[hpsdr] A/D

[Henry Vredegoor]

Hi Ben, All,

<
With the addition of an anti-aliasing low-pass filter to cutoff input 
below the 2X limit (IE: for a 60 MHz ADC, the filter is 30 MHz), you're 
absolutely correct.
>

Assumed there IS significant content above 30 MHz....... ;-)

<
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.
>

Is this true? I thought it would be possible to reconstruct it to a 100 % ?
I didn't know this, I am still studying the sample theory in more detail as
I get along.
But I get frustrated by the mathematics....... :-0
I will have to study this more.

<
To me, the saving grace is that I believe we're looking not at direct 
sampling at 10 meters, but some sort of down-conversion of the 10 meter 
signal to something easily processed, correct?
>

I think HPSDR is doing direct sampling up to 6 meters/50MHz (low pass filter
at 55 MHz) at a sampling rate of 125 MHz, (62,5 MHz final bandwidth) in
Mercury.
After this there is indeed a DDC to a lower sample rate.

Maybe one of the Phil's can comment on this, being the creators of Mercury.

I think the saving grace might be that we look more at the information coded
in the phase and amplitude rather than the exact form of the carrier wave?


73's,
Henry.


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