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Any ADS gurus to help create a time-domain avalanche transistor simulation?


Full Member level 6
Dec 7, 2010
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Canberra, Australia
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Good evening folks,

I'm helping put a course together using avalanche transistors to explore time domain reflectometry etc techniques and I could really use a hand with creating a simulation model based around ADS. I've packaged up the (paid) task and posted it to freelancer(.)com for anyone with skills to spare in the next 2 weeks or so.

Since posting the job my students have told me there are much newer versions of ADS out there now, as well as a stack of other simulation tools that may be suitable. At this stage I don't actually care what it's simulated with (*except* LT/pSpice) - as long as we can numerically capture the transistor's interaction with surrounding transmission line structures.

/beg-for-help :)
Please note that if the transistors have been modelled by SPICE, transient simulations in ADS might create some troublesome convergence errors beyond of breakdown region. Because transient simulation capability of ADS is not so perfect and it creates frequently convergence problems.
In fact, as a RF/MW Engineer, transient simulation is not my preferred and lovely tool but I believe Cadence transient is slightly powerful than ADS.
I would offer that the task and methods of modeling are
also ripe for learning experiences, and perhaps more time
spent on that path would benefit your students (not much
more useless, than a designer who doesn't know their
devices, how to tell whether models express the reality at
hand, and how to move forward when corporate CAD
lets them down).

If you have a time domain result then fitting a macromodel
(subcircuit to it) should be practical. I would use a diode
model (w/ reverse params populated) across the transistor
C-B, and start nudging params.

From what I've seen of freelancer projects, I don't bother.
My days are full and my rate is high.

But if you take an interest in making this a learning project
in itself, you can find advice here.
Does standard bipolar SPICE model (Gummel-Poon) cover avalanche breakdown? I'm not aware of, please correct me if I'm wrong.
We would rather use a behavioral avalanche generator model. Generator modeling is only loosely related to the TDR circuit, I think.
Heya folks, cheers for the feedback - it's been years since I've been in this forum and it's nice to see many of the familiar faces still around :)

To cover off on some of your suggestions:
* Aye, the freelancer site doesn't seem an encouraging place to do professional engineering unless you really feel that $12 is adequate compensation for 40 hours work! I confess this job was a way to use funds previously uploaded (and I deemed otherwise irretrievable ;) ...and maybe some good will come of it.

* The goal of the exercise (for the students) is:

1. I give them an SMA connector that I've built the Williams pulse generator on. It's a marvellous configuration that costs nothing, *just works* and launches lovely short pulses into whatever transmission line you screw on.
2. From here they can look at the oscilloscope while crushing the coax cables, loosening connectors, transitioning through other adapters, trying different grades of coax, open/short circuits etc. The objective is to implant an intuitive feel for transmission lines and their behaviour.
3. The simulation is actually beyond the class scope (and available time and their present understanding), but I want to be able to wave my hands at a real model with transmission line elements now familiar to them (after their experimentation) and encourage them to see the intellectual connection between the physical and simulation domains (and plant the seed of curiosity). Twiddle numerical parameters, and see the same effects they just observed physically.

* I can't comment on how LTSpice internally uses the published ( avalanche models, but the supplied model exhibits *remarkable* agreement with what I measured on the workbench. I'm generally of the "Bob Pease spice opinion" group ( so I'm genuinely impressed & gobsmacked!

* I know absolutely nothing about Cadence. Sounds like I've got some reading to do!

Thanks all :)

Spice simulations are fine but the problem is some people get stuck with them.
The same problem is using any other simulator.
I know start-up companies which went bankruptcy because their design engineers got stuck with one simulator.

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