Practical Approach to Frequency Modulation and Demodulation

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Published on International Journal of Engineering & Industry
Publication Date: July 18, 2019

Wai Phyo Aung, Nang Tee Shar & Lay Nandar Soe
Ph.D (Electronics), Department of Electronic Engineering, TU (Loikaw)
Demonstrator, TU (Loikaw)
Lecturer, TU (Loikaw)
Loikaw, Kayah State, Myanmar

Journal Full Text PDF: Practical Approach to Frequency Modulation and Demodulation.

The aim of this paper is to practice the experiments with true mathematical modeling hardware, designed specifically for implementing various communication theories. The Emona DATEx telecommunication trainer kit with NI ELVIS II PROTOYPING Platform draws on a well established experimental methodology that brings to life the “universal language” of telecommunication, the BLOCK DIAGRAMS. Focus examples of Emona DATEx trainer kit: Amplitude Modulation (AM), Double Sideband (DSBSC) modulation, Frequency modulation and demodulation, PCM coding and encoding, etc. Through this dissertation, this paper pointed out the operation of FM modulation and demodulation theoretically. Also, FM transmitter-receiver is indeed an electronic project that places great emphasis on practical work.

Keywords: Emona DATEx, telecommunication trainer kit, NI ELVIS II, PROTOTYPING Platform, frequency modulation ,demodulation

Many different circuits have been devised to produce FM and PM signals. There are two different types of frequency modulator circuits, direct circuits and circuits that produce FM indirectly by phase modulation techniques. Methods of generating FM signals but they all basically involve an oscillator with an electrically adjustable frequency. But for this experiment, we will generate a real FM signal using the modules like voltage-controlled oscillators (VCOs) on the Emona DATEx trainer kit. And there are several methods of demodulating an FM signal for example: using slope detector, the Foster-Seeley discriminator, the ratio detector, the phase-locked loop and the zero crossing detectors. We can analyze the waveforms and do comparisons with the theory due to the simulation results are performed by waveforms. (Louis E. Frenzel Jr,2016).

1.1 Amplitude Modulation
As the name suggests, in AM, the information signal varies the amplitude of the carrier sine wave. The instantaneous value of the carrier amplitude changes in accordance with the amplitude and frequency variations of the modulating signal. The carrier frequency remains constant during the modulation process, but its amplitude varies in accordance with the modulating signal. (Louis E. Frenzel Jr,2016).

1.2 Phase Modulation
A type of modulation where the phase of the carrier signal is varied accordance to the low frequency of the message signal is called phase modulation. (Louis E. Frenzel Jr,2016).

1.3 Frequency Modulation
It is the modulation where the frequency of the carrier signal is modulated (changed) in proportion to the message signal while the phase and amplitude and amplitude are constant. (Louis E. Frenzel Jr,2016).

Frequency modulation (FM) is one of the angle modulation techniques. It can be found in any kind of commercial radios and music synthesis, some systems that use video-transmission and also for magnetic tape recording system. That kind of technique is mostly used in the field of telecommunications and signal processing. For generating a frequency modulated signal, change in the frequency of radio carrier is brought to be in line with the amplitude of the given incoming signal, there is up and down movement in frequency of the newer frequency signal is the amount by which the signal moves up and down. FM generation is quite easy. All that is needed a linear voltage to frequency converter, something likes voltage controlled oscillator (VCO). A voltage controlled oscillator is an electronic oscillator whose oscillation frequency is controlled by a voltage input. The applied input voltage determines the instantaneous oscillation frequency.

Picture 1. Frequency modulation (Barry Duncan,2008)

In order to use the modulation on an FM signal, also it is necessary to extract the modulated signal, i.e. demodulate or detect the signal. The frequency variations are converted into voltage for the radio receiver in demodulation. A simple effective FM demodulator is one which takes a time average of the zero crossings of the FM signal. Each pulse in the pulse train is fixed width, and is located at a zero crossing of the FM signal. This is a pulse- repetition-rate modulated signal. If the pulse train is passed through a low pass filter, the filter will perform an averaging operation. The rate of change of this average value is related to the message frequency.

Picture 2. Waveforms resulting from modulating FM, comparator’s output and ZCD output
(Barry Duncan, 2008)

For this experiment, the Emona DATEx experimental add-in module is used to help the operation of electronic systems (like a radio transmitter for example). Block diagrams are used to explain the principles without having to describe the details of how the circuit works. Each block represents a part of the circuit that performs a separate task and is named according to what it does. Examples of common blocks in communication equipment include the adder, filter, phase shifter and so on.

Picture 3. Block diagram of recovering message signal from signals. (Barry Duncan,2008)

At the same time, there would be very few electronics laboratories or workshops that don’t also have a DC power supply and Function Generator. So, the NI ELVIS II PROTOTYPE PCI SLOT can be used to set up the trainer add-in module in this experiment. The NI ELVIS II sends the information via USB to a personal computer where the measurements are displayed on the screen. On the computer, the NIELVIS II devices are called “visual instruments”. However, don’t let the term mislead. The digital multimeter and scopes are real measuring devices.

3.1 Generating FM signal
The master signals module is used to provide a 2 kHz square wave message signal and the VCO is the FM modulator with a 20 kHz carrier. This procedure can be represented as shown in Picture 4.

Picture 4. Block diagram of generating FM signal. (Barry Duncan,2008)

By the idea of block diagram in Picture 4, we can connect the probes at EMONA DATAx telecommunication with NI ELVIS II Protoype as shown in Picture 5.

Picture 5. Circuit Connection of FM using EMONA DATAx on NI ELVIS II Protoype

3.2 Setting up to complete the FM demodulation
Firstly, the comparator module on the Utilities module is used to clip the FM signal, effectively turning it into a square wave. And the edge-triggered Twin-pulse generator module is used to implement the zero-crossing detector. To complete the FM demodulator, the RC low pass filter module and tunable low-pass filter combination is used to pick-out the changing DC component of the twin-pulse generator module’s output. Block Diagram of this Demodulation Process is shown in Picture 6 and we can connect the probes of EMONA DATEx on NI ELVESII Prototype as shoen in Picture 7.

Picture 6. Block diagram of generating message signal from demodulation. (Barry Duncan,2008)

Picture 7. FM Demodulation Circuit connected in EMONA DATEx

As testing result of this experimental approach , result photos are mentioned in Picture 8 to Picture 12.

Picture 8. Simulation result of 2 kHz square wave signal

Picture 9. Simulation result of generating FM signal

Picture 10. Simulation result from the comparator on the Utilities module

Picture 11. Simulation waveforms resulting from Twin-pulse generator

Picture 12. Simulation of regenerating a message signal (a 2 kHz sine wave here) from demodulation

As a conclusion, communication is a subject that really doesn’t touch in practically very much. By this research paper, communication subject which is teaching for not only second year (EcE) students but also other (EcE) courses can be improved for practical approach teaching. Without external devices, likes oscilloscopes, multi-meters, function generators and spectrum analysizers, the research only requires PC with appropriate software installed, the NI ELVIS II board and trainer module. And also the DATEx has a collection of blocks (called modules) to implement dozens of communication and telecommunications. The main empower person to be carried out this eperimental research paper was second and third author, the first author is a supervisor of this Communication Practical Approach.

[1] Barry Duncan, Emona DATEx Telecommunictions Trainer Lab Manual Volume 1-Experiments in Modern Analog and Digital Telecommunications for NI ELVIS II, 2008
[2] Louis E. Frenzel Jr., Principles of Electronic Communication Systems, 4th Ed, march, 2016
[3] Leon W. Couch II, Digital and Analog Communication systems, 8th Ed
[4] B.P. Lathi, Modern Digital and Analog Communication Systems, 3rd Ed, Oxford University Press, 1998
[5] “Communication Systems” 4th Ed.Simon Haykin.2001.