Download Lab Seven - Amplitude Modulation and Demodulation | EE 451 and more Lab Reports Digital Signal Processing in PDF only on Docsity! EE451L Fall 2008 ______________________________________________________________________________ EE 451 – LAB 7 Amplitude Modulation and Demodulation This laboratory is divided into two parts. In the first part you will learn how to design an AM modulator using the C6713 DSK environment, and in the second part you will implement the demodulation of an AM signal. Introduction A very common method of transmitting information is through Amplitude Modulation (AM) of a message. The transmission of a low-frequency signal over a channel requires a process to transform the signal to a high-frequency. At the receiver end, the signal is demodulated and filtered to extract the low-frequency signal. There are four major types of modulation of signals: amplitude modulation, frequency modulation, phase modulation, and pulse amplitude modulation. In amplitude modulation a transmitted signal includes the carrier signal. An AM signal has the following form )cos()](1[)( twtmkAts cac += Where Ac is the amplitude of the carrier, ka is the amplitude sensitivity of the modulator, m(t) is the message, and cos(wct) is the carrier. In standard AM modulation, 1+kam(t) ≥ 0 for all t, so the message can be recovered from the envelope to within a scale factor and constant offset. Two methods can be used for envelope detection and are particularly suited for DSP implementation: the square-law and Hilbert transform detection. The square-law demodulation method consists of squaring the signal, passing it though a lowpass filter, obtaining the square root and finally, scaling and removing an offset. The first step can be expressed as )2cos()](1[5.0)](1[5.0)(cos)](1[)( 22222222 twtmkAtmkAtwtmkAts cacaccac +++=+= The right-hand side of the equation above consists of a lowpass signal whose cutoff frequency has been modified to 2ωs by the squaring operation, and a second term that has a spectrum centered at ± 2ωc. For positive frequencies, the spectrum of the signal has a range of 2ωc ± 2ωs. The spectra of these two terms must not overlap ωc > 2ωs The lowpass filter has a cutoff frequency of 2ωs and its output is 22 )](1[5.0 tmkA ac + . Square- root of this signal results in an output signal that is proportional to m(t) with a DC offset, which can be removed by a highpass filter.
