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How to obtain Discrete-Time (DT) response from a switched-cap (SC) circuit (integrator)?

bernd2700

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

Parents
  • SpiceMonkey

    You have got the right DT bode plot.

    For time-averaged PAC, you have got a response looks like sinc function (green one, it looks right)
    For sampled PAC, you should get a replica response in every Fs frequecncy (blue one, it also looks right)

    The blue one seems not that accurate, if you change you frequncy display type, or try linear with smaller step, or logrithm with 100 points per decade, you will get a more accurate response: exact replica every fs and each has fs/2-symmetry .

    For CT circut, I think you could also get a sampled PAC: just place a vpulse without any connection,  then use vpulse's frequecy as pss-beat-frequency and use it's posedge as sampled event. (make sure your circuit is periodic or just simply set the input transient signal = zero).

  • ShawnLogan
    in reply to SpiceMonkey

    Dear bernd2700,

    Well, after reading through your conversations with Mr. Kundert and your prior posts, I must also agree with Mr. Kundert's comment to one of your posts:

    "Too many questions all at once.  I need to focus on one at a time.  So lets focus on #1."

    bernd2700 said:
    It rather is: How to obtain the Bode diagram of the _DT_ response of any circuit.

    If you are just trying to find the Bode diagram, it seems as if spectreRF is not the most efficient simulation tool to use. Clearly you can view the second and third order effects of a transistor implementation of a discrete-time system, but it seems that i will take a lot more time and effort to find just its transfer function and Bode plot than using one of your MatLab models or a simulator designed for SC circuits such as Switcap.

    However, in spite of my trying to read through all your posts, I still do not know what your end goal is. So perhaps my comment is not relevant for which I apologize.

    Shawn

Reply
  • ShawnLogan
    in reply to SpiceMonkey

    Dear bernd2700,

    Well, after reading through your conversations with Mr. Kundert and your prior posts, I must also agree with Mr. Kundert's comment to one of your posts:

    "Too many questions all at once.  I need to focus on one at a time.  So lets focus on #1."

    bernd2700 said:
    It rather is: How to obtain the Bode diagram of the _DT_ response of any circuit.

    If you are just trying to find the Bode diagram, it seems as if spectreRF is not the most efficient simulation tool to use. Clearly you can view the second and third order effects of a transistor implementation of a discrete-time system, but it seems that i will take a lot more time and effort to find just its transfer function and Bode plot than using one of your MatLab models or a simulator designed for SC circuits such as Switcap.

    However, in spite of my trying to read through all your posts, I still do not know what your end goal is. So perhaps my comment is not relevant for which I apologize.

    Shawn

Children
  • bernd2700
    in reply to ShawnLogan

    Dear Shawn,

    also thanks to you for your answer!

    Of course, I took to my heart Mr. Kunderts recommendation “not too many questions at once” and therefore reduced it to only 2 questions: Both – in my opinion – formulated as precise as possible. Honestly, I do not have an idea at the moment how to formulate my 2 questions even more precisely than: “Q1: What to do in SpectreRF PAC analysis to get a real DT Bode plot = symm. around fs/2” and “Q2: What to do in PNoise analysis to get the DT response, so what exactly to “invoke” there? (as there is no “sampled” option available)”

    My overall goal is quickly described: “Simply” (I thought, haha) to bring a Matlab model in agreement with the reference, a real switched-cap circuit in Cadence and from these, the Bode plots of the DT response, since I am interested in this one, as Mr. Kundert writes. So, my goal is reached if I see the _same_ result from both finally (apart from small deviations where I understand from where they come from). Therefore it is not helpful for me to just do the game in one of those either in Matlab or in Cadence. (And besides, it is my "specification" to use Cadence and Matlab.)

    Finally: Every comment I try to consider thoroughly and I am happy about!

    Thanks a lot so far and I am curious about your answer(s),

    bernd2700

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