Hi,

I am trying to design a crystal oscillator with AGC circuit built in, in order to decrease the total power consumption of the circuit. The difficulty we are facing right now is the stability analysis of the whole loop.

The topology is a classical Pierce oscillator, where a Pmos current source feeds current into a Nmos device. This Nmos device acts as a gain element. We are AC coupled to the crystal signal. After AC coupling we detect the peak of this oscillation and compare the peak value to a known reference voltage. Depending on the difference between the peak value and the reference voltage, we adjust the value of the Pmos current source. Since the whole system is pretty non linear, an AC analyis in Spice is unsufficient.

Does anybody have any suggestion how to analyse this kind of circuit for any potential unstabilities? Any help is welcome, even a title of a paper.

Thanks.

Thanks ACWWong.

I went through the papers by Vittoz. There is not much about the stability analysis as far as I can tell.

What would be the concern, if we artifically de Q crystals to speed up the simulations?

That is a good point.

Thanks for your inupts.