Dear Alberto,
On Thu, 24 Feb 2005, Alberto di Bene wrote:
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.
Essetial point is that due to digital filtering you can recieve the signal
much weaker then one lowest sign meaning. Let's consider 10-bit ADC
(plus 1 sign bit) for simplicity. There is only abt 1000 (1024 exectly)
'quantization steps'. Let code 3ff (1024) corresponds 1V (we remember
about 'broadcasting stn') then 'quantization step' is 1mV. Does it mean
signal of say 10uV can't be resieved? Answer is no, such a signal can be
recieved! If sampling rate is 100MHz and bandwith of signal is say 100Hz
this 10uV signal will be very nice recieved after digital filtering.
Certanly if this 10uV signal is the only signal in ADC input then
ADC gives zero code only. Nothing more. But we can add a noise with
spectrum out of signal spectrum (so called 'dithering'). Then signals
can be digitized by the ADC. Added noise can be canceled by digital
filtering then.
73 de RA9MB/Alex
http://www.qsl.net/ra9mb
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