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Low frequency oscillator - problem (Read 5190 times)

tg Junior Member Offline Posts: 11	Low frequency oscillator - problem 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 3invs and 2rc 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.
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HdrChopper Community Fellow Offline Posts: 493	Re: Low frequency oscillator - problem Reply #1 - 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
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tg Junior Member Offline Posts: 11	Re: Low frequency oscillator - problem Reply #2 - Jan 19^th, 2008, 10:12am of course, already considered, rc extraction. Beside, the resistor been measured, its only 10% larger then typ models.
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HdrChopper Community Fellow Offline Posts: 493	Re: Low frequency oscillator - problem Reply #3 - 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
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tg Junior Member Offline Posts: 11	Re: Low frequency oscillator - problem Reply #4 - 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).
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HdrChopper Community Fellow Offline Posts: 493	Re: Low frequency oscillator - problem Reply #5 - 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
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tg Junior Member Offline Posts: 11	Re: Low frequency oscillator - problem Reply #6 - 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
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tg Junior Member Offline Posts: 11	Re: Low frequency oscillator - problem Reply #7 - 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
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