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Design >> Analog Design >> Low frequency oscillator - problem
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Message started by tg on Jan 18^th, 2008, 12:35pm

Title: Low frequency oscillator - problem
Post by tg on Jan 18^th, 2008, 12:35pm

Hi,
I have a problem with a ring oscillator. It works @ ~ half the frequency.
Its a simple ring oscillator 3*invs and 2*rc delay to get ~2MHz.
The result is far from the worstcase simulation by 30%!!!
Does anyone have an idea what could be the reason ?
Thanks.

Title: Re: Low frequency oscillator - problem
Post by thechopper on Jan 18^th, 2008, 6:54pm

Hi tg,

Did you check the parasitic r/C that you might have in the caps nodes you are using for the oscillator? That might be a good reason to make the oscillator run at a lower frequency and not to show up in your WC corners; unless you considered those when simulating the circuit.

Hope this helps
Tosei

Title: Re: Low frequency oscillator - problem
Post by tg on Jan 19^th, 2008, 10:12am

of course, already considered, rc extraction.
Beside, the resistor been measured, its only 10% larger then typ models.
:-/

Title: Re: Low frequency oscillator - problem
Post by thechopper on Jan 19^th, 2008, 6:53pm

Then I guess you are left with either a larger than expected capacitance or inverter thresholds. Were you able to check for those two?

Regards
Tosei

Title: Re: Low frequency oscillator - problem
Post by tg on Jan 20^th, 2008, 3:51am

I cannot measure the capacitor because of its small size (~0.5pF).
I can ask the fab what was the vt in thier measurements, but I think such a change in vt would kill other digital and analog modules (everything works fine but this oscillator).

Title: Re: Low frequency oscillator - problem
Post by thechopper on Jan 20^th, 2008, 3:45pm

0.5pf sounds marginally small to me in case you are connecting it between the inverter output and gnd. If that's the case then I would double check your rc parasitic extraction...may be something not modeled in such tool. Layout visual inspection might also help.

Tosei

Title: Re: Low frequency oscillator - problem
Post by tg on Jan 20^th, 2008, 7:21pm

Hi,
The capacitor is connected between the resistor (and next inv input) to ground.
I dubble checked parasitics and found their added value is small, an ammount of 20fF was added to the capacitor (wires).
Regards,
tg

Title: Re: Low frequency oscillator - problem
Post by tg on Jan 30^th, 2008, 11:30pm

Hi,
parasitic capacitors on polysilicon resistor fingers added to total capacitance (not seen in simulation and extraction)
Issue closed
Thanks for the support
tg