On 09/16/2015 01:15 PM, Joe Subich, W4TV wrote:
On 9/16/2015 2:02 PM, Alan wrote:
The amplitude distribution of a large number of signals of different
frequencies and amplitudes closely approximates Gaussian noise (see
note 1 below). As a rule of thumb the peak to RMS voltage ratio of
Gaussian noise is about 5 or 6. Of course, theoretically it is
infinity, but peaks over about 5-6 are statistically rare (note 2),
so that's a good practical rule of thumb.
We know N > 3 based on NC0B's test with the Flex-6700. 5-6 may be
reasonable if all signals are the same strength but what happens when
a very strong signal (that neighbor half mile away) preloads the ADC?
In that case the peak to RMS ratio actually gets less. In the limit
where the strong signal is much greater than the rest, the ratio
approaches 0 dB.
...
The real issue is that the -10 to -17 dBm clip point for the current
class of DDC devices is simply too low for full performance from MDS
to clipping.
Yes, the blocking dynamic range of direct-sampling receivers using
current state-of-the-art ADCs is 15-20 dB worse than a top superhet
receiver like a K3. In addition to that, the "soft clipping" of the
analog circuitry probably gives some additional advantage to the superhet.
On the other hand, for casual operation you don't really need all that
dynamic range anyway. A QRP rig based on a NE602 has horrible dynamic
range, but you can still make a lot of contacts with it and have a lot
of fun. (For a lot less money. :=)
Alan N1AL
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