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<DIV><SPAN class=250492422-27032011><FONT color=#0000ff size=2 face=Arial>A good
question. Multiplication by saturated diodes in a conventional DBM
yields a best-case conversion loss of 3.9 dB -- 3 dB for the opposite
sideband which is unused, and 0.9 more dB lost in various distortion
products arising from square-wave switching. I'd expect the
"noise figure" of an ideal DDC to be simply 3 dB, because
sine-wave multiplication wouldn't be accompanied by all the
distortion. </FONT></SPAN></DIV>
<DIV><SPAN class=250492422-27032011><FONT color=#0000ff size=2
face=Arial></FONT></SPAN> </DIV>
<DIV><SPAN class=250492422-27032011><FONT color=#0000ff size=2 face=Arial>In
real life, the multiplier is presumably followed by one or more CIC filters, and
the picture gets murkier there due to bit allocation. You can lose
up to ~6 dB (almost a full bit) if the MSB's precision isn't
fully utilized. In other words, if the MSB is toggled by
only the very strongest signals, it will contribute less than the usual 6.02 dB
of dynamic range, and that deficit is effectively part of the conversion
loss.</FONT></SPAN></DIV>
<DIV><SPAN class=250492422-27032011><FONT color=#0000ff size=2
face=Arial></FONT></SPAN> </DIV>
<DIV><SPAN class=250492422-27032011><FONT color=#0000ff size=2
face=Arial>Another concern might be DC offset from the ADCs. If the full
dynamic range isn't available going into the DDC it won't be available at
baseband, either. </FONT></SPAN></DIV>
<DIV><SPAN class=250492422-27032011><FONT color=#0000ff size=2
face=Arial></FONT></SPAN> </DIV>
<DIV><SPAN class=250492422-27032011><FONT color=#0000ff size=2 face=Arial>--
john, KE5FX</FONT></SPAN></DIV>
<DIV><SPAN class=250492422-27032011><FONT color=#0000ff size=2
face=Arial></FONT></SPAN> </DIV>
<BLOCKQUOTE
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<DIV dir=ltr class=OutlookMessageHeader align=left><FONT size=2
face=Tahoma>-----Original Message-----<BR><B>From:</B>
hpsdr-bounces@lists.openhpsdr.org
[mailto:hpsdr-bounces@lists.openhpsdr.org]<B>On Behalf Of </B>Edson
Pereira<BR><B>Sent:</B> Sunday, March 27, 2011 12:30 PM<BR><B>To:</B>
hpsdr@openhpsdr.org<BR><B>Subject:</B> [hpsdr] DDC Noise
Figure<BR><BR></FONT></DIV><BR>
<DIV>Hello,</DIV>
<DIV><BR></DIV>
<DIV>I have been trying to educate myself about SDR using direct sampling and
am having some difficulty in understanding how to calculate the noise figure
of a DDC based SDR system. Since the noise figure takes into account the
bandwidth, would it be correct to say that even though the noise figure of the
ADC is high, the total noise figure of the receiver system can be improved (by
the various decimation stages)? Putting a pre-amp in front of the ADC would
seems to help considerably, at the cost of lower dynamic range, but if my
understanding is correct, the total noise figure will be much better than that
of the ADC alone. Could anyone shed some light? Some math will be ok.</DIV>
<DIV><BR></DIV>
<DIV>73,</DIV>
<DIV><BR></DIV>
<DIV>-- Edson, pu1jte, n1vtn, jf1afn, pu2mwd�</DIV></BLOCKQUOTE></BODY></HTML>