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<H2><A ID="SECTION001243000000000000000"></A>
<A ID="sect8.singlesideband"></A>
<BR>
Single Sideband Modulation
</H2>
<P>
As we saw in Chapter 5, multiplying two real sinusoids together results
in a signal with two new components at the sum and difference of the
original frequencies. If we carry out the same operation with complex
sinusoids, we get only one new resultant frequency; this is one result of
the greater mathematical simplicity of complex sinusoids as compared to
real ones. If we multiply a complex sinusoid <!-- MATH
$1, Z, {Z^2}, \ldots$
-->
<IMG
WIDTH="81" HEIGHT="34" ALIGN="MIDDLE" BORDER="0"
SRC="img664.png"
ALT="$1, Z, {Z^2}, \ldots$">
with another one, <!-- MATH
$1, W, {W^2}, \ldots$
-->
<IMG
WIDTH="90" HEIGHT="34" ALIGN="MIDDLE" BORDER="0"
SRC="img998.png"
ALT="$1, W, {W^2}, \ldots$"> the result is
<!-- MATH
$1, WZ, {{(WZ)}^2}, \ldots$
-->
<IMG
WIDTH="128" HEIGHT="39" ALIGN="MIDDLE" BORDER="0"
SRC="img999.png"
ALT="$1, WZ, {{(WZ)}^2}, \ldots$">, which is another complex sinusoid whose
frequency, <IMG
WIDTH="55" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1000.png"
ALT="$\angle(ZW)$">, is the sum of the two original frequencies.
<P>
In general, since complex sinusoids have simpler properties than real ones, it
is often useful to be able to convert from real sinusoids to complex ones. In
other words, from the real sinusoid:
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
x[n] = a \cdot \cos (\omega n)
\end{displaymath}
-->
<IMG
WIDTH="120" HEIGHT="28" BORDER="0"
SRC="img1001.png"
ALT="\begin{displaymath}
x[n] = a \cdot \cos (\omega n)
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
(with a spectral peak of amplitude <IMG
WIDTH="27" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img409.png"
ALT="$a/2$"> and frequency <IMG
WIDTH="14" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img27.png"
ALT="$\omega $">) we would like
a way of computing the complex sinusoid:
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
X[n] = a \left ( \cos (\omega n) + i \sin (\omega n) \right )
\end{displaymath}
-->
<IMG
WIDTH="210" HEIGHT="28" BORDER="0"
SRC="img1002.png"
ALT="\begin{displaymath}
X[n] = a \left ( \cos (\omega n) + i \sin (\omega n) \right )
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
so that
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
x[n] = \mathrm{re} (X[n])
\end{displaymath}
-->
<IMG
WIDTH="105" HEIGHT="28" BORDER="0"
SRC="img1003.png"
ALT="\begin{displaymath}
x[n] = \mathrm{re} (X[n])
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
We would like a linear process for doing this, so that superpositions of
sinusoids get treated as if their components were dealt with separately.
<P>
Of course we could equally well have chosen the complex sinusoid with
frequency <IMG
WIDTH="26" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img1004.png"
ALT="$-\omega$">:
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
X'[n] = a \left ( \cos (\omega n) - i \sin (\omega n) \right )
\end{displaymath}
-->
<IMG
WIDTH="214" HEIGHT="28" BORDER="0"
SRC="img1005.png"
ALT="\begin{displaymath}
X'[n] = a \left ( \cos (\omega n) - i \sin (\omega n) \right )
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
and in fact <IMG
WIDTH="31" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img80.png"
ALT="$x[n]$"> is just half the sum of the two. In essence we need a filter
that will pass through positive frequencies (actually frequencies between
0 and <IMG
WIDTH="13" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img41.png"
ALT="$\pi $">, corresponding to values of <IMG
WIDTH="15" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img20.png"
ALT="$Z$"> on the top half of the complex
unit circle) from negative values
(from <IMG
WIDTH="25" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img576.png"
ALT="$-\pi$"> to 0, or equivalently, from <IMG
WIDTH="13" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img41.png"
ALT="$\pi $"> to <IMG
WIDTH="21" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img16.png"
ALT="$2\pi $">--the bottom
half of the unit circle).
<P>
One can design such a filter by designing a low-pass filter with cutoff
frequency <IMG
WIDTH="29" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img5.png"
ALT="$\pi /2$">, and then performing a rotation by <IMG
WIDTH="29" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img5.png"
ALT="$\pi /2$"> radians using the
technique of Section <A HREF="node143.html#sect8.twopolebandpass">8.3.4</A>. However, it turns out to be
easier to do it using two specially designed networks of all-pass filters
with real coefficients.
<P>
Calling the transfer functions of the two filters <IMG
WIDTH="23" HEIGHT="30" ALIGN="MIDDLE" BORDER="0"
SRC="img1006.png"
ALT="$H_1$"> and <IMG
WIDTH="23" HEIGHT="30" ALIGN="MIDDLE" BORDER="0"
SRC="img1007.png"
ALT="$H_2$">, we design
the filters so that
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
\angle({H_1}(Z)) - \angle({H_2}(Z)) \approx
\left \{
\begin{array}{ll}
\pi/2 & {0 < \angle(Z) < \pi} \\
-\pi/2 & {-\pi < \angle(Z) < 0}
\end{array}
\right .
\end{displaymath}
-->
<IMG
WIDTH="353" HEIGHT="45" BORDER="0"
SRC="img1008.png"
ALT="\begin{displaymath}
\angle({H_1}(Z)) - \angle({H_2}(Z)) \approx
\left \{
\be...
...pi} \\
-\pi/2 &amp; {-\pi &lt; \angle(Z) &lt; 0}
\end{array} \right .
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
or in other words,
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
{H_1}(Z) \approx i {H_2}(Z) , \; 0 < \angle(Z) < \pi
\end{displaymath}
-->
<IMG
WIDTH="221" HEIGHT="28" BORDER="0"
SRC="img1009.png"
ALT="\begin{displaymath}
{H_1}(Z) \approx i {H_2}(Z) , \; 0 &lt; \angle(Z) &lt; \pi
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
{H_1}(Z) \approx -i {H_2}(Z) , \; -\pi < \angle(Z) < 0
\end{displaymath}
-->
<IMG
WIDTH="246" HEIGHT="28" BORDER="0"
SRC="img1010.png"
ALT="\begin{displaymath}
{H_1}(Z) \approx -i {H_2}(Z) , \; -\pi &lt; \angle(Z) &lt; 0
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
Then for any incoming real-valued signal <IMG
WIDTH="31" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img80.png"
ALT="$x[n]$"> we simply form a complex number
<IMG
WIDTH="80" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1011.png"
ALT="$a[n] + i b[n]$"> where <IMG
WIDTH="30" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img503.png"
ALT="$a[n]$"> is the output of the first filter and <IMG
WIDTH="28" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1012.png"
ALT="$b[n]$"> is
the output of the second. Any complex sinusoidal component of <IMG
WIDTH="31" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img80.png"
ALT="$x[n]$">
(call it <IMG
WIDTH="24" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img653.png"
ALT="$Z^n$">) will be transformed to
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
{H_1}(Z) + i {H_2}(Z) \approx
\left \{
\begin{array}{ll}
2 {H_1}(Z) & {0 < \angle(Z) < \pi} \\
0 & \mbox{otherwise}
\end{array}
\right .
\end{displaymath}
-->
<IMG
WIDTH="318" HEIGHT="45" BORDER="0"
SRC="img1013.png"
ALT="\begin{displaymath}
{H_1}(Z) + i {H_2}(Z) \approx
\left \{
\begin{array}{ll}
...
...ngle(Z) &lt; \pi} \\
0 &amp; \mbox{otherwise}
\end{array} \right .
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
<P>
Having started with a real-valued signal, whose energy is split equally into
positive and negative frequencies, we end up with a complex-valued one with
only positive frequencies.
<P>
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