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Design >> Analog Design >> Varactor simulation
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Message started by Visjnoe on Sep 14^th, 2007, 6:18am

Title: Varactor simulation
Post by Visjnoe on Sep 14^th, 2007, 6:18am

Dear all,

I have a simulation set-up problem when it comes to simulating a varactor (actually 2) of which I want to determine the capacitance versus the tune voltage using following SPICE setup:

Code:

Xvar1 tune test nvar_rf finger=nf w=wvar l=lvar
Xvar2 tune vss nvar_rf finger=nf w=wvar l=lvar

Vtune tune vss dc 'vtune'

Itest test vss dc 0 ac 1

.ac dec 100 100 100e6 sweep vtune -0.9 0.9 0.1

The problem is that the Vtune source acts as a short to ground during an AC analysis. Therefore, my setup returns the effective capacitance C of 1 varactor instead of the correct C/2.

I can solve this problem by inserting a 1GOhm resistor between the vtune source and the controlling node of the varactors. I was wondering if there is another approach to tackle this (setup) problem?

Kind Regards

Peter

Title: Re: Varactor simulation
Post by monte78 on Sep 14^th, 2007, 8:51am

Visjnoe wrote on Sep 14^th, 2007, 6:18am:

Xvar1 tune test nvar_rf finger=nf w=wvar l=lvar
Xvar2 tune vss nvar_rf finger=nf w=wvar l=lvar

Vtune tune vss dc 'vtune'

Itest test vss dc 0 ac 1

.ac dec 100 100 100e6 sweep vtune -0.9 0.9 0.1

Hello Peter,

I think a better solution to bias a generic circuit and to exclude this effect in in the ac analysis is to put a large inductance instead of the 1GOhm resistance (1T Henry for example...). By this way the dc bias will be correct while during ac analysis the inductor can be considered as an open circuit.

Bye
Monte