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Simulators >> Circuit Simulators >> preciseness of tstep, tsrobe in transient analysis
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Message started by yvkrishna on May 10^th, 2010, 8:36am

Title: preciseness of tstep, tsrobe in transient analysis
Post by yvkrishna on May 10^th, 2010, 8:36am

hi all,

This is regarding fft,snr calculations in spice for T/H circuit of an ADC.
For finding the fft of a sine wave we choose freq of input (fin) such that there are prime number of cycles of input in N clock cycles(which is 2^n, n is integer).
So strobing one point per each clock cycle would be sufficient for fft . however tstep of .TRAN need to be lesser than that. (correct me if i am wrong)

my doubt is how to choose tstep , how small should it be compared to fin or fs(clock freq) ?

also I have seen people(in some of matlab scripts) taking some 256 points per clock cycle for both tstep and tstrobe .

So should the tstep and tstrobe always be that small, that too a multiple of 2^x ?? some one please help me out in understanding this ..

ex: fs=80e6 , nfft=2^14 , and here fin =5.219726563e6 ( when 1069 cycles of input is chosen for 2^14 cycles of clock)
and the tstep,tstrobe here was 48.828125ps . this number(which is 1/(256*fs) ) made me to raise this doubt.

thanks for the replies..

regards,
yvkrishna

Title: Re: preciseness of tstep, tsrobe in transient analysis
Post by boe on May 25^th, 2010, 9:09am

yvkrishna,
one output point per sample clock (at appropriate position wrt clock edges) is sufficient for the FFT. However, strobing more often will help you debugging your simulation(proper sample position, settling time etc). Of course, strobing frequency needs to be an integer multiple of sampling frequency.

Hope this helps
B.O.E.

Title: Re: preciseness of tstep, tsrobe in transient analysis
Post by yvkrishna on May 26^th, 2010, 6:10am

Thanks for the reply B.O.E.,

Yes that would help in debugging, The strobing frequency needs to be an integer multiple of sampling frequency, but is it necessarily a power of 2 ? In my example it was 256 i.e, 2^8 ..

Thanks,
yvkrishna