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Design >> RF Design >> Ring oscillator start-up
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Message started by Visjnoe on Oct 17^th, 2007, 9:17am

Title: Ring oscillator start-up
Post by Visjnoe on Oct 17^th, 2007, 9:17am

Dear all,

I'm at the moment assessing some ring oscillator topologies (no prior experience). On the single-ended front, a
certain topology is rather popular. It basically uses a ring build up of 3-5 inverters, which are fed from the top (or bottom) by a current mirror. Just assume that this is a PMOS current mirror for the moment.

The frequeny control of the ring oscillator is than taken care of by an NMOS transistor below the steering part of this PMOS current mirror. The gate of this NMOS transistor is than controlled by the tune voltage (coming from the loop filter). The tune voltage thus sets the current and the current sets the oscillation frequency.

I now have following question: will this always lock in a PLL configuration? Because when the tune voltage is below the VT of the NMOS, the current mirror will be conducting very little current.

Of course, one could argue, that at this very small frequency, the PLL locking will drive the tune voltage up and up until the nominal frequency is reached.

I'm just wondering if anyone has some experience with this type of ring oscillator topology and can discuss this (potential) problem.

Kind Regards

Peter

Title: Re: Ring oscillator start-up
Post by Stefan on Oct 17^th, 2007, 9:38am

At a certain point, your ring oscillator will simply stop oscillating. At least at it will look like (very!! small frequency).
Now everything depends on your PFD if your PFD detects this as the necessity to ramp up the control voltage, your circuit will work.
If not ... well - it will simply hang there :)

I've seen other designs that prevent this by adding a small current to the control voltage that will be used as startup sequence (negligible small compared to the currents that are provided my the PFD/loop filter).

Regards,

Stefan

Title: Re: Ring oscillator start-up
Post by loose-electron on Nov 2^nd, 2007, 10:03am

suggestion - use a fully differential system there -

see the papaer by John Maneatis (IEEE JSSC - November 1996) for a good architecture for the VCO.