The Designer's Guide Community Forum https://designers-guide.org/forum/YaBB.pl Simulators >> Circuit Simulators >> Sampled PAC (Spectre RF) Analysis - Strange results ? https://designers-guide.org/forum/YaBB.pl?num=1590925194 Message started by Michael.S on May 31st, 2020, 4:39am

 Title: Sampled PAC (Spectre RF) Analysis - Strange results ? Post by Michael.S on May 31st, 2020, 4:39am Hello everyone,I was following the "Simulating Switched-CapacitorFilters 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

 Title: Re: Sampled PAC (Spectre RF) Analysis - Strange results ? Post by Frank Wiedmann on 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/explanation-for-sampled-pxf-analysis for a more detailed explanation of the sampled analysis.

 Title: Re: Sampled PAC (Spectre RF) Analysis - Strange results ? Post by Michael.S on 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.

 Title: Re: Sampled PAC (Spectre RF) Analysis - Strange results ? Post by Frank Wiedmann on Jun 4th, 2020, 3:33am