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Jul 7th, 2020, 3:01am
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Sampled PAC (Spectre RF) Analysis - Strange results ? (Read 142 times)
Michael.S
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Sampled PAC (Spectre RF) Analysis - Strange results ?
May 31st, 2020, 4:39am
 
Hello everyone,
I was following the "Simulating Switched-Capacitor
Filters with SpectreRF" tutorial/paper and tried to replicate some of the simple S&H results.

So I'm using Virtuoso 6.1.6 (which is quite old AFAIK).
Below is the testbench I've used (Clock frequency is 1 MHz):


I've set the PAC analysis to be "sampled" at 750 ns (which is the middle of the "hold" time), below are the results for the net "out":


I have expected the sampled pac result to show the sinc() behaviour as well as shown in figure 6, in tutorial (https://designers-guide.org/analysis/sc-filters.pdf).

What's going on with the sampled pac anaylsis ?
Another question - with regards to this simple sample and hold circuit, what is the meaning of all the harmonics of the "regular" continuous time PAC analysis ?

Thanks in advance,
Michael
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Frank Wiedmann
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Re: Sampled PAC (Spectre RF) Analysis - Strange results ?
Reply #1 - Jun 3rd, 2020, 3:51am
 
Sampled pac gives you the result at the output of an ideal sampler, not an ideal sample/hold like in Ken's paper (which was written when sampled pac was not available in SpectreRF yet). It's the hold function that is responsible for the sinc() behavior. Please see https://community.cadence.com/cadence_technology_forums/f/custom-ic-design/45055... for a more detailed explanation of the sampled analysis.
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Michael.S
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Re: Sampled PAC (Spectre RF) Analysis - Strange results ?
Reply #2 - Jun 3rd, 2020, 7:02am
 
Thanks for the response.

So the sampled pac analysis shows the response of the output being sampled by an ideal pulse train at the beat frequency of the PSS ?
And that's why the frequency response is flat up to the RC pole (1k & 1p) ?

Thinking intuitively about this, I can't imagine how the amplitude of the signal would be the same (after sampling) for both fin < fs and fin > fs/2.
I can't see (intuitively) the difference between a zero order hold sampling or ideal pulse train sampling.

Any help understanding this better is appreciated.

Thanks in advance,
Michael.
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Frank Wiedmann
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Re: Sampled PAC (Spectre RF) Analysis - Strange results ?
Reply #3 - Jun 4th, 2020, 3:33am
 
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