# [time-nuts] Harmonics suppression in ring oscillators

Attila Kinali attila at kinali.ch
Wed Mar 18 04:23:27 EDT 2015

```On Tue, 17 Mar 2015 22:04:49 +0100
Florian Teply <usenet at teply.info> wrote:

> funnily I stumbled across that very question just a few weeks ago while
> doing my very first ring oscillator designs myself.

Good, I am not alone.. I felt stupid not being able to find something
this basic.

> The explanation I have come to is essentially the following: In
> principle, you're right, a ring oscillator CAN oscillate at a number of
> frequencies. Given the way the ring oscillator works, the list of
> possible fundamental frequencies (not considering the spectrum due to
> the rectangular waveform with some duty cycle) is essentially given by
> (1+2k)/(2*n*td), with n being the number of stages (odd integer), td
> being the time delay of one stage (assuming all stages are identical),
> and k being any integer between 0 and (n-1)/2.

Hm.. so only odd harmonics? What prevents the even harmonics?

> The trick here is to have one element in the chain that can be used to
> create a steady internal state from which oscillation can be started
> predictably. In the 11 stage ring oscillator mentioned above, that
> might be a NAND or NOR gate together with 10 inverters. With one input
> of that NAND or NOR being tied to the output of the chain and the other
> one being tied to a reset input, which can be used to enable or disable
> oscillation. A steady state would be reached within 11 gate delays in
> the sample oscillator mentioned above after DISABLING oscillation. One
> oscillation is reenabled, it will ONLY oscillate at 1/(22*td).

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?

> up three more ;-)

Oh.. I have many questions! Too many actually ^^;
Is there any good literature on ring oscillators? I have not been able
to find anything substantial yet. It's just papers and books that highlight
one particluar feature, but nothing else.

BTW: Depending on how this project goes, we might work toghether with IHP on it.
So I might potentially come over to Frankfurt....

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
```