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Question about Ansoft Designer Models - RF Cafe Forums

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john-ee

Post subject: Question about Ansoft Designer Models Posted: Tue Aug 23, 2005 7:06 pm

Lieutenant

Joined: Tue Aug 23, 2005 6:33 pm

Posts: 3

Hello everyone. I'm new to the forum and like what I have browsed so far.

In a nutshell, I'm designing a 480MHz square-wave oscillator circuit somewhat resembling PECL logic with the inclusion of a T-Line for delay and was planning to use the Infineon BFP405 due to its high 25GHz fT, I have used it previously in other amplifier circuits, and I still have a few lying around.

When analyzing in the frequency domain, everything is great, but for the square-wave design, I have been using the transient analysis in the time-domain and have found that the simulations take forever and I get horrible ringing in the output.

However, when I use the BFP620, with a 70GHz fT, the simulations fly through the transient analysis in no time and produce nearly perfect results.

The BFP620s seem hard to come by and I don't want to waste time building this circuit and have it perform horribly due to some overly simplistic model, but I also cannot spend 20min waiting for each new simulation run with the BFP405 only to get the ringing.

By the way, I am taking into account and testing different termination techniques on the T-Line.

Any comments or help would be appreciated.

Thanks,

-John-

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Guest

Post subject: OscillatorPosted: Tue Aug 23, 2005 7:27 pm

Hi!

What simulator are you using? Whose models are you using?

Assuming that you're using SPICE, the standard versions generally take a long time to simulate oscillators in particular - and it's made worse by inadequate vendor models. And you really have to model your own parasitics - no way around that!

Even though it's not aimed at your application, an example of a version of SPICE that's been optimized for oscillators is Linear Technology's SwitcherCAD/LTSpice. They've gone in and set the various parameters that most users don't even know about, in order to get fast simulation for their switching power supplies.

If you've got the '405 amplifiers on hand, maybe prototyping would be a good approach - if you have the necessary instruments. (You'll want them for the final product verification, anyway...)

Good Luck!

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john-ee

Post subject: Posted: Wed Aug 24, 2005 11:39 am

Lieutenant

Joined: Tue Aug 23, 2005 6:33 pm

Posts: 3

I'm using Ansoft Designer and the associated models that come with it.

-John-

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Graham

Post subject: Posted: Tue Oct 25, 2005 8:10 am

Colonel

Joined: Fri Sep 02, 2005 7:25 pm

Posts: 34

Location: Hampshire UK

I can attest to the SWCAD speed. I used it to check out how far one could press things with a two-FET symmetrical oscillator which I knew would start, but simulation attempts were failing. The SWCAD ran straight off, and allowed me to sim the first few microseconds (startup) at 670MHz. Also - its free!

It has to be said that it does not come with easy bells and whistles if you want to put in any models it did not come with. You have to get down and dirty with the netlist file.

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