Hello everybody!

Before I start my questions, first I have to cite from Mr. Ken Kundert’s excellent paper “sc-filters - Simulating with Cadence SpectreRF” the following: “Usually, with switched-capacitor circuits, designers are interested in the discrete-time behavior as switched-capacitor filters are embedded in sampled-data systems. SpectreRF is set up to report on the continuous-time behavior. As such, you must take steps to get the discrete-time behavior if that is what you need. The __PNoise analysis does provide a built-in feature that can be invoked in order to determine the noise of a filter acting as a discrete-time system__. Otherwise you will need to add a idealized sample-and-hold to your circuit.”

He wrote to me at forum “designers-guide.org/.../YaBB.pl” the following: “I'm afraid I have no experience with sampled PAC”, because this was implemented later. So I count on you, Cadence guys that hopefully YOU have an answer to me:

Q1: I thought (wrongly) I would obtain the DT response of a system if I specify the special option “SAMPLED” of SpectreRF Periodic-AC (PAC). This is not the case. So, if anyone please could tell me what to do to get the DT frequency response for the PAC analysis? (see graphs below)

Q2: Do YOU have an idea which feature can be invoked to obtain the DT system response in the PNoise analysis? As you see above, Ken Kundert indicated so, but did not say which feature. Once, he wrote to me: “I think they confused autonomous sampling (sample very T seconds) and triggered sampling (sample at a threshold crossing), and assumed that triggered sampling in PNoise was enough”.

Ad Q1:

You see an CT (Conttinuous-Time) analog Low-Pass-Filter (LPF) which is simulated with Cadence SpectreRF PAC _sampled and the result of it below.

Ok, this time, it's not a SC circuit, but this is not the topic. It rather is: How to obtain the Bode diagram of the _DT_ response of any circuit.

You can recognize the obtained spectrum is NOT symmetrical around half the sampling frequency fs/2, as I was expecting, but it still seems to show the CT response.

Many thanks in advance and best regards,

bernd2700