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Simulators >> RF Simulators >> 3 tone QPSS simulation vs. 2 tone QPSS/QPAC simulation accuracy for a Mixer
https://designers-guide.org/forum/YaBB.pl?num=1238150066

Message started by Murat H. Eskiyerli on Mar 27^th, 2009, 3:34am

Title: 3 tone QPSS simulation vs. 2 tone QPSS/QPAC simulation accuracy for a Mixer
Post by Murat H. Eskiyerli on Mar 27^th, 2009, 3:34am

I am simulating an upconversion mixer, which requires fairly high linearity. When I simulate with 3-tone QPSS using flexible-balance algorithm in SpectreRF, I get lower IIP3 number compared to 2-tone QPSS followed by QPAC analysis. I am quite far from IP1dB point (at least 10dB) so I don't think the circuit compresses. Any ideas about why this discrepancy could be happening?

Title: Re: 3 tone QPSS simulation vs. 2 tone QPSS/QPAC simulation accuracy for a Mixer
Post by pancho_hideboo on Mar 27^th, 2009, 4:06am

eskiyerli wrote on Mar 27^th, 2009, 3:34am:

Any ideas about why this discrepancy could be happening?

Did you plot IM3 power vs input power and compare results between 3tones-HB and 2tones-HB/QPAC ?
See http://www.designers-guide.org/Forum/YaBB.pl?num=1205223090

Although I don't know new Spectre such as version 7.0 or 7.1, you must not specify "selectharm" and "maximorder" in HB-QPSS of Cadence Spectre.

Did you increase "maxharms" until IM3 can not be changed ?
Generally I recommend you to use 2tones-HB/QPAC not 3tones-HB, if you want to evaluate only IM3 not IM5, IM7, etc.
Even in Agilent simulator, I use 2tones-HB-SS when I have to evaluate only IM3.

Title: Re: 3 tone QPSS simulation vs. 2 tone QPSS/QPAC simulation accuracy for a Mixer
Post by Murat H. Eskiyerli on Mar 27^th, 2009, 4:53am

Well, I haven't really swept the input power as we are really interested in S/IM3 at one particular input power level. S/IM3 is around 70dB and thus any numerical errors in IM3 will be important in the final result. I don't fiddle with harmonic settings and just use default setting. In HB-QPSS, I use 5 large signal harmonics with 3x oversampling ratio.

Murat

Title: Re: 3 tone QPSS simulation vs. 2 tone QPSS/QPAC simulation accuracy for a Mixer
Post by pancho_hideboo on Mar 27^th, 2009, 4:59am

eskiyerli wrote on Mar 27^th, 2009, 4:53am:

I don't fiddle with harmonic settings and just use default setting. In HB-QPSS, I use 5 large signal harmonics with 3x oversampling ratio.

Increase "maxharms" in your case. Even though you increase oversampling factor, it is never helpful in your case.
I always feel HB-QPSS of Cadence Spectre require large "maxharms" to get reasonable results compared to HB Analysis of Agilent.
http://www.designers-guide.org/Forum/YaBB.pl?num=1223563305/1#1
http://www.cadence.com/Community/blogs/rf/archive/2009/03/18/setting-up-harmonic-balance-part-1.aspx

Title: Re: 3 tone QPSS simulation vs. 2 tone QPSS/QPAC simulation accuracy for a Mixer
Post by Murat H. Eskiyerli on Mar 27^th, 2009, 5:09am

I am now trying with 7 harmonics. I looked at your old posting and I think you believe that QPSS/QPAC is a bit more reliable compared to 3-tone QPSS. Am I right?

My company does not have much experience with QPSS/QPAC combination, so any results from that type of simulation would not be acceptable anyway. Still, I look to know if I can trust it to keep it in my arsenal as a sanity check.

Title: Re: 3 tone QPSS simulation vs. 2 tone QPSS/QPAC simulation accuracy for a Mixer
Post by pancho_hideboo on Mar 27^th, 2009, 5:22am

eskiyerli wrote on Mar 27^th, 2009, 5:09am:

I am now trying with 7 harmonics.

IM3 is not changed even if you increase maxharms=[7 7 7] ?
http://www.designers-guide.org/Forum/YaBB.pl?num=1191464825/3#3

eskiyerli wrote on Mar 27^th, 2009, 5:09am:

I looked at your old posting and I think you believe that QPSS/QPAC is a bit more reliable compared to 3-tone QPSS. Am I right?

Right.

eskiyerli wrote on Mar 27^th, 2009, 5:09am:

My company does not have much experience with QPSS/QPAC combination, so any results from that type of simulation would not be acceptable anyway. Still, I look to know if I can trust it to keep it in my arsenal as a sanity check.

I use HSPICE and ADSsim as Golden Standard Simulators.
http://www.designers-guide.org/Forum/YaBB.pl?num=1227668750/4#4
http://www.designers-guide.org/Forum/YaBB.pl?num=1208334972/1#1
http://www.designers-guide.org/Forum/YaBB.pl?num=1208334972/20#20

You can get same results as above Golden Standard Simulators even if you use Spectre if you tweak setting parameters of PSS and QPSS, here simulation speed might be slow and memory consumption might be huge compared to Golden Standard Simulators.

I admit usefulness of Shooting PSS of Cadence Spectre.
But I don't use Shooting-QPSS at all. Shooting-QPSS is not reliable at all.

I'm always very honest.
But maybe some counter opinions from Cadence Guys will be posted in defense of their products.

Title: Re: 3 tone QPSS simulation vs. 2 tone QPSS/QPAC simulation accuracy for a Mixer
Post by Andrew Beckett on Mar 28^th, 2009, 8:51am

I'm not going to append a "defence" of Spectre, because much of what has been said is not generally correct. It may be correct for specific circumstances (I'm not doubting Panchoo Hideboo's honesty), but is not correct in general. Otherwise all the large numbers of SpectreRF users would be getting incorrect results which don't match with silicon, and that isn't the case. We've got a lot of very vocal customers, and believe me, they tell us! Similarly if the simulator was vastly slower, Cadence would hear about that too.

Quick rebuttal - I don't have the patience to go into all of the points in detail:

I don't think you should need to use oversampling most of the time. maximorder/selectharm is OK providing you use them sensibly. Shooting QPSS is appropriate for appropriate circuits - but large, high frequency, weakly non-linear, multitone simulations is not its strength. 3-tone HB should be OK, but 2-tone HB+PAC should be OK too - again, which is appropriate depends on your needs. Similarly you could use PSS+RapidIP3 - it depends on your needs.

Regards,

Andrew.

Title: Re: 3 tone QPSS simulation vs. 2 tone QPSS/QPAC simulation accuracy for a Mixer
Post by Murat H. Eskiyerli on Mar 30^th, 2009, 2:04am

My worry about QPAC is that I think it inherently assumes that the interferer is a small signal. IIP3 is normally the result of very low signal levels. What if both signal and interferer are quite large, at the order of -13dBV? Is QPAC still reliable?

Title: Re: 3 tone QPSS simulation vs. 2 tone QPSS/QPAC simulation accuracy for a Mixer
Post by pancho_hideboo on Mar 30^th, 2009, 2:20am

eskiyerli wrote on Mar 30^th, 2009, 2:04am:

My worry about QPAC is that I think it inherently assumes that the interferer is a small signal.
IIP3 is normally the result of very low signal levels.
What if both signal and interferer are quite large, at the order of -13dBV? Is QPAC still reliable?

Generally reliable valid input level for small signal analysis subjected to master large signal analysis is under 10dB from 1dB Gain Compression point.

Again you should plot both Fundamental and IM3 powers vs input power and compare results between 3tones-HB and 2tones-HB/QPAC.
See http://www.designers-guide.org/Forum/YaBB.pl?num=1205223090
Show me your results.

If you have other simulators such as Nexxim, Aplac, ADSsim, GoldenGate, eldoRF, HSPICE-RF, SmartSpice-RF, etc.
show me results of comparison with Cadence Spectre.

http://www.ansoft.com/products/hf/nexxim/datasheet.cfm?f=Nexxim_Flysheet.pdf