Hello All,

I am trying to analyze phase noise for a design involving cross-
coupled LC oscillator. I am able to get the PSS working, however, the
pnoise analysis throws up a strange warning. I have attached the
simulator log file below.

I tried implementing the suggestion posted at

http://www.cadence.com/Community/blogs/rf/archive/2008/08/26/guidelines-for-simu...

but it didnt seem to make any difference in the results.

Has anyone faced this kind of issue while simulating their oscillator
design? Any inputs is highly appreciated.

Regards,
VP
________________________________________________________________________________
_____________

SIMULATOR log file :

**************************************************************
Periodic Steady-State Analysis `pss': guessed fund = 106.2 GHz
**************************************************************

Warning from spectre during IC analysis, during periodic steady state
analysis `pss'.
   WARNING (SPECTRE-16255): Initial condition of 0 V between nodes
OUTp and 0 converted into initial guess (nodeset) because there is no
capacitive path to ground.

=================================
`pss': time = (0 s -> 2.04708 ns)
=================================
Important parameter values in tstab integration:
   start = 0 s
   outputstart = 0 s
   stop = 2.04708 ns
   period = 9.4162 ps
   step = 2.00942 ps
   maxstep = 376.648 fs
   ic = all
   skipdc = no
   reltol = 1e-06
   abstol(I) = 1 pA
   abstol(V) = 1 uV
   temp = 27 C
   tnom = 27 C
   tempeffects = all
   method = gear2only
   lteratio = 3.5
   relref = sigglobal
   cmin = 0 F
   gmin = 1 pS

   pss: time = 51.22 ps     (2.5 %), step = 376.6 fs    (18.4 m%)
   pss: time = 153.7 ps    (7.51 %), step = 376.6 fs    (18.4 m%)
   pss: time = 256.1 ps    (12.5 %), step = 376.6 fs    (18.4 m%)
   pss: time = 358.6 ps    (17.5 %), step = 376.6 fs    (18.4 m%)
   pss: time = 460.6 ps    (22.5 %), step = 376.6 fs    (18.4 m%)
   pss: time = 563.1 ps    (27.5 %), step = 376.6 fs    (18.4 m%)
   pss: time = 665.5 ps    (32.5 %), step = 376.6 fs    (18.4 m%)
   pss: time = 767.7 ps    (37.5 %), step = 9.276 fs     (453 u%)
   pss: time = 870 ps      (42.5 %), step = 29.51 fs    (1.44 m%)
   pss: time = 972.4 ps    (47.5 %), step = 46.02 fs    (2.25 m%)
   pss: time = 1.075 ns    (52.5 %), step = 36.8 fs      (1.8 m%)
   pss: time = 1.177 ns    (57.5 %), step = 29.57 fs    (1.44 m%)
   pss: time = 1.279 ns    (62.5 %), step = 30.46 fs    (1.49 m%)
   pss: time = 1.382 ns    (67.5 %), step = 24.38 fs    (1.19 m%)
   pss: time = 1.484 ns    (72.5 %), step = 34.03 fs    (1.66 m%)
   pss: time = 1.586 ns    (77.5 %), step = 29.02 fs    (1.42 m%)
   pss: time = 1.689 ns    (82.5 %), step = 32.04 fs    (1.57 m%)
   pss: time = 1.791 ns    (87.5 %), step = 21.18 fs    (1.03 m%)
   pss: time = 1.894 ns    (92.5 %), step = 31.87 fs    (1.56 m%)
   pss: time = 1.996 ns    (97.5 %), step = 28.98 fs    (1.42 m%)
The Estimated oscillating frequency from Tstab Tran is = 106.176 GHz
(Hz).

=======================================
`pss': time = (2.04708 ns -> 2.0565 ns)
=======================================
Important parameter values in pss iteration:
   start = 2.04708 ns
   outputstart = 0 s
   stop = 2.0565 ns
   period = 9.41835 ps
   steadyratio = 10e-03
   step = 2.00942 ps
   maxstep = 94.1835 fs
   ic = all
   skipdc = no
   reltol = 1e-06
   abstol(I) = 1 pA
   abstol(V) = 1 uV
   temp = 27 C
   tnom = 27 C
   tempeffects = all
   errpreset = liberal
   method = gear2only
   lteratio = 3.5
   relref = sigglobal
   cmin = 0 F
   gmin = 1 pS

   pss: time = 2.047 ns    (2.63 %), step = 27.66 fs     (294 m%)
   pss: time = 2.048 ns    (7.55 %), step = 27.02 fs     (287 m%)
   pss: time = 2.048 ns    (12.7 %), step = 27.51 fs     (292 m%)
   pss: time = 2.049 ns    (17.6 %), step = 29.57 fs     (314 m%)
   pss: time = 2.049 ns    (22.7 %), step = 31.31 fs     (332 m%)
   pss: time = 2.05 ns     (27.7 %), step = 35.86 fs     (381 m%)
   pss: time = 2.05 ns     (32.5 %), step = 18.33 fs     (195 m%)
   pss: time = 2.051 ns    (37.5 %), step = 10.9 fs      (116 m%)
   pss: time = 2.051 ns    (42.5 %), step = 9.163 fs    (97.3 m%)
   pss: time = 2.052 ns    (47.8 %), step = 31.33 fs     (333 m%)
   pss: time = 2.052 ns    (52.6 %), step = 29.66 fs     (315 m%)
   pss: time = 2.053 ns    (57.6 %), step = 28.94 fs     (307 m%)
   pss: time = 2.053 ns    (62.5 %), step = 29.41 fs     (312 m%)
   pss: time = 2.053 ns    (67.8 %), step = 31.73 fs     (337 m%)
   pss: time = 2.054 ns    (72.5 %), step = 33.36 fs     (354 m%)
   pss: time = 2.054 ns    (77.9 %), step = 38.53 fs     (409 m%)
   pss: time = 2.055 ns    (82.6 %), step = 17.55 fs     (186 m%)
   pss: time = 2.055 ns    (87.5 %), step = 9.45 fs      (100 m%)
   pss: time = 2.056 ns    (92.6 %), step = 9.585 fs     (102 m%)
   pss: time = 2.056 ns    (97.5 %), step = 31.47 fs     (334 m%)
Pinning node: 46, harm: 1, name: U1.M3.Mpx:int_b, value: (0.242847,
0.343320)

==============================
Harmonic balance
==============================
Important harmonic balance parameters:
   RelTol=1.00e-06
   abstol(I)=1.00e-12 A
   abstol(V)=1.00e-06 V
   ResidualTol=1.00e+00
   LTE_Ratio=3.50e+00
   Steady_Ratio=1.00e+00
   MaxNewtonIter=50
********** initial residual **********
Resd Norm=2.18e+05  at node U1.M4.Mpx:int_b  harm=(10)

********** iter = 1 **

Hi,

I am sorry, I posted the incorrect CDS.log file. Attached is the correct one.

Regards,
VP

Unfortunately your log file doesn't contain the spectre subversion information, so it's slightly tricky to check, but this _may_ have been fixed in MMSIM701 (the Update release of MMSIM70).

I believe (from what I found from doing some searching) that the warning may be benign, but I'd recommend first checking the latest MMSIM70 Hotfix (or MMSIM71). If it still does it, please contact Cadence Customer support with the netlist that produces the problem so that it can be investigated further.

Thanks,

Andrew.

VP,

That's fairly recent, so you should contact customer support about this - ideally with an example that reproduces the problem.

Best Regards,

Andrew.