Telecommunication Principles Engineering

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  • Topic: Frequency modulation, Modulation, Amplitude modulation
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  • Published : March 13, 2013
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FM Modulator and Demodulator
Nor Ayu Hidayah Binti Mohd Safri (31743) Department of Electronic Engineering (Computer) Universiti Malaysia Sarawak, Malaysia Kuching, Sarawak, Malaysia Email : nor_ayu92@yahoo.com

Abstract This paper presents about FM modulator and Abstractdemodulator.In electronic and telecommunication, modulation process of varying one or more properties of a high frequency periodic waveform , called carrier signal with a modulating signal which typically contain information to be transmitted. The modulator and demodulator contain an optimized direct digital frequency synthesizer (DDFS) based on quarter-wave symmetry technique for generating the carrier frequency with spurious free dynamic range (SFDR) of more than 64 dB. 64  In this article also will have about the performance in FM. Also have an example for each demodulator and modulator for FM.

it can be processed and used on the receiver end.

II. FM Modulator Modulator
In angle modulation, the amplitude of the signal is held constant and the phase is being varied with the message.An angle modulated signal is of the form xc(t) = Ac cos(Ωct + φ(t)).......... (1) The instantaneous phase of xc(t) is given by θi(t) = Ωct + φ(t).................(2) and the instantaneous frequency is given by Ωi(t) = dθi(t)/dt = Ωc + dφ(t)/dt........... (3) Using this approach, if the message is proportional to φ(t), which is the phase deviation, then we have phase modulation. If the message is proportional to dφ(t)/dt , which is the frequency deviation, then we have frequency modulation.In order to have phase modulation, φ(t) = kpm(t).............(4) where kp is known as the deviation constant. For frequency modulation, dφ(t)/dt = kfm(t)..............(5) where kf is known as the frequency deviation constant. Consequently, an FM modulated signal is of the form xc(t) = Ac cos(Ωct + kf

I. Introduction
FM is the one of info signal is used to control the frequency of the carriers waves. The frequency of the carrier is made to increase as the voltage in the info signal increase and to decrease in freq as it reduces. The larger the amplitude of the info signal, the further the frequency of the carrier signal is shifted from its starting point. As its name implies, frequency modulation (FM) uses a message’s amplitude to vary the frequency of a carrier instead of its amplitude. This means that the FM demodulator is designed to look for changes in frequency instead. As such, it isles affected by amplitude variations and so FM is less susceptible to noise. This makes FM a better communications system in this regard. However, in AM, the info signal is used to vary the amplitude of the carrier wave and then, at the receiver, these variations in the amplitude were detected and the info recovered. A demodulator is a circuit that is used in amplitude modulation and frequency modulation receivers in order to separate the information that was modulated onto the carrier from the carrier itself. A demodulator is the analog part of the modulator. A modulator puts the information onto a carrier wave at the transmitter end and then a demodulator pulls it so



m(α)dα)......(6)

An FM signal is shown in Figure 1

Figure 1: Frequency modulation

VARACTOR TUNED MODULATION
Varactor operation behave as a voltage controlled capacitor.When operated in circuit,make sure the varactor remain in reverse bias. If reverse bias increased, the depletion region increases, less capacitance.

The frequency of resonance depends on value of inductor, L and capacitor, C. Adding an additional capacitor in parallel will cause the total capacitance to increase and this will result in a decrease in the resonance frequency. The tuned circuit is part of the oscillator used to generate the carrier frequency. If the capacitance changes, then so will the carrier frequencies. This is demonstrated in Figure 2.

Figure 2: Frequency Modulated Carrier If reverse bias decreased, the depletion region in...
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