[hpsdr] A/D

[Lyle Johnson]

Hello Ben!

> However, Nyquist theory gives me some heartburn:
> 
> The 2X rule is true - you can use a 60 MHz ADC to capture a 30 MHz 
> signal and reconstruct the signal.  But, you've got to assume a wave 
> form shape, as sampling at 2X gives you no information as to the shape 
> of the waveform, as you're only looking at two points per waveform 
> period and don't have enough information to ascertain the waveform's 
> shape.  At a 2X sample rate, you can't tell if the signal recorded is a 
> sine wave, a square wave, a triangle wave, or what.


If it is other than a pure sine wave, it will have frequency components 
above 30 MHz.  This violates the 2x rule.  If,however, the 30 MHz 
low-pass filter is perfect, so 30.0000000001 MHz and above are 
completely cut off, while 30.0000000 MHz and below pass with no 
attenuation, then sampling at 60 MHz will give you *all* the information 
you need to accurately reconstruct the  signal(s) that made it through 
the filter.

In general, you want to sample somewhat faster than 2x, or dither the 
sampling clock in a known way that you can "un-dither" afterwards.  2.2X 
or so enough on a practical basis, depending on how good your low-pass 
input filter is.


Enjoy!

Lyle kK7P


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