+ Post New Thread
Results 1 to 2 of 2
  1. #1
    Member level 3
    Points: 584, Level: 5

    Join Date
    Mar 2017
    Posts
    54
    Helped
    1 / 1
    Points
    584
    Level
    5

    Ring oscillator Q factor

    Let's take an N stage ring oscillator.

    If the Q factor is defined as,

    Q=2pi * maximum energy stored/ energy dissipated per cycle

    since the max energy stored in the cap is CV2/2 and energy lost in the resistors as heat every cycle is CV2, I get Q < pi ~ 3.14. But I read that ring oscillators have Q ~ pi/2 as N-> ∞.
    What's wrong here? (is it something to do with the harmonics?) Does it depend on number of stages in the ring, as I don't see an obvious dependence on 'N' since for each cycle, all inverters store and dissipate the amount described before.

    •   AltAdvertisment

        
       

  2. #2
    Advanced Member level 5
    Points: 38,613, Level: 48

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,250
    Helped
    1810 / 1810
    Points
    38,613
    Level
    48

    Re: Ring oscillator phase noise

    Ring oscillators are not really resonant, I wonder whether
    you are just setting yourself a task with no useful result.
    They certainly are not small-signal resonant once you get
    to a significant number of stages - at some point the
    signals become bang-bang, square, and there is nothing
    linear going on except momentarily as stages cross the
    inverter's linear region one after the other.

    Ask yourself what use any calculated Q number would
    have.

    If you want to get at phase noise, you should look to
    stage edge-rates and supply / ground rejection (slim
    to none), the edge rate slope transforms voltage noise to
    "time noise" (jitter, phase noise depending on how you
    care to express it) during the linear intervals of the
    stage's cycle.

    I wouldn't expect (but could be wrong) a Q:PN relation
    based on a small signal linear type tank oscillator to
    hold much relevance to a RO's behaviors. Transistor
    noise is going to be dwarfed by all of the thrashing.



--[[ ]]--