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PAC Sweep type: absolute or relative? (Read 203 times)
iVenky
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PAC Sweep type: absolute or relative?
Dec 23rd, 2019, 5:04pm
 
Hi,

I would like to know the difference between PAC sweep types: Absolute & Relative. Cadence manual says it's recommended to use "relative" for autonomous systems like oscillators and "absolute" for driven systems like mixers. Can you please explain me the difference and why it is circuit dependent?

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Life is a nonlinear system. The goal is to find a solution with the least error and not an exact solution.
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Ken Kundert
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Re: PAC Sweep type: absolute or relative?
Reply #1 - Dec 23rd, 2019, 11:51pm
 
Relative is relative to the fundamental frequency. Absolute is relative to 0. So, if you sweep from 1Hz to 1MHz on a 1GHz oscillator and sweep type is absolute, then the noise will be measure from 1Hz to 1MHz. However, if the sweep time is relative, then it is measured from 1GHz + 1Hz to 1GHz + 1MHz.

The reason you use relative for oscillators is that you almost always want to know the noise near the carrier frequency, and the carrier frequency is not known precisely. In my example I said the oscillation frequency is 1GHz, but it is only approximately 1GHz. So you need a relative sweep, an absolute sweep is useless for oscillators.

With driven circuit, you do know the fundamental frequency precisely, so the need for relative sweeps is reduced. However, you are free to use either sweep type on driven circuits as suits your needs.

-Ken

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iVenky
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Re: PAC Sweep type: absolute or relative?
Reply #2 - Dec 24th, 2019, 3:53pm
 
Got it! Thanks
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Life is a nonlinear system. The goal is to find a solution with the least error and not an exact solution.
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