[hpsdr] General question - sampling rate vs bits on ADC

[Chris Albertson]

On Tue, May 17, 2011 at 7:50 AM, Dr. David Kirkby
<david.kirkby at onetel.net> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> This is a general question, about SDRs and the use of a suitable A/D.
>
> If one uses an audio card, sampling rates are limited to about 196 kHz, but
> 24-bit units are available.
>
> In contrast, if one uses a faster card (say 100 MHz), the number of bits of
> the A/D decrease. Typically 14-bit, though there are some 16-bit cards.

I think the main reason to use a faster A/D is because you want to
receive a wider bandwidth.  Posable you'd like to monitor many CW
transmitters over a few Mhz simultaneously.  A demo I saw a while back
had a computer displaying 6 live UHF TV channels from an antenna and
one A/D.  With many Mhz of bandwidth you can do different things that
are imposable with an audio "sound card".

But even if the goal is only to receive one narrow band channel then
you do have some reduction in noise from oversampling.  I think (?) it
is reduced by the square root of the sample rate.  So doing 4X faster
gives a factor of two.

I think also if you use a 100Mhz A/D then you can design the RF front
end differently and there may be something to gain there



-- 
=====
Chris Albertson
Redondo Beach, California

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