For example, the signal in a cell phone is a voice signal with a bandwidth of about 4 kHz. But we may want to view low-frequency modulation with an analyzer that does not have sufficient resolution. The total argument of the cosine is 2πf c t+φ(t), an angle with units of radians (or degrees).When φ(t)=0, we simply have a cosine and the angle 2πf c t is a linear function of time. Think of this angle as the angle of a phasor rotating at a constant angular velocity. spectrum is the same for sinewave modulation. filename: fmsidebands.mcd avo 09/21/04 last edit date:2/27/07 β=.4, Sideband Level =β/2 for Narrowband FM 2 10 1 2 0.2 0 0.2 0.4 0.6 0.8 Spectrum Single Sided Spectrum Peak Volts Frequency versus Amplitude Modulation l Amplitude Modulation (AM) l Changes the amplitude of the carrier signal according to the amplitude of the message signal. Phase and Frequency Modulation Think about what it means to modulate the phase of a cosine. • For tone modulation: Modulation index, b = Df/ f m • BW = 2(k f A m + f m) • PM: • For tone modulation: Modulation index, b = k p A m • BW = 2(k p A m +1)f m • Increasing frequency has a more profound effect on the BW of PM 10/14/08 21 Theory Consider a carrier signal, S(t) = A cos (ω c t + θ ) (6.1) where A, ϖc, and θ denote the amplitude, frequency, and … Though less intuitive than amplitude modulation, frequency modulation is still a fairly straightforward method of wireless data transmission. Chapter 4 - Radio Frequency Modulation PDF Version. l All info is carriedin the amplitude of the carrier l There is a linear relationship between the received signal quality and received signal power. Since the frequency change of a sinusoid in time results in alternation of phase in time, FM may also be referred to as phase modulation, or PM, as well. Fig.1: Frequency modulation waveforms Frequency demodulator, also called frequency discriminator, is a circuit, which converts instantaneous frequency variations to linear voltage changes. l AM systems usually occupy less bandwidth then FM the frequency of the transmitted signal must be quite high compared to the frequency of the information signal. Frequency Modulation So far, the assumption has been that the spectrum analyzer has a resolution bandwidth narrow enough to resolve the spectral components of the modulated signal. • In modulation, one characteristic of a signal (generally a sinusoidal wave) known as the carrier is changed based on the information signal that we wish to transmit (modulating signal). Phase Modulation: ( ) ( ), Frequency Modulation: ( ) ( ) t GAM C p f t A t m h t d h t k t h t k u t M Z D D D G f ªº «» ¬¼ ³ We shall focus more on Frequency Modulation in this course and less on Phase Modulation. 2 Theory 2.1 Introduction A sinusoidal carrier c()t =Acos(ωct +θ0) has three parameters that can be modified (modulated) according to an information signal f.(t) 1. Frequency Modulation 1 Objective To understand the principles of frequency modulation and demodulation. set to Δf/fm for frequency modulation. Note: h(t) is the unit impulse response For example, a common modulation test tone is 400 Hz. • That could be the amplitude, phase, or frequency, which result in Amplitude modulation (AM), Phase modulation (PM), or Frequency modulation (FM). Its amplitude A, which leads us to the class of systems designated as amplitude modulation.
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