PCB). I used soldering techniques to install each device on the board. I also installed a 70 Ohm FM antenna. After I connected the audio amplifier and a speaker, I was able to detect 88.5, 89.3, and 91.3MHz. I also used a strong university antenna to see if my receiver could pick other stations which was a success to a certain extent and for some other station I only received noise which was due to the constraints of my design.
CE1.4) Summary
This laboratory assignment was a very interesting opportunity to introduce me to the field of telecommunication systems. I applied a significant level of mathematical knowledge to analyse the role of electronic devices in filters, detectors, modulators, etc. Moreover, I was able to practice manual skills in assembling a telecommunication device which could actually detect signals and demodulate them. The final report and receiver was submitted to lab demonstrators and I was able to accomplish all the requirement of this project.
