[Elecraft] K4 DSP bit width

[Jeff Scaparra]

Comparing a 24 bit ADC at a 32 KHz sample rate to a 16 Bit ADC at 122 Msps
and saying it is less bits and therefore will have worse performance isn't
a great comparison. The high speed ADC would be highly oversampling the
signal at the second IF. This allows the ADC to use some neat tricks to
effectively get more bits.

https://en.wikipedia.org/wiki/Oversampling
https://www.silabs.com/documents/public/application-notes/an118.pdf

I don't think we can determine theoretically if the K4 can perform better
or worse than the K3s from the current data that we have.

On Wed, May 29, 2019 at 12:08 AM Bill Frantz <frantz at pwpconsult.com> wrote:

> A friend and I were discussing the K4HD and he said that it
> sounds a lot like the K3S. Well, we both agreed that since the
> K3S has world class receiver specs, that would not be a bad thing.
>
> I started comparing the information about the K4HD with the K3S
> manual and found:
>
> K3S       K4HD   Feature
> 8.215MHz  ~8MHz  First IF
> 15KHz      ?     Second IF (if used in the K4)
> 24         16    ADC width
> ~30KHz?   122MHz ADC sample rate (WAG for the K3S)
> 5         2+1    Crystal filters (one empty slot is from a post
> on this list.)
>
>
> Note: In the K4 FAQ there are forward references to a K4HD
> section, but I couldn't find it on the web site.
>
>
> Analysis and wild speculation
>
> Both DSPs and ADC technology have had at least 10 years to
> improve between the K3S and the K4. WHile I don't track ADC
> tech, DSPs are computers and we all know what has happened in
> computers. The basic processors are somewhat faster, and there
> are many more cores on a chip. The K4 certain to have taken
> advantage of these improvements.
>
> The K3S uses a 32 bit floating point DSP, and I don't see any
> reason to change that specification for any of the K4 models.
> Not changing means that much of the K3 DSP code should easily
> port to the new DSP.
>
> I don't understand how the basic K4 can get good dynamic range
> with a 16 bit DAC. The K3's 24 bit DAC seems a better choice,
> although getting high speed and wide bit width at the same time
> is hard and expensive.
>
> In the wild speculation department, there seem to be several
> approaches for the K4HD. (1) Run a K3 like superhet with an
> ~8MHz IF and digitize that into the DSP. (2) Duplicate the K3's
> 2nd IF and use the 16 bit ADC at 15Khz. (3) Use a wider ADC at
> either 8MHz or at 15KHz. Note that one of the features of the K4
> is ease of upgrade to new ADCs.
>
> 73 Bill AE6JV
>
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