tuning curve vco
Thanks for the reply.
I am using a
MOS transistor in inversion-mode as varactor, due to its scalabillity with lower supply voltages, and I know Q-factor may be higher for the MOS varactor, than for the reverse biased diode varactor.
If i increase the length of the MOS transistor, the capacitance range will increase due to increase in the transistor areal, and also the resonanse frequency will be shiftet to a higher frequency.
However, the increase in the MOS transistor length will degrade the LC-tank Q-factor, due to increased of resistance in the transistor channel.
Also, i don't think the Q-factor is the main problem for the ideal w=1/sqrt(Lc(v)) characteristic, but rather the large amplitude swing at the oscillator output, that influence the effective capacitance of the varactor.
This tank amplitude can be controlled by the bias current, when in the current limited region.
I have tried to reduce the tank amplitude to approach the w=1/sqrt(L(C)), and to some extent the tuning curve of the oscillator was corrected..... but still, there is need for improvement, so im wondering what other steps that should be taken?
For instance, how should the MOS transistor lenghth and width be set? (I know this depends on what what resonsanse frequency, and tuning range i want, but to obtain the ideal characteristic is the primary objective.).
Still hoping someone can give me some ideas
Saber890