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Design >> Analog Design >> Ring Oscillator Pre vs. Post Layout Frequency Shift
https://designers-guide.org/forum/YaBB.pl?num=1574724558

Message started by melvin1109 on Nov 25^th, 2019, 3:29pm

Title: Ring Oscillator Pre vs. Post Layout Frequency Shift
Post by melvin1109 on Nov 25^th, 2019, 3:29pm

Hello All,

I am trying to layout a ring oscillator that oscillates at ~300MHz in pre layout simulations. The issue I am having is that in post layout simulations the oscillation frequency is ~100MHz.

This is my first layout, so I do not know any good layout practices for this frequency range.

I assume that propagation delay is being increased becasue various parasitic resistances/capacitances in the layout.

Can you guys suggest some things I can look at in my layout, in general, that I could tweak in order to reduce parasitics and bring my post layout frequency closer to my pre layout frequency?

Also, what are things are typically the biggest contributors to a shift in frequency when laying out oscillators?

Thanks

Title: Re: Ring Oscillator Pre vs. Post Layout Frequency Shift
Post by kumar.g on Dec 3^rd, 2019, 4:59am

The layout parasitics can not increase the RC time constant by 300% unless it is done in a complete amateurish way. You can search it on the web and you will find general guidelines to do a proper layout.

Title: Re: Ring Oscillator Pre vs. Post Layout Frequency Shift
Post by Maks on Dec 7^th, 2019, 1:58pm

This shift is most likely caused by layout parasitics.

Title: Re: Ring Oscillator Pre vs. Post Layout Frequency Shift
Post by Horror Vacui on Apr 3^rd, 2020, 5:36pm

kumar.g wrote on Dec 3^rd, 2019, 4:59am:

I assume he is a desperate student, and this is his first IC design homework, where he used minimum sized devices and did not abutted the devices, used only the first two metals, etc. In that case I can imagine a big frequency shift, but 2/3 difference in frequency is really something spectacular.