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Design >> Analog Design >> What are the pros of pseudo-differential ring oscillators compared with single-e
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Message started by zczc999 on May 17^th, 2014, 7:39am

Title: What are the pros of pseudo-differential ring oscillators compared with single-e
Post by zczc999 on May 17^th, 2014, 7:39am

Hi,

I am designing a VCO, but confused with single-ended and pseudo-differential RO.
What are the main difference between them, with respect to phase noise, PSRR, common mode rejection, et al.

As I know, single-ended RO is less power consuming, better phase noise performance(because PMOS and Nmos can be turned off ??), but its PSRR and common mode rejection are lousy.

For pseudo differential RO, transistors can also be turned off, so the phase noise is better than full differential, but it doesn't have a tail current source, so the common mode reject is also not good.

Since single-ended and pseudo-differential ROs both are bad at common mode rejection PSRR, what is the advantage of pseudo-differential compared with single-ended.

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Title: Re: What are the pros of pseudo-differential ring oscillators compared with single-e
Post by loose-electron on May 22^nd, 2014, 4:04pm

strongly suggest that you use delay cells in the ring oscillator that are differential (or complementary) in signal output and differential in reception.

Without that you will have some ugly jitter and sensitivity to both ground and power rail noise.

Title: Re: What are the pros of pseudo-differential ring oscillators compared with single-e
Post by rf-design on Jun 6^th, 2014, 1:42am

If the supply noise induced jitter is not tolerable for a simple inverter based ring oscillator the current controlled differential is possible not much better for supply noise frequencies which are typical in digital systems.

A better solution is to complete isolate the supply of the ring oscillator and using the supply direct as frequency control for the oscillator. Then using a decoupling cap on this supply and using a voltage regulator which a current output stage driving this isolated supply. This circuit give you isolation, linear frequency control and best trade of current vs. phase noise. The downside is that you have to level shift the ring oscillator swing to you regular digital supply. But be aware, if the further osillator signal distribution is jitter sensitive you need to run this on a further isolated or regulated supply.