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Measurements >> RF Measurements >> Large Signal S-Parameters
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Message started by Ken Kundert on Sep 12^th, 2002, 1:54pm

Title: Large Signal S-Parameters
Post by Ken Kundert on Sep 12^th, 2002, 1:54pm

I have been hearing about large signal s-parameters for years, but have never seen a precise definition. Can anyone give the definition or point me to a paper that defines them and describes how they are used? I'm also interested in finding out how commonly they are used.

Thanks,
-Ken

Title: Re: Large Signal S-Parameters
Post by flip_flop on Sep 12^th, 2002, 7:32pm

Ken,

You can try to read this:
"Computation of large-signal S-parameters by harmonic-balance techniques
Rizzoli, V.; Lipparini, A.; Mastri, F.
Electronics Letters , Volume: 24 Issue: 6 , 17 March 1988
Page(s): 329 -330"

As for me, I like the following definition: "a brutal attempt to force nonlinear circuits to obey linear circuit theory" ;)

Title: Re: Large Signal S-Parameters
Post by Ken Kundert on Sep 16^th, 2002, 8:37am

Thanks flip_flop. I love the quote.

Once one has the large signal s-parameters, what does one do with them? Can you apply the standard small-signal metrics (max available power gain, stability factor, etc.)?

-Ken

Title: Re: Large Signal S-Parameters
Post by Bob Pau on Sep 18^th, 2002, 11:01pm

Hello Ken,
>
> I have difficulties to reply from the website you attached below, so I
> do it here.
>
> LSSP is originally for matching purpose, because s11 and s22 is major
> parameters determines operation point of PA. Also the harmonics
> distortion, and most important, the output power.
>
> Since you can measure S-para of PA at high power level directly, then
> we use an indirect format to measure S11 and S22, in this way, s21 and
> s12 is not measured. Actually, S21 is wrong anyway (or not valid) in
> testbench measurement.
>
> The indirect measurement is connect PA input and output with two
> slotted line stub, (it behaves like our port-Adaptor), then we tuned
> two slotted line to different power level, and then disconnect the
> slotted line, we measure the slotted line instead. On end of the
> slotted connected to source is terminated to 50Ohm, the other end of
> slotted line which is connected to the PA is actually s11*. We do the
> same thing for output.
>
> Since PA Z0 is usually working below 10 Ohm, so that the Smith chart
> is actually normalized to 10 Ohm or PA Zo instead of 50Ohm.
>
> The beauty of smith chart is only A Z-plot, but also a Reflection
> coefficient plot, and VSWR plot.
>
> The input and output impedance of PA is determined in this way. It
> cannot use to do stability or other small signal S-para features. Of
> course there is way to extract the linear S-para from Non-linear one,
> but no body do that.
>
> The modern application of LSSP is mainly for modelling of PA (input
> and output Impedance modulated the behavior of PA) and give a clue of
> ACPR and IM3, because you can find a sweet spot of input and output
> matching condition that product maximum power, highest PAE and minimal
> distortion (ACPR), etc.
>
> To understand more, you can search MTT-S proceeding, conference, there
> are lot of stuff on this topic. Following I attach one for you.
> (loadpull a.k.a. LSSP)

Title: Re: Large Signal S-Parameters
Post by Adrian Sutinjo on Mar 27^th, 2003, 9:32am

I refer to p 14 of Cripps book "RF Power Amplifiers for Wireless Communications".
The whole of chapter 1 of the book does an excellent job of clarifying misconceptions such as large signal S parameter use for PA designs :).

The only large signal S parameter that I find
useful as an approximation is Large S11, which is really the input impedance under the desired drive power. This assumes, of course, that the drive is sinusoidal. In some cases it is not. So, use with care.

Regards,

Adrian

Title: Re: Large Signal S-Parameters
Post by RC on Dec 29^th, 2005, 9:13am

i use LSSP in ADS very often (last job), and the measured result is quite good. it is for finding matching indeed. i may need to run PSS/PSP for this purpose in spectre.

Title: Re: Large Signal S-Parameters
Post by Ken Kundert on Dec 29^th, 2005, 9:22am

Spectre's PSP does not compute large signal S-parameters. Rather, it computes the small-signal S-parameters when frequency translation is involved. So the SP analysis would be used to compute the S-parameters of an amplifier, and PSP would be used when analyzing mixers.

To compute large signal S-parameters with SpectreRF, you would use a simple PSS analysis.

-Ken

Title: Re: Large Signal S-Parameters
Post by Andrew Beckett on Jan 2^nd, 2006, 10:57pm

There's now an LSSP wizard in ADE to set up the PSS simulations and post-processing
to output large signal S parameters. It's under Tools->RF->Wizards->LSSP; however, it
currently requires a "magic" setting in your .cdsinit file since it is in engineering release.

It's covered in the PA tutorial in the MMSIM60 USR2 installation -
<mmsimInstDir>/tools/spectre/examples/SpectreRF_workshop
The example there has the setting in the .cdsinit file needed.

I don't recall exactly when it was introduced, but it's there in IC5141 USR3/MMSIM60 USR2.

Regards,

Andrew.

Title: Re: Large Signal S-Parameters
Post by mg777 on Jan 19^th, 2007, 11:08am

Thanks for the question & the replies. Is it correct to say then that large signal S11 is a sort of average reflection as a sinusoidal signal takes a large excursion about the bias point? That is, the answer is sensitive to the ccdf of the signal, which for a sine is sec^-1 or something like that. What if the signal was an RF carrier with a different ccdf, like OFDM?

M.G.Rajan
www.eecalc.com

Title: Re: Large Signal S-Parameters
Post by PWDN on Jan 30^th, 2007, 11:42am

Only large signal S11 generated by either ADS or SpectreRF LSSP simulation is useful. Large signal S22 would be useful too if it was calculated under input-driven condition (of course, in that case, S22 would be a function of input power, but so is S11), but both ADS and SpectreRF LSSP would set the input to be passive (meaning just 50 ohm termination) when calculate large signal S22. I don't know how other people make use of that S22. :-?

Title: Re: Large Signal S-Parameters
Post by Ken Kundert on Jan 30^th, 2007, 4:36pm

I would modify you statement to say that only large signal s₁₁ and s₂₁ are useful.

If you want s₂₂ with the input driven, that might be a job for PSP. In that case you would apply the input and perform a PSS analysis. Then you would perform a PSP analysis and examine s₂₂. This would compute the output small signal reflection coefficient when a large signal is present due to the input.

-Ken