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HREF="node131.html">Equalizing filters</A>
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<B> Up:</B> <A ID="tex2html2516"
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HREF="node128.html">Taxonomy of filters</A>
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HREF="node129.html">Low-pass and high-pass filters</A>
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<B> <A ID="tex2html2520"
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<H2><A ID="SECTION001212000000000000000"></A>
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<A ID="sect8.bandpassdef"></A>
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<BR>
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Band-pass and stop-band filters
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</H2>
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<P>
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A
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<A ID="10086"></A><A ID="10087"></A><I>band-pass filter</I>
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admits frequencies within a given band, rejecting frequencies below it and
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above it. Figure <A HREF="#fig08.03">8.3</A> shows the frequency response of a band-pass
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filter, with the key parameters labelled. A stop-band filter
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does the reverse, rejecting frequencies within the band and letting through
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frequencies outside it.
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<P>
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<DIV ALIGN="CENTER"><A ID="fig08.03"></A><A ID="10092"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 8.3:</STRONG>
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Terminology for describing the frequency response of band-pass and
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stop-band filters. The horizontal axis is frequency and the vertical
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axis is gain. A band-pass filter is shown; a stop-band filter would have a
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contiguous stopband surrounded by two passbands.</CAPTION>
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<TR><TD><IMG
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WIDTH="491" HEIGHT="326" BORDER="0"
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SRC="img856.png"
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ALT="\begin{figure}\psfig{file=figs/fig08.03.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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In practice, a simpler language is often used for describing bandpass filters,
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as shown in Figure <A HREF="#fig08.04">8.4</A>. Here there are only two parameters: a
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<A ID="10096"></A><I>center frequency</I>
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and a
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<A ID="10098"></A><I>bandwidth</I>.
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The passband is considered to be the region where the filter has at least half
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the power gain as at the peak (i.e., the gain is within 3 decibels of its
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maximum). The bandwidth is the width, in frequency units, of the passband.
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The center frequency is the point of maximum gain, which is approximately the
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midpoint of the passband.
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<P>
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<DIV ALIGN="CENTER"><A ID="fig08.04"></A><A ID="10102"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 8.4:</STRONG>
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A simplified view of a band-pass filter, showing bandwidth and
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center frequency.</CAPTION>
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<TR><TD><IMG
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WIDTH="288" HEIGHT="226" BORDER="0"
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SRC="img857.png"
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ALT="\begin{figure}\psfig{file=figs/fig08.04.ps}\end{figure}"></TD></TR>
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</TABLE>
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