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ring and opamp-based RC relaxation oscillator

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nodame123

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Hi all,
I have a 5 stage ring oscillator and a comparator based RC relaxation oscillator.
I want to analyze both of them on these following factors:
My freq of interest : from 5MHz to 25Mhz.
-Power
-Freq stability
-Variation across corners etc.

Which one is more suitable for my application? Any good differences between them?
Relaxation osc has an active element in it,where as the ring is a passive one. So, in the view of operating points for the active element,is the variation in freq for the relaxation osc worse?
Please point out any other differences,if any.

Thank you.
 

Never have seen an oscillator without active elements. Since when are inverter gates or whatever forms your ring oscillator considered passive?

How do you implement a 5 stage ring oscillator at only 25 MHz?
 

I meant,no gain element in ring.
 

Inverter gates are surely gain elements, not linear amplifiers but gain elements.
 

Yes,you are right. My bad. They do have power gain.Leaving that point,could you enlighten me on the question please?
 

Op amps are usually biased as constant-current as possible,
so should be first-order supply-rejecting. Inverters in a RO
have square-law drive strength and straight-ratio threshold
movement w/ supply so should vary more. Also consider the
phase noise effect of supply voltage ripple / self-injected
noise (if PN or short term frequency deviation matters to you)
as again the op amp has PSRR as a prime spec and an inverter
almost never. I have seen ring oscillators fail to work on-wafer
because the decoupling was off-chip and internal supply
bounce fought against oscillation. Op amps don't make such
spikes.
 

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