Designer’s Guide Community

supermoment

Senior Member

Offline

Posts: 167

dfferent freq results in pss vs transient
Aug 07^th, 2010, 1:40am

in the QLCVCO (cmos type with bottom bias), freq vs vctrl (vc) are plotted using pss and transient methods.
At the vc=0.8V, transient result has a dip to much lower frequency, however the pss results looks ok. In the instantaneous freq plot (transient sim), it confirms that the vco has settled down.

However, which result is reliable? At the end, i got to simulate whole pll in transient mode.

Back to top

pss_vs_transient2.png

IP Logged

pancho_hideboo

Senior Fellow

Offline

Posts: 1424
Real Homeless

Re: different freq results in pss vs transient
Reply #1 - Aug 7^th, 2010, 2:32am

I can't find out any design issue in your post.

supermoment wrote on Aug 7^th, 2010, 1:40am:

At the vc=0.8V, transient result has a dip to much lower frequency, however the pss results looks ok.

Show me time domain waveforms at vc=0.8V both for PSS and Transient Analysis.

supermoment wrote on Aug 7^th, 2010, 1:40am:

However, which result is reliable?

Result of PSS is reliable.

Tighten accuracy of Transient Analysis, e.g. "reltol".

See http://www.designers-guide.org/Forum/YaBB.pl?num=1237627988/5#5

Back to top

« Last Edit: Aug 7^th, 2010, 11:44am by pancho_hideboo »

Kita━━━━━━(ﾟ∀ﾟ)━━━━━━ !!!!!
http://www7.plala.or.jp/ungeromeppa/flash/kita.html
http://www.youtube.com/watch?v=mjIxGh55bMM&feature=related

IP Logged

supermoment Senior Member Offline Posts: 167	Re: dfferent freq results in pss vs transient Reply #2 - Aug 7^th, 2010, 4:04am i have tried 2 methods to confirm that PSS run is more accurate 1) change reltol from 1e-3 to 1-5 2) sim conservative run the diagram shows the conservative run overlapping with pss result. thx
Back to top	pss_transient_compare2.png IP Logged

pancho_hideboo

Senior Fellow

Offline

Posts: 1424
Real Homeless

Re: different freq results in pss vs transient
Reply #3 - Aug 7^th, 2010, 4:08am

supermoment wrote on Aug 7^th, 2010, 4:04am:

the diagram shows the conservative run overlapping with pss result.

I think you can understand you have to pay attention to accuracy of Transient Analysis of Cadence Spectre if you use it for VCO analysis.

Most serious issue is such irregular large deviation from correct value.

I often encounter irregular large deviation from correct value when I use Cadence Spectre.
You could not notice such large deviation, if you simulate only one point such as vc=0.8V.

Even if I use Golden Standard HSPICE, I can observe deviation from correct value if I don't tighten accuracy such as "delmax".
But I never encounter such extraordinal deviation when I use Golden Standard HSPICE.

Again see http://www.designers-guide.org/Forum/YaBB.pl?num=1237627988/5#5

I always set "maxstep" for VCO analysis if I use Cadence Spectre.

Back to top

« Last Edit: Aug 7^th, 2010, 11:43am by pancho_hideboo »

Kita━━━━━━(ﾟ∀ﾟ)━━━━━━ !!!!!
http://www7.plala.or.jp/ungeromeppa/flash/kita.html
http://www.youtube.com/watch?v=mjIxGh55bMM&feature=related

IP Logged

sheldon

Community Fellow

Offline

Posts: 751

Re: dfferent freq results in pss vs transient
Reply #4 - Aug 7^th, 2010, 8:24pm

Casual,

� Can you explain more about your measurement methodology?
1) How long do you allow for the VCO to settle to steady-state?
2) How do you measure period?
� �Do you measure for one period to calculate the frequency?
� �Do you average over several periods to calculate the frequency?
� �Do you use the frequency meter?

� �The results for both tools should be consistent. However, it is more
difficult to perform the measurement using transient analysis.

� � � � � � � � � � � � � � � � � � � � � � � � � � � � �Best Regards,

� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � Sheldon

Back to top

IP Logged

vp1953 Senior Member Offline Posts: 172	Re: dfferent freq results in pss vs transient Reply #5 - Aug 8^th, 2010, 10:35am Hi Sheldon, Why would be period be different with and without averaging for transient analysis (assuming that noise sources are not enabled)?
Back to top	IP Logged

vp1953 Senior Member Offline Posts: 172	Re: dfferent freq results in pss vs transient Reply #6 - Aug 8^th, 2010, 11:57am Hi Sheldon, Come to think of it, the presence of higher tones in the spectrum can create a difference between a single period and averaged period - is this what you had in mind?
Back to top	IP Logged

supermoment

Senior Member

Offline

Posts: 167

Re: dfferent freq results in pss vs transient
Reply #7 - Aug 9^th, 2010, 8:59pm

Seldon,

The first method I use is to measure frequency of the clk after a long settling time about 30ns in the transient analysis since I still new in PSS. You could see from the instantaneous freq waveform that it has settle down to around 4.88GHz. So one freq and average freq will be almost the same (small variation).

I do not know what the freq meter about? is it written in verilogA?

from the study here, found that sweep PSS is the best method to extract vco gain behaviour.
However, in the close loop simulation, it is still running in transient mode. So it is great to check in transient mode (my point of view) and adjust the accuracy setting.

Back to top

IP Logged

sheldon

Community Fellow

Offline

Posts: 751

Re: dfferent freq results in pss vs transient
Reply #8 - Aug 10^th, 2010, 6:22pm

Casual,

� With adaptive time stepping, the simulator will not necessarily stop at
the instance in time required for the frequency measurement and as a
result, there will be interpolation error. By averaging the effect of
interpolation error is reduced.

� �The freq_meter is an element that was created for the noise aware
PLL flow and is in the pllMMLib, (requires Spectre RF license). The
component is used to measure the frequency at the output of a VCO
or PLL in the time domain. It also enables time domain measurement
of phase noise, Direct Plot --> main form --> tran --> PLL Noise PSD.

� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �Best Regards,

� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �Sheldon � � �

Back to top

IP Logged

supermoment Senior Member Offline Posts: 167	Re: dfferent freq results in pss vs transient Reply #9 - Aug 10^th, 2010, 9:56pm sadly, i do not have noise aware pllflow for mmsim61 ic5.1.41 Quote: time domain measurement of phase noise, Direct Plot --> main form --> tran --> PLL Noise PSD. Could i do this in manual way with the calculator?
Back to top	IP Logged

sheldon

Community Fellow

Offline

Posts: 751

Re: dfferent freq results in pss vs transient
Reply #10 - Aug 11^th, 2010, 5:05am

Casual,

� No, this functionality is not supported in the calculator. I suspect that
the approach is similar to the approach described in Ken's paper on
PLL modeling and simulation.

� � http://www.designers-guide.org/Analysis/PLLnoise+jitter.pdf

� � � � � � � � � � � � � � � � � � � � � � � � � � � � � Best Regards,

� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �Sheldon

Back to top

IP Logged

Ken Kundert Global Moderator Offline Posts: 2386 Silicon Valley	Re: dfferent freq results in pss vs transient Reply #11 - Aug 11^th, 2010, 7:51am You could use a verilog-a module to accurately measure the frequency. You can probably use the one on this site (http://www.designers-guide.org/VerilogAMS/functional-blocks/periods.tgz). You can plot the internal variable 'freq' to show frequency versus time. -Ken
Back to top	IP Logged

supermoment Senior Member Offline Posts: 167	Re: dfferent freq results in pss vs transient Reply #12 - Aug 12^th, 2010, 1:41am thank you very much.
Back to top	IP Logged