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

Attila Kinali attila at kinali.ch
Wed Oct 10 10:54:25 EDT 2018

Salut Mattia,

On Thu, 20 Sep 2018 12:31:01 +0200
Mattia Rizzi <mattia.rizzi at gmail.com> wrote:

> > People talk of aliasing and sampling,
> > but do not describe where the sampling happens in the first place.
> > After all, it's a time-continuous system and as such, there is no
> > sampling.
> I would say that the sampling occurs when you're using only a slice of an
> input signal.  For instance, If you're using only the zero-crossing slice
> of a sinewave to produce a divided version rather than the full envelope.
> It's a matter of how you process information in your circuit.

Yes. That's the basic way how the sampling/noise-aliasing happens.
I just wonder why nobody (as far as I am aware of) has described
this process in detail. It looks obvious and if you look at the
general information theory/signal processing literature, it almost
falls out of the basic text books.

> >"A Physical Sine-to-Square Converter Noise Model,"
> > by Kinali, 2018
> I read the paper, very interesting as well!
> I have a  minor remark, in the paper you relate the ISF (let's say
> "sampling window") to the output slew rate of the comparator. I would say
> that the sampling window should be related to the comparator input stage
> bandwidth. If you have an high bandwidth input stage (e.g. 5 GHz) followed
> by a slew rate limited output stage (e.g. 100 MHz) , high frequency noise
> will trigger the output circuit and aliasing it. Viceversa, if you have a
> low bandwidth input stage, even if the output stage is very fast, you don't
> get input noise aliasing.

Yes, exactly! Though, you have to look at a comparator IC as a multi-stage
system, where each gain-stage represents one "comparator" in my paper.
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.

Hmm.. I probably should have made it more clear that the model I
defined applies only to single gain stages and not to whole components.

			Attila Kinali

Science is made up of so many things that appear obvious 
after they are explained. -- Pardot Kynes

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