[time-nuts] Harmonics suppression in ring oscillators

[Tim Shoppa]

Attila,
  Aren't you just talking about the simple fact, that the perfect
no-rise-time-gate ring oscillator will put out square waves? (Fundamental
and all odd harmonics).

  "Phase shift oscillators" in the analog domain, especially the
prototypical
inverting-active-device-followed-by-three-60-degree-phase-shift-RC-lowpass
networks, will typically have high harmonic content at the output of the
active device and low harmonic content at the input to the active device.

Tim N3QE

On Tue, Mar 17, 2015 at 6:28 AM, Attila Kinali <attila at kinali.ch> wrote:

> Hi,
>
> I stumbled over something that does not seem to be properly documented
> anywhere. A ring oscillator (like any delay line oscillator) has an
> infinte number of poles (on the complex plane), which are on a straight
> line (disregarding the effect that the transistor acts like a first
> order low pass filter, as f_t is usually a lot higher than the oscillation
> frequency). This means that a ring oscillator will always excite more than
> just one mode and oscillate on multiple frequencies.
>
> While for (optical/electrical) delay line oscillators, the way to go
> is to add a frequency selective element, this is not done for ring
> oscillators.
>
> So, how do people keep ring oscillators from oscillating at higher modes?
>
> So far, my google skills have failed me to turn up any answer.
>
>                         Attila Kinali
>
> --
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