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Design >> RF Design >> IIP3 measurement for filter at 3MHz
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Message started by jobless on Dec 31^st, 2006, 6:44am

Title: IIP3 measurement for filter at 3MHz
Post by jobless on Dec 31^st, 2006, 6:44am

Hi,

I want to find IIP3 of a Gm-C bandpass filter (3 MHz center freq.) by giving two closely-spaced 1mV tones at f1, f2 and observe the output at 2*f1-f2 and 2*f2-f1.

I tried PSS with spectre RF, but the magnitude of the output signal at 2*f1-f2 and 2*f2-f1 is changing each time I vary tstab (the additional time for stabilization) and rerun the simulation.

Should I rather use tran analysis+fft or should I try QPSS?

Also, I don't understand the difference between PSS and QPSS analysis.
Please give show me a good reference to QPSS Vs PSS analysis and when to use what.

Thanks a lot.

Title: Re: IIP3 measurement for filter at 3MHz
Post by ACWWong on Jan 2^nd, 2007, 2:37am

I think your issue is that your input tones are too small, so the intermodulation products on the output are somewhere near the simulator noise floor.
Try increasing the input signal levels (upto 10dB backed off from Gain compression point) to say 10mV and try again. As you have now increased the tones by 20dB, the third order distortion product will now increase by 60dB and should now be big enough to give you a stable reading given an adequate tstab (check tstab on a transient run). Anyway you would probably want to do a couple of signal levels to confirm a consistant IIP3, say for a 100mV gain compression, i would be trying say input tones of 5mV, 10mV and 20mV to confirm the third order linearity.
If you needed to lower the pss numerical noise floor use "high order" option at the cost of simulation time.
Transient and dft should give the same result, and with good use of time step control and dft point you can get very low numerical noise floor.
for the differences between pss and qpss check out the cadence documentation int he instatllation path, or spectre -h pss or qpss for qucik summary.

cheers
aw

Title: Re: IIP3 measurement for filter at 3MHz
Post by Ken Kundert on Jan 2^nd, 2007, 8:50am

You should start by reading the paper on measuring IP₃: http://www.designers-guide.org/Analysis/intercept-point.pdf.

You should also check the date on you copy of SpectreRF. Several years ago Spectre used to occasionally produce results that varied with tstab, but I believe that has been resolved for quite a while now.

Finally, use PSS when there is one large periodic tone, use QPSS if there are more than one and they are not harmonically related.

On amplifiers you can get away with using PSS combined with PAC to measure IP₃.

-Ken

Title: Re: IIP3 measurement for filter at 3MHz
Post by aaron_do on Jan 2^nd, 2007, 5:07pm

hi all,

if the circuit has not reached the DC operating point, then the results for IIP3 are supposed to vary with tstab right? i.e. you need to know he settling time of your circuit...

Aaron

Title: Re: IIP3 measurement for filter at 3MHz
Post by Ken Kundert on Jan 2^nd, 2007, 6:47pm

No, the results should not vary with tstab. The PSS analysis should compute the steady-state solution. If there is only one, then tstab should not affect it.

-Ken

Title: Re: IIP3 measurement for filter at 3MHz....
Post by jobless on Jan 10^th, 2007, 10:44pm

Dr. Kundert,
Thanks a lot. The icfb version we are using is IC 5.0.33. Do you think it has the tstab problem which you were talking about in the earlier posts?

Title: Re: IIP3 measurement for filter at 3MHz
Post by Ken Kundert on Jan 10^th, 2007, 10:48pm

That is pretty old. I would not be surprised if that version of Spectre had the problem. You only need to upgrade Spectre.

-Ken

Title: Re: IIP3 measurement for filter at 3MHz
Post by aaron_do on Jan 10^th, 2007, 11:45pm

Hi Ken,

you said that the PSS analysis will compute the steady state solution right? Does that mean that if tstab is not long enough then the simulator will adjust tstab? i.e. simulation time will be longer...

I'm also using 5.0.33 and I found tstab affected my P1dB simulation and transient analysis revealed that tstab was not long enough for the circuit to reach steady state. I guess this is the problem you are talking about right?

thanks,
Aaron

Title: Re: IIP3 measurement for filter at 3MHz
Post by Ken Kundert on Jan 11^th, 2007, 8:17am

It is not necessary for the tstab interval to take you all the way to steady-state. It was initially envisioned as a way of allowing the designer specify that the circuit needed a certain amount of time to initialize itself. For example, a shift register might need a certain number of clock cycles after the reset was lifted before it was in the right state to support steady-state behavior. It was quickly found to also be a nifty way to help the convergence of the PSS analysis. It allows you to get the circuit to close to its steady-state behavior before starting the PSS. In that sense, it is kind of like nodesets and the DC analysis. If you give the DC analysis a complete set of nodesets that are close in value to the DC solution, then the DC analysis converges quickly and easily. The tstab interval is kind of like a way of automatically generating nodesets for the PSS analysis. And like nodesets, they do not have to be actual solution. Just like the DC analysis takes nodesets and uses it as a starting point in the process of calculating the true DC solution, so does the PSS analysis take the values at the end of the tstab interval as the starting point for computing the steady-state solution. Once PSS converges and produces a result, it is the steady-state solution regardless of how good or how poor the starting point was. The starting point (or tstab) should only affect whether PSS converges, not how accurate the result is.

Older versions of Spectre have a bug that makes the final solution overly sensitive to tstab in some cases. In these cases the results computed by Spectre will be inaccurate. Therefore you should upgrade to the latest version.
-Ken

Title: Re: IIP3 measurement for filter at 3MHz
Post by aaron_do on Jan 11^th, 2007, 5:37pm

thank you for clearing that up for me

Aaron