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Design >> RF Design >> Mutual inductance estimation
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Message started by butcher_tekkotsu on Jan 24^th, 2007, 1:58am

Title: Mutual inductance estimation
Post by butcher_tekkotsu on Jan 24^th, 2007, 1:58am

Hi,

Now I'm designing a single ended narrow band LNA. Operating frequency is 1.5GHz.
The topology is the cascode amplifier w/ inductor load.
I'm also using an inductor for the input gate for input impedance matching.

I'm conserned the unnecessary oscilation because of output to input feedback. :-?
So I'd like to estimate mutual inductance between input and load inductors from actual layout.

Does anyone know good method for that?

Title: Re: Mutual inductance estimation
Post by ACWWong on Jan 24^th, 2007, 4:33am

Most em simulators can give you the magnetic coupling between your spirals.
As a quick (free) tool which I have used in the past to minimise unwanted k by better placement of inductors is ASITIC. Other tools you might try are ADS(momentum), Sonnet, etc. etc. These tools won't though work so well on a completed layout... you'll need to extract the spirals out.
For layout based extraction, Andrew Beckett on this forum, mentioned not so long ago that Assura has a high frequency option available to extract L, not sure if Mutual is available.... but this might depend on whether your Assura deck supports the option ?

cheers
aw

Title: Re: Mutual inductance estimation
Post by sheldon on Jan 26^th, 2007, 3:10am

Greetings,

There are a couple of other options. SpectreRF includes a tool for
synthesis of passive components, VPCM, and it also has the ability
to analyze the coupling between inductors. Or, you can use the field
solver in VPCM to perform a high accuracy extraction.

Best Regards,

Sheldon

Title: Re: Mutual inductance estimation
Post by Andrew Beckett on Jan 26^th, 2007, 6:10am

The Assura parasitic inductance extraction can also extract mutual inductance. The requirement is that the Assura RCX files have the data to extract resistance - nothing extra is needed for inductance and mutual inductance extraction in terms of PDK support.

Regards,

Andrew.

Title: Re: Mutual inductance estimation
Post by mg777 on Jan 28^th, 2007, 8:12pm

Let's see if we can estimate this mutual inductance from first principles, the Neumann formula:

M = (μ₀/4π) ∫ dl1.dl2 / r

For simplicity take two coplanar square wire loops (one turn only) of side 'a' and distance 'd' between centers. By hand calculation I get

k = M/L ≈ a² / (π.d.(d+a))

For d = a (worst case) I get k = 1/2π = 0.16. If d = 2a then k = 0.05.

N turns should not matter as the N² factor would enter both M and L. Now that we know the spatial dependence we can sense that our estimate will upper bound the k for a spiral. It's a pessimistic upper bound, though. I'm guessing for d = 2a, a typical spiral would have k = 0.01.

Bottom line: if d = a (and it would be, right?) then you can't ignore mutual.

M.G.Rajan
www.eecalc.com

Title: Re: Mutual inductance estimation
Post by ACWWong on Jan 29^th, 2007, 3:33am

yes, mutual generally isn;t a problem, BUT when one is using several large spirals, the tendancy to pack them in as close a possible without thought coule be dangerous. So often "d" is only greater than "a" by as little as possible and its a rare test-chip luxury to have d>= 2a.

Title: Re: Mutual inductance estimation
Post by pancho_hideboo on Feb 15^th, 2007, 7:52am

Hi.

Coupling of inductors is influenced by surrounding layout patterns.
But VPCM(Virtuoso Passive Component Modeler) coupling analysis ignores them.
For example, ground or power metal lines could affect coupling.

It seems that VPCM don't use any file from "capgen" although VPCM reads procfile and p2lvsfile.
For VPCM, there is no meaning in the following sections inside "procfile".
metal_variations
endmetal_variations

metal_combinations
endmetal_combinations

Also VPCM ignores contact resistance defined in "p2lvsfile". :'(

We have to rely on Assura RCX.

Title: Re: Mutual inductance estimation
Post by sheldon on Feb 16^th, 2007, 8:31am

Pancho,

The original question was how to extract coupling between inductors, the
issue you mention is different. Depending on the the tools you have access
to and the PDK you are using, you might want to try the following:

Option I:
Use Assura as Andrew suggested. Assura is intended this type of application
and will allow you to include more of the circuit in the extraction.

Option II:
Use the VPCM full-wave solver, EMSS, to extract the coupling between
inductors and the surrounding layout. This approach is accurate and
useful for analyzing the inductor and a few critical nets, However, using
the full-wave solver limits the size of a circuit because of the time
required for the EM analysis.

Best Regards,

Sheldon

Title: Re: Mutual inductance estimation
Post by pancho_hideboo on Feb 16^th, 2007, 8:51am

Hi.
Starter of this thread, butcher_tekkotsu is my colleague.

His Concern is inductors coupling including surrounding layout patterns.
So we will adopt Assura RCX.

If we need more precise analysis, we use Momentum in RFDE not EMSS in VPCM.
We can choose and control thick metal modeling and via modeling scheme in Momentum.

On the other hand, EMSS is very unclear regarding thick metal modeling and via modeling.
As far as quasi-static mode, via resistance doesn't affect VPCM synthesis and analysis results at all.