Problem with the results of simulating diff. VCO at 0.15u

[cmos_dude]

VCO help

Hi All
I have designed a differential VCO at 0.15u tech. I simulated the design by gradually increasing the control voltage and got oscillations where the oscillating frequency is increasing with the increase in the control voltage.
but the oscillating swing decreases at low control voltages.
I have attached the simulation result pic.
Please look into and tell whether the simulation results are what they are supposed to be.
Any kind of inputs will be highly appreciated.

Regards
cmos dude

 

[willyboy19]

Re: VCO help

We can not give you any useful feedback without looking at your circuitry first -- it is needed to debug a problem in the design.

 

[neoflash]

VCO help

Normally, good question will result in good answers. Do you need junk answers?

 

[gdhp]

Re: VCO help

i think you can add a level shift at the output of VCO, such the output of vco will tuggled with 0 to vdd.

Added after 3 minutes:

i think you can add a level shift at the output of VCO, such the output of vco will tuggled with 0 to vdd.

 

[cmos_dude]

Re: VCO help

Hi All
I really apologise if my earlier post was not clear enough.
Here is the differential cell that I am using for my circuit. It is a ring oscillator design. I am also confused as to what should be the input voltage at the Vcp terminal in the differential cell. Presently I am keeping it at a constant voltage.

Regards
cmos dude

 

[paulux]

Re: VCO help

You can keep control voltage constant to see the oscillation. Maybe you can check whether the differential pair is out of saturation region or not.