[time-nuts] Noise of digital frequency circuits (was: Programmable clock for BFO use....noise)

Azelio Boriani azelio.boriani at gmail.com
Wed Oct 10 15:55:32 EDT 2018


How many samples of the Loran input signal in that 1/6th of the period?
On Wed, Oct 10, 2018 at 5:22 PM Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
>
> --------
> In message <20181010165425.df6d24aa3825ca765f301c0d at kinali.ch>, Attila Kinali w
> rites:
>
> >> >"A Physical Sine-to-Square Converter Noise Model,"
> >> > by Kinali, 2018
> >>
> >[...]
> >Hence the first gain stage already aliases the noise from its whole
> >bandwidth, which can be a lot of noise if the BW is large.
>
> Some years ago I spent a lot of time trying to find a way to
> "oversample" single-shots of the 3rd zero-crossing of Loran-C
> signals.
>
> My finding was that of all the technologies available, the simple
> comparator was the worst, because it only "looks" at a very tiny
> time-slice around the actual zero-crossing, and thus is needlessly
> sensitive to noise.
>
> To make matters worse, the window is always late, it cannot be
> symmetric, because at least some electrons have to move in the
> opposite direction before the comparator changes state.  HP has
> interesting info about this in an old app-note on TI counters.
>
> If the incoming curve-shape is unknown, that is the only thing one
> can do, but when the curve-shape is known to be a sine or a loran-C,
> better results can be had with a wider time window.
>
> The final version of my code (This was SDR with an ADC directly on
> the antenna) found the optimal least-square match between the sampled
> signal and the theoretical signal for a configurable time-window,
> produced the zero crossing from the theoretical signal.
>
> Best performance was around 1/6-th period, which I'm sure there was
> a reason for, but I gave up looking for it.
>
> I have not found a way to implement it in the analog domain.
>
> --
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by incompetence.
>
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