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Design >> Analog Design >> Problem with performing DFT in SPECTRE cadence
https://designers-guide.org/forum/YaBB.pl?num=1298281879

Message started by Nandish Mehta on Feb 21^st, 2011, 1:51am

Title: Problem with performing DFT in SPECTRE cadence
Post by Nandish Mehta on Feb 21^st, 2011, 1:51am

Hi Friends,

I am design a boot strapped switch and trying to measure distortion by looking at the DFT.

My input is 3 MHz sinusoid and the sampling clock is 20 MHz.

When i do dft of input, i expect only single tone. However to my surprise i find multiple tones.

Can any one please comment on this? Please suggest a solution on how to take a proper DFT.

Regards
Nandish

Title: Re: Problem with performing DFT in SPECTRE cadence
Post by aaron_do on Feb 21^st, 2011, 5:10am

Hi Nandish,

are you probing the voltage of an ideal voltage source? Also over what period of time did you take the DFT? Why don't you try PSS or QPSS analysis?

cheers,
Aaron

Title: Re: Problem with performing DFT in SPECTRE cadence
Post by Nandish Mehta on Feb 21^st, 2011, 6:27am

hi aaron,

I am not sure how to use .pss analysis for calculating the distortion of a boot strap switch.

My simulations setting information is as below:

1) Input is from ideal sinusiodal voltage source with 3 MHz freq.
2) Sampling frequency for sample and hold is 20 MHz
3) Simulation is ran for 10us
4) 1024 point DFT is calculated with rectangular window

I have came to know that the error i am facing is called spectral leakage. I am not sure how to manage it. I tried changing the spectral window but it didn't work.

Please suggest me some solution if you know.

Thanks for your response.

Regards
Nandish

Title: Re: Problem with performing DFT in SPECTRE cadence
Post by Frank Wiedmann on Feb 22^nd, 2011, 12:30am

Take a look at http://www.rfcafe.com/references/articles/Spectral_Leakage_FFT_4_10_07.ppt.

Title: Re: Problem with performing DFT in SPECTRE cadence
Post by Nandish Mehta on Feb 22^nd, 2011, 5:02am

Thanks for the link Frank.