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PSS and TRAN jitter results not matching for very simplistic testcase

HoWei

In order to better understand the PSS and TRAN noise simulations, I created a very simple testbench "Vsource+Buffer" (see below) and compared the PSS and TRAN phasenoise and jitter results.

The phasenoise curves are matching okay, but the jitter results (Jcc) are completely off and not matching at all.

The VCO (Vsource) frequency is 100MHz with 0.75V amplitude and the phase noise values are given as source parameters.

Transient noise is enabled with a PSD frequency range from 1M to 1G.

The jitter intergation bandwidth is from 1M to 1GHz as well.

The TRAN phasenoise is the PSD of the absolute jitter, normalized to the 0.75V amplitude (-5.51dBVrms).

Some questions I do have:

1. Why do the jitter values differ so significantly between PSS and TRAN ?

2. What does the PSD averaging value mean in the TRAn setup (did not find any useful description in the documentation) ?

All the settings are shown in the pictures below:

And here the results:

Phasenoise results are matching well from 1MHz to 50MHz, but

3) Why do I not see the harmonics in the TRAN result ? Is it due to sinx/x of the PSD/DFT ?

Below you see the expressions and the huge difference in the Jitter results:

Why are those jitter results differing so much ?

How can I get reliable and trustworthy jitter results ?

Parents

  • Update:

    After reducing the integration bandwidth from 1GHz to 80MHz in both TRAN and PSS (to avoid the harmonics), the PSS jitter is in the same range as the TRAN result, but still ~4 times higher.

    The TRAN result did not change much.

    1) To measure the jitter Jcc, which integration bandwidth has to be used - is there any rule of thumb ?

    2) Why do the harmonics increase the jitter in PSS so much - they should be in phase and not cause jitter, right ?

  • in reply to HoWei

    No time to even look at this today beyond this response, but the thing I can spot (although it's difficult because the screenshots of your forms are rather small) - you have your frequency sweep for the sampled/jitter mode going up to 1G, whereas your PSS fundamental is 100MHz. You should not sweep beyond half the PSS fundamental because there sampled/jitter mode works by having an ideal sampler at the PSS fundamental rate at the output of the circuit, and hence you will be summing up all the aliases. The band up to 50MHz will contain all the noise from the entire band - sweeping beyond that starts double counting (and more) noise because the sidebands overlap.

    Andrew

  • in reply to Andrew Beckett

    Thanks Andrew - I was assuming this could be the issue, but I did not find it stated in any documentation so far.

    I am really wondering what other things I am missing or doing wrong.

    Isn't there a book or tutorial of "what newbies do wrong" in PSS and TRAN noise .... I would be interested;-)

  • in reply to Andrew Beckett

    This might also be the reason why the phase-noise plots are matching well up to 50MHz/2 (F_fundamental/2).

    So can we use this as a rule of thumb, that phase-noise and jitter calculations are only valid up to a bandwidth of (F_fundamental/2) ?

    What if we want to simulate a PLL with 100MHz output-frequency and 1MHz comparison (beat-) frequency  - is the jitter-bandwith and phase-noise only reliable up to Ffund/2=500kHz ?

    That does not sound correct to me, because I think the jitter at the output-node (100MHz) shall be integrated at least up to 10MHz or so.

  • in reply to HoWei

    This is covered (if my memory is correct) in the Spectre RF training, plus in numerous rapid adoption kits and articles on support.cadence.com - sorry, can't spend the time finding them and posting links here (vacation looming...). There's also an option on the pnoise form to stop at half the PSS fundamental when using sampled/jitter. My guess is that you must be using an old version - it was added in IC618 ISR15.

    Andrew

  • in reply to Andrew Beckett

    Okay, will once again digging thru all the tutorials and documentations.

    I am using IC6.1.8.500.13 <-- This is ISR13, right ?

    So I have to update to the latest.

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