[hpsdr] DDC Noise Figure

[John Miles]

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.

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.

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.

-- john, KE5FX

  -----Original Message-----
  From: hpsdr-bounces at lists.openhpsdr.org
[mailto:hpsdr-bounces at lists.openhpsdr.org]On Behalf Of Edson Pereira
  Sent: Sunday, March 27, 2011 12:30 PM
  To: hpsdr at openhpsdr.org
  Subject: [hpsdr] DDC Noise Figure




  Hello,


  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.


  73,


  -- Edson, pu1jte, n1vtn, jf1afn, pu2mwd?
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