Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Ring oscillator phase noise

Status
Not open for further replies.

venn_ng

Member level 5
Joined
Mar 26, 2017
Messages
87
Helped
1
Reputation
2
Reaction score
1
Trophy points
18
Activity points
645
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.
 

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.
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top