Hi,
I am designing an RF VCO with switching capacitors, whose oscillation frequency is around 3GHz.
The switching cap cell in the off-state is simplified as:
![](http://img405.imageshack.us/img405/8104/swcap.png)
The voltage
VD is precharged to a value near 0.8 V to assure the NMOS is off when its gate is switched to ground.
However, when I do the transient analysis and set a very long stop time, say 10us,
the voltage
VD starts to drop to near 0V.
I don't think the voltage drop comes from the leakage current of the NMOS, if so,
the voltage should be pulled up to 1.2V but not drop to 0V.
I also find that the speed of voltage drop is related to the accuracy of transient analysis.
The simulation results is shown below
(liberal, moderate, conservative are the errpreset, pointlocal is conservative with relref=pointlocal, reltol=1e-5 is conservative with reltol=1e-5
The dropping speed is liberal>moderate>conservative>pointlocal>reltol=1e-5)
![](http://img403.imageshack.us/img403/3638/swcapaccuracy.png)
I cannot figure out how the accuracy setting affects this simulation result.
Does anybody have any idea about this phenomenon?
Thanks in advance.
YCY