[hpsdr] DDC

[jeff millar]

FRANCIS CARCIA wrote:
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> A friend told me DDCs have poor dynamic range. 
If by DDC you mean digital down converter, then their dynamic range is 
determined by their numerical precision and possibly by the spurs of the 
Numerically Controlled Oscillator.  Any competent implementation will 
have much more dynamic range than the A/D.
> Am I blind but can't seem to find anything about dynamic range in the A.D. data.
Look for the term SFDR (spur free dynamic range)  That's the difference 
in level between the strongest signal the A/D can handle to the 
strongest spur generated by the test signals, typically around 80-100 dB 
these days.  Another measure of dynamic range comes from the noise 
specification.  Typical A/D may have a noise floor of 65-80 dB below max 
signal.  But that measures total noise across the whole Nyquist band.  
The noise floor in bandwidths used by hams usually fall below the spur 
levels.
>   I was told they work great in cell phones because int receiver input power is fairly constant. fran
Hmmm...cell phone basestation systems shoot for about 80 dB SFDR with 
AGC only used for cases of extreme overload.  The typical signal range 
at the antenna runs from -120 to -40 dBm, with power control shooting 
for about -70 dBm from most handsets.  Handsets don't have to perform 
nearly as well.

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