📄 quadrature amplitude modulation 1.htm
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<H2>Quadrature Amplitude Modulation</H2>
<P>Quadrature amplitude modulation (QAM) is a modulation scheme in
which two sinusoidal carriers, one exactly 90 degrees out of
phase with respect to the other, are used to transmit data over a
given physical channel. Because the orthogonal carriers occupy the
same frequency band and differ by a 90 degree phase shift, each
can be modulated independently, transmitted over the same frequency
band, and separated by demodulation at the receiver. For a given
available bandwidth, QAM enables data transmission at twice the rate
of standard pulse amplitude modulation (PAM) without any degradation
in the bit error rate (BER). QAM and its derivatives are used in
both mobile radio and satellite communication systems.</P>
<P>You can use the numerically controlled oscillator (NCO) Compiler
to design a dual-output oscillator that accurately generates the
in-phase and quadrature carriers used by a QAM modulator. The
carrier frequency of each sinusoid can be set to any precision by
defining the phase increment input to the NCO. A block diagram of
the QAM modulator is shown Figure 1. A raised cosine finite impulse
response (FIR) filter is used to filter the data streams before
modulation onto the quadrature carriers. When passed through a
band-limited channel, rectangular pulses suffer from the effects of
time dispersion and tend to smear into one another. This pulse
shaping filter eliminates inter-symbol interference by ensuring that
at a given sampling instance, the contribution to the response from
all other symbols is zero.</P>
<P><B><I>Figure 1. QAM Modulator Block Diagram</I></B></P>
<P><IMG height=217 alt="QAM Modulator Block Diagram"
src="Quadrature Amplitude Modulation 1_files/t-alt-qam_modulator.gif"
width=500 border=0></P>
<H3>Design Files Provided</H3>
<P>The QAM reference design files are located in the
<B>\refdesigns\qam\*</B> folder. Table 1 describes the files
provided with this reference design.</P>
<P>To download the reference design files, go to the <A
href="http://www.altera.com/products/ip/altera/m-alt-ncocompiler.html">NCO
product page</A> and click on the free test drive icon.</P>
<TABLE border=1>
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<TD colSpan=2><B><I>Table 1. QAM Reference Design
Files</I></B></TD></TR>
<TR>
<TD><B>File</B></TD>
<TD><B>Description</B></TD></TR>
<TR>
<TD><B>qam.gdf</B></TD>
<TD>Top-level design file, which can be compiled and simulated
in the MAX+PLUS<FONT size=-1><SUP>⌨️ 快捷键说明
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