Voltage controlled capacitor

Hello everybody,

I am a newbee in this forum, but I really enjoyed following your conversations.
Here it is my question. I was wondering if it was possible to simulate a "voltage controlled capacitor" in ADS.
Essentially, I would like to be able to change a capacitor value according to an external voltage (not to the voltage across the same capacitor, like in a varactor). Then I would like to run some Harmonic Balance and noise simulations of my circuit.
Many thanks in advance for any hint!

Define an equation that relates your voltage changing to capacitor value and put this variable in capacitor component as its value.
For instance MyCap=0.5*Vtune+10*log(Vtune) etc.Then put this variable into cap value like C=MyCap.
It's so straightforward..

Dear BigBoss,

many thanks for the prompt reply.

Actually, I have already tried what you suggested without good results (also with R, L, etc).

I am sure I am missing something, but if I define an equation using the name of a node voltage i get the error "expression has not been defined" for the component value.

Many thanks in advance for any other hint.

It seems to me that in ADS VarEqn can only be used to initialize some value before the actual simulation, and MeasEqn to calculate a formula based on the simulation results.
Is there any way to feed a component value (like a capacitor capacitance) with a node voltage value during the simulation itself (in a sort of closed loop)
Thanks!

But you should assign an initial value for that voltage.
For instance
Vtune=1
MyCap=0.5Vtune+10*log(Vtune) after that C=MyCap..

So your capacitor value will be changed in according with your formulae.

Dear BigBoss,

I already tried your solution, but it doesn't reflect my needs. In your example the capacitor value will reflect the assigned initial value, but not the value calculated by the simulator (for example, as already mentioned, a voltage in a node of the circuit).

Anyway, after some search in the litterature, it seems the problem is not really straightforward. Now I am trying the so called Symbolically-Defined Devices... I'll let you know.
Thanks.

Thanks also for the attached pictures. Anyway, as said before, your simulation simply sweep a parameter (modified by the equation), does not use a voltage value (or current) from the same circuit.

Hello everybody,

i just solved my problem using the so called "Symbolically-Defined Devices" available in the "Eqn. Based NonLinear" library of ADS. For all of you interested in doing these kind of stuff, I found very inspiring this doc.: http://cp.literature.agilent.com/lit...d_Devices.html
Have fun! :)

Hi augustot,

I would like to design varactor diode in ADS. However I want to control varactor diode with time domain signal or node voltage from same circuit. I used SDD block to model varactor diode. But I could not solve the problem. Could you send the ADS schematic of the solution of your problem ? Thank you.