on HPSDR.org. It appears that a CW SDR lashup that would work is a
dual 9951 for quadrature VFO, LVDS squarer, 3125 with ~1uf
integrating caps and then directly into a 24bit a/d such as the
CS5381.
The input impedeance requirements of the 5381 normally require
buffering but the low output impedance of the mixer appears to
satisfy that directly. The signal levels appear to be in the
ballpark as well.
The dual 9951 should give the best currently available spur
performance and a stable quadrature output.
The nice part about this is that it requires no additional amps to
introduce noise and nonlinearity.
The 5381 isn't a full 24bit and has input noise but seems like it
might be an interesting point along the way especially if the
sampling rate is kept low.
Note that I'm assuming that the bandwidth presented to the A/D is
~1Khz and the DDS is doing the tuning rather than a spectrum type
display which requires much greater sample rate that the HPSDR folk
are pursuing.
The PC or DSP would then be responsible for controlling the 9951
frequency including any phase tweaks and providing the audio signal
processing for additional filtering and compression.
Are the available software bits capable of processing in this more
limited way?
thoughts?
jim ab3cv
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