Message-ID: <421D20BA.7090108@usa.net>
Date: Thu, 24 Feb 2005 01:32:58 +0100
From: Alberto di Bene <dibene@usa.net>
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Subject: LF: Re: Off Topic  Ft101zd
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Alexander S. Yurkov wrote:

>Why one need 24-bit? Though 16 bit is adequate for most SW,MW,LW recievers.
>16 bit yelds abt 90 dB dynamic range (every bit exept sign bit yelds 6dB).
>But this is dynamic range if there is no filtering. When you make banwidth 
>narrow by DSP then dynamic range will be improved because noise decreases.
>If frontend sampling rate is, say 100 MHz (there is such an ADC) and
>bandwidth of DSP filter is about, say 10 kHz this yelds 40 dB of noise
>decreasing. Thus RX dynamic range to be about 90+40=130 dB!
>[...]
>  
>
Alex,

  I am not quite sure I do agree with you. While it is true that 
narrowing the bandwidth the energy
of the noise inside that bandwidth do decrease while the signal doesn't 
(provided it is entirely confined
into that bandwidth), the problem is another, namely dynamic range. You 
need a great dynamic range
to dig out a faint CW (or SSB) signal near a much stronger one, when 
both are inside the preselector
filter bandpass. AGC is of no help here. A good receiver needs ideally 
at least 100 dB (better more)
of so-called spurious-free DR  And a 16-bit ADC running at  tens of MHz 
I bet doesn't offer more
than 14 bits of useable amplitude resolution, which translates into 
about 84 dB dynamic range. 

If the problem were only that of a good SNR, then your solution would 
work, but here what is needed
is the capability to receive a 0.1 uV signal near to a broadcast station 
with 1V signal at the input of
your receiver....

73  Alberto  I2PHD