[time-nuts] Harmonics suppression in ring oscillators

Attila Kinali attila at kinali.ch
Thu Mar 19 09:17:58 EDT 2015

On Wed, 18 Mar 2015 21:19:55 +0100
Florian Teply <usenet at teply.info> wrote:

> > Good, I am not alone.. I felt stupid not being able to find something
> > this basic.
> >
> Maybe we are stupid not being able to find something this basic, but
> then we're stupid together, so at least we have company ;-)

> > Hm.. so only odd harmonics? What prevents the even harmonics?
> Now you got me thinking...
> Based on my train of thought of yesterday, the prevention of even
> harmonics is caused by the need of an odd number of stages. Now as I
> rethink about it I'm no longer sure that there couldn't possibly any
> even harmonic. From what it seems to me now, it doesn't even need to
> have an odd number of stages, it just happens to need to have an odd
> number of INVERTING stages for it to self-start oscillation reliably.

Yes, I have seen reports of ring oscillators with even number of
inverters. But all of them said that they needed to kick the
oscillator to reliably start it. 
And yes, if you look at an inverter as an analog amplifier with
a negative amplification that is non-linearly dependent on its
input, then it is not clear at all, why there aren't any harmonics,
both odd or even. 

My only guess is, that the R_DS_on of the transistors, together
with the C_GS of the next stage form a first order low-pass filter
that dampens the higher modes, such that the Barkhausen Criteria
is violated.

I would like to do a simulation of this to get some understanding, 
but i'm lacking the right simulation software and data of real
transistors. (the disadvantage of being in an theoretical
computer science group)

> > Ok, so you are saying, that if you start the ring oscillator
> > in the right way, you get only the fundamental mode. What prevents
> > higher modes from apearing during runtime? What happens if a particle
> > passes trough the oscillator and switches one of the transistors?
> > 
> Well, assuming that we are talking about sane environments (which your
> mentioning of particle strikes basically renders null and void,
> pointing to either high energy physics or space applications which can
> not be considered sane in this context due to their posssibility to
> switch logic states in circuits), all possible causes of introduction of
> higher order oscillation are excluded by definition ;-)
> Joking aside, the case that one cell is switched should be covered
> above.

Well.. SEUs are common enough on earth that, if you are talking
about high reliability, you need to take them into account. Of course
if you are going into space, then it's a rather common event.
The goal of what I am doing here is to have a clocking system that
can withstand arbitrary faults and self-stabilize again after a
number of nodes (not necessarily all) regain their composure and
start working correctly again.

Part of that is asking all those "what if..." questions that usually
get discarded because they have low probability or because people
think that the system will stabilize again in that case... maybe.

> Hmm, I'm already mentally sorting the list of past and potential
> project partners to see where this might lead. In any case, should you
> come close to Frankfurt (or Berlin for that matter), notify me so we can
> have a beer together. If it's on my boss, even better ;-)

I definitly will :-)

			Attila Kinali

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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