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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

    This too is killing me - I did the next test by adding a div-by-8 after the VCO:

    - beat-freq=12.5MHz

    - tstab is changed from 100ns to 800ns

    - PNOISE is setup with multiple noise simulations 2x phasenoise and 2xsampled-jitter for VCO and DIV8 out.

    - PNOISE harmonic for VCO-out is set to "8", harmonic for DIV8-out is set to "1"

     


    Now the pnoise2 result is NOT showing the proper phase-noise plot anymore - it is selecting the harm=1 instead of harm=8 when plotting with direct plot and the values is constant at -50dBc/Hz - why ?

    The phasenoise for the DIV8-output is matching well with the transient phasenoise.

  • in reply to HoWei

    Doing a single (not multiple )noise analysis with the same settings, then the phasenoise plot selects the 8th harmonic and shows the correct results

    This clearly seems to be a bug, right ?

  • in reply to HoWei

    It looks like the plotting interface is getting confused by the multiple noise simulations. You should probably report this to Cadence Support.

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