The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design Languages >> Verilog-AMS >> ddt during ac analysis
https://designers-guide.org/forum/YaBB.pl?num=1202481407

Message started by king on Feb 8^th, 2008, 6:36am

Title: ddt during ac analysis
Post by king on Feb 8^th, 2008, 6:36am

Hi,

I'm new to ac analysis in Verilog.

Models in Verilog has

Q = ddt(q)
I(p,n) <+ Q

What happens to ddt(q) during an ac analysis?
Can I display the value of Q as a function of f
by strobing Q while running an AC?

thanks.

Title: Re: ddt during ac analysis
Post by Ken Kundert on Feb 8^th, 2008, 2:55pm

ddt() effectively gets replaced by jω in an AC analysis. You cannot strobe during an AC analysis because the model is not actually executed during the AC analysis. Rather it is evaluated during the preceding DC analysis and linearized.

-Ken

Title: Re: ddt during ac analysis
Post by king on Feb 12^th, 2008, 4:34am

Thanks Ken.

In your varactor example, how will I then
check the frequency characteristics of q, ddx(q)
and i(p,n)?

Title: Re: ddt during ac analysis
Post by Ken Kundert on Feb 12^th, 2008, 12:04pm

I don't understand your question.

Title: Re: ddt during ac analysis
Post by king on Feb 13^th, 2008, 4:24am

after an ac analysis, the only outputs are the
Re and Imag parts of node currents and voltages
as a function of frequency.

how can i output other variables like
capacitances as a function of frequency?
in varactor, for example,
how can i extract dq/dV as a function of freq?

Title: Re: ddt during ac analysis
Post by Ken Kundert on Feb 13^th, 2008, 9:14am

Isolate the capacitor, drive it with a voltage source that has an AC magnitude of 1, and plot its current. The current will equal the admittance. Divide by jω to get the capacitance.

-Ken