[hpsdr] A/D

[Henry Vredegoor]

Hi Frank, All,
 
Being basically an analog guy too and not at all an expert on this, I can
point you only to the Nyquist Theorem or Sampling Theorem, e.g. 
 
http://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem
 
A lot of mathematic's!
 
What this all comes down to, as I understand it, is that this guy (and
others) proved mathematically that EVEN WITH ONLY TWO samples per period of
the signal to be be digitized, it can be reconstructed correctly from these
two samples!
 
So for 10 meters / 30 MHz you would need a minimum of two samples per one
single period of 30 MHz.
So your minimum sampling frequency would be 2 X 30 MHz = 60 MHz.
 
This is why you always see the sampling frequency of e.g. an ADC is divided
by 2 to get the highest possible signal frequency that can be digitized
correctly.
 
Confusing can be the fact that within SDR's we have QSD's with I and Q
signals, each coming from their own ADC. (e.g. as is the case in a PC with a
stereo sound card)
In that case you have 2 channels and thus 2 samples each moment you sample,
so in fact doubling the sample rate/frequency.
Then you have a maximum signal frequency of the signal that can be digitized
correctly equal to the full sampling frequency of a single channel, so no
division by 2.
 
Hope this helps; Guru's, please correct me if I am telling this not
correctly...... ;-)
 
73's,
 
Henry.
 
 
 

-----Original Message-----
From: hpsdr-bounces at hpsdr.org [mailto:hpsdr-bounces at hpsdr.org] On Behalf Of
FRANCIS CARCIA
Sent: zaterdag 21 april 2007 2:23
To: hpsdr at hpsdr.org
Subject: [hpsdr] A/D


Correct me if I am wrong but how well can we expect a 130 Ms A/D to work at
10 meters?
4 samples per cycle sounds a bit lean. I'm sure it will really kick butt at
455 KHz.
an analog guy trying to fit in.....frank 

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