[time-nuts] Building a DMTD/phase noise set in the 21st century (was: Keysight N5511A - phase noise measurements down to theoretical-177 dBm/Hz)

[Jan-Derk Bakker]

On Sat, Aug 24, 2019 at 10:31 PM Attila Kinali <attila at kinali.ch> wrote:
<snip>

> Another important thing to know with audio ADCs is, that they are
> almost always some sigma-delta variant. This gives them a high linearity
> and low noise with moderate cost. But that also means that the higher
> the bandwidth you are using, the higher the noise floor becomes, as
> you are "averaging" over less and less samples. I.e. while an audio
> ADC might be spec'ed to be sampling at 192kHz, you will be only able
> to use up to 20kHz with the nice low noise floor that is specified.
> If you go beyond that, you will have a corresponding increase in
> noise.
>

 That really depends on the order of the modulator and the digital filter.
Compare, for example, figure 18 of the PCM4222 data sheet (
http://www.ti.com/general/docs/suppproductinfo.tsp?distId=10&gotoUrl=http%3A%2F%2Fwww.ti.com%2Flit%2Fgpn%2Fpcm4222
) with figure 2.1.3 of the AK5394 dev board manual (
https://www.akm.com/akm/en/file/ev-board-manual/AK5394AVS.pdf ). And while
there may be some flicker noise present, I've not personally seen the poor
DC performance in many S/D audio converters either (apart as artefacts from
the front-end electrolytic coupling capacitors).

JDB
[The SDR community has rather a bit of experience with using audio ADCs as
generic samplers, before faster converters were feasible.]