need help for LC-VCO on @DS

[Rekresreb]

Hi, I am newbie for RF design, now I am making a simplest cross-coupled LC-VCO that can be find in any paper.
I made the output voltage oscillated alright, and I want to plot frequency vs control voltage curve.
I think I should use Harmonic balance and the OscPort in order to do it, however I have no idea how to put my OscPort.

anybody can tell me how to do it? thanx!

 

[khouly]

put the Oscport in serise with tank circuit "in the feedback path"

and use harmonic balanced simulator and sweep variable vtune
and plot the frequency

 

[Rekresreb]

thanx for replying!
I am not very sure about where the feedback path is :?
the attachment picture is what I did... please tell me if it is right
and somehow the frequency vs vtune looks weird...
any idea why?

 

[divan]

something is not correct , frequency must be increased with control voltage .

 

[Rekresreb]

yes, I know it is wrong...
but I don't know why this happens, anybody has any idea?

 

[CTT]

Hi,

did you use the differential Oscport ? I can't see your picture so I can't
really tell you what's wrong in your schematic. The feedback path is the connection between the differential LC tank circuit and the negative cross coupled Gm that you use. A way to have things work correctly is to connect the two negative input of the differential oscport to the the differential transconductor and the two positive output of OSCport to the tank differential input. In harmonic balance, you should use specify "yes" for the oscport name in ADS2003A or C.

I'm not sure about your frequency vs. vtune curve. The slope depends on P or N well varactor if you use MOS varactors.

Let me know if this work,

CTT

 

[khouly]

look to this photo and see , where is the oscillation port
the feedback path is the inductor and varactor

and about the varactors u using it a accumulation mode varactors
i think it is better to use inversion mode varactor