[Elecraft] Analog vs. Digital Front Ends

[Alan]

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