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Aug 12th, 2020, 9:56pm
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Ring Oscillator Pre vs. Post Layout Frequency Shift (Read 406 times)
melvin1109
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Ring Oscillator Pre vs. Post Layout Frequency Shift
Nov 25th, 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
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kumar.g
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Re: Ring Oscillator Pre vs. Post Layout Frequency Shift
Reply #1 - Dec 3rd, 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.
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Maks
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Re: Ring Oscillator Pre vs. Post Layout Frequency Shift
Reply #2 - Dec 7th, 2019, 1:58pm
 
This shift is most likely caused by layout parasitics.
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Horror Vacui
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Re: Ring Oscillator Pre vs. Post Layout Frequency Shift
Reply #3 - Apr 3rd, 2020, 5:36pm
 
kumar.g wrote on Dec 3rd, 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.


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.
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