📄 qpsk modulation demystified - maxim-dallas.htm
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src="QPSK Modulation Demystified - Maxim-Dallas.files/T126Eqn9.gif"
width=71 align=absBottom> yields a "demodulated"
waveform with an output frequency double that of
the input frequency, whose dc offset varies
according to the phase shift, <IMG height=15
src="QPSK Modulation Demystified - Maxim-Dallas.files/T126Eqn10.gif"
width=11 align=absBottom>.</P>
<P>To prove this,</P>
<P> <IMG height=297
src="QPSK Modulation Demystified - Maxim-Dallas.files/T126Eqn11.gif"
width=348></P>
<P>Thus, the above proves the supposition that
the phase shift on a carrier can be demodulated
into a varying output voltage by multiplying the
carrier with a sine-wave local oscillator and
filtering out the high-frequency term.
Unfortunately, the phase shift is limited to two
quadrants; a phase shift of <IMG height=9
src="QPSK Modulation Demystified - Maxim-Dallas.files/Pi.gif"
width=10>/2 cannot be distinguished from a phase
shift of -<IMG height=9
src="QPSK Modulation Demystified - Maxim-Dallas.files/Pi.gif"
width=10>/2. Therefore, to accurately decode
phase shifts present in all four quadrants, the
input signal needs to be multiplied by both
sinusoidal and cosinusoidal waveforms, the high
frequency filtered out, and the data
reconstructed. The proof of this, expanding on
the above mathematics, is shown below.</P>
<P>Thus,</P>
<P> <IMG height=248
src="QPSK Modulation Demystified - Maxim-Dallas.files/T126Eqn12.gif"
width=349></P>
<P>A SPICE simulation verifies the above
theory.<B> </B>Figure 1A shows a block diagram
of a simple demodulator circuit. The input
voltage, QPSK IN, is a 1MHz sine wave whose
phase is shifted by 45°, 135°, 225°, and then
315° every 5µs. </P>
<CENTER><IMG height=299
src="QPSK Modulation Demystified - Maxim-Dallas.files/T126Fig1A.gif"
width=383> </CENTER>
<P>Figures 2 and 3 show the "in-phase" waveform,
Vi, and the "quadrature" waveform,
V<SUB>q</SUB>, respectively. Both have a
frequency of 2MHz with a dc offset proportional
to the phase shift, confirming the above
mathematics. </P>
<TABLE cellSpacing=0 cellPadding=0 width="40%"
align=center border=0>
<TBODY>
<TR>
<TD>
<CENTER><IMG height=290
src="QPSK Modulation Demystified - Maxim-Dallas.files/T126Fig2.gif"
width=289> </CENTER></TD></TR>
<TR>
<TD> </TD></TR>
<TR>
<TD>
<CENTER><IMG height=290
src="QPSK Modulation Demystified - Maxim-Dallas.files/T126Fig3.gif"
width=289> </CENTER></TD></TR></TBODY></TABLE>
<P>Figure 1B is the phasor diagram showing the
phase shift of QPSK IN and the demodulated data.
</P>
<CENTER><IMG height=244
src="QPSK Modulation Demystified - Maxim-Dallas.files/T126Fig1B.gif"
width=262> </CENTER>
<P>The above theory is perfectly acceptable, and
it would appear that removing the data from the
carrier is a simple process of low-pass
filtering the output of the mixer and
reconstructing the 4 voltages back into logic
levels. In practice, getting a receiver local
oscillator exactly synchronized with the
incoming signal is not easy. If the local
oscillator varies in phase with the incoming
signal, the signals on the phasor diagram will
undergo a phase rotation, its magnitude equal to
the phase difference. Moreover, if the phase
<I>and</I> frequency of the local oscillator are
not fixed with respect to the incoming signal,
there will be a continuing rotation on the
phasor diagram. </P>
<P>Therefore, the output of the front-end
demodulator is normally fed into an ADC and any
rotation resulting from errors in the phase or
frequency of the local oscillator are removed in
DSP.</P>
<P>With the advances in monolithic silicon
germanium (SiGe) technology, all of the above
front-end circuitry can be integrated to reduce
the problems outlined. A good example of how
much of the front-end circuitry can be
integrated is illustrated in the MAX2450,
ultra-low-power quadrature modulator/demodulator
IC. This is one of many devices from Maxim
Integrated Products that incorporates the
quadraphase shifter, the on-chip oscillator, and
the mixer. Once the data has been demodulated,
the output can be applied to a high-frequency
dual-channel ADC (such as the MAX1002 or the
MAX1003) before processing the signal in
DSP.</P>
<P>As the MAX2450 is designed to be used at an
IF of 35MHz to 80MHz, RF signals up to 2.5GHz
can be downconverted using the MAX2411A. This is
a high-frequency up/downconverter with a
low-noise amplifier (LNA) local oscillator, and
it has access to the output of the LNA for
image-reject filtering.</P>
<P>Alternatively, an effective way of converting
straight to baseband is using a
direct-conversion tuner IC. The MAX2102 is
designed to take RF inputs from 2150MHz and
convert directly down to baseband I and Q
signals, thus providing cost savings over
multiple-stage devices. </P>
<P>The above devices are part of the rapidly
expanding RF chipsets from Maxim Integrated
Products. With five high-speed processes, more
than 70 high-frequency standard products, and 52
ASICs in development, Maxim is committed to
being a major player in the RF/wireless,
fiber/cable, and instrumentation markets.</P>
<P align=right><FONT
face="Arial, Helvetica, sans-serif"><FONT
size=-1>T126, last revised: October
2000</FONT></FONT></P><!-- END: DB HTML -->
<P>
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<TABLE cellSpacing=0 cellPadding=0 width="100%"
border=0><!-- BEGIN: MORE INFO -->
<TBODY>
<TR vAlign=top>
<TD><B>More Information</B></TD>
<TD> </TD>
<TD noWrap align=right>APP 686: Oct 13,
2000 </TD></TR>
<TR vAlign=top>
<TD colSpan=3>
<TABLE cellSpacing=0 cellPadding=2 border=0>
<TBODY>
<TR class=tablebodyalt>
<TD vAlign=top><A
href="http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1740">MAX1002</A></TD>
<TD vAlign=top width="60%">Low-Power, 60Msps,
Dual, 6-Bit ADC</TD>
<TD vAlign=top><A
href="http://www.maxim-ic.com/getds.cfm?qv_pk=1740">Full
Data Sheet</A><BR>(PDF, 120k) </TD>
<TD vAlign=top><A
href="https://shop.maxim-ic.com/storefront/searchsample.do?event=Sample&menuitem=Sample&Partnumber=MAX1002">Free
Samples</A> </TD></TR>
<TR class=tablebody>
<TD vAlign=top><A
href="http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1725">MAX1003</A></TD>
<TD vAlign=top width="60%">Low-Power, 90Msps,
Dual 6-Bit ADC</TD>
<TD vAlign=top><A
href="http://www.maxim-ic.com/getds.cfm?qv_pk=1725">Full
Data Sheet</A><BR>(PDF, 128k) </TD>
<TD vAlign=top><A
href="https://shop.maxim-ic.com/storefront/searchsample.do?event=Sample&menuitem=Sample&Partnumber=MAX1003">Free
Samples</A> </TD></TR>
<TR class=tablebodyalt>
<TD vAlign=top><A
href="http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1758">MAX2102</A></TD>
<TD vAlign=top width="60%">Direct-Conversion
Tuner ICs for Digital DBS Applications</TD>
<TD vAlign=top><A
href="http://www.maxim-ic.com/getds.cfm?qv_pk=1758">Full
Data Sheet</A><BR>(PDF, 160k) </TD>
<TD vAlign=top><A
href="https://shop.maxim-ic.com/storefront/searchsample.do?event=Sample&menuitem=Sample&Partnumber=MAX2102">Free
Samples</A> </TD></TR>
<TR class=tablebody>
<TD vAlign=top><A
href="http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3212">MAX2361</A></TD>
<TD vAlign=top width="60%">Complete Dual-Band
Quadrature Transmitters</TD>
<TD vAlign=top> </TD>
<TD vAlign=top></TD></TR>
<TR class=tablebodyalt>
<TD vAlign=top><A
href="http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1820">MAX2411A</A></TD>
<TD vAlign=top width="60%">Low-Cost RF
Up/Downconverter with LNA and PA Driver</TD>
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