loose-electron wrote on Jun 18th, 2009, 6:47pm:IMHO - the best way to start a crystal quickly may be a system with a VCO, (think a current into a capacitor and a ramp slope to control the VCO) that you sweep across the band, and monitor the amplitude coming out of the crystal. When the amplitude spikes (Q = 10E5 here!) you are on frequency, and freeze the VCO.
Hi Jerry,
While this sounds like a good way of atleast speeding things up a bit, won't it also suffer from the same fundamental issue that Yawei already pointed out? I imagine that the amplitude of the signal coming out of the crystal won't really change that fast as you sweep the slope of the VCO control voltage.
Basically, the high Q of the crystal means that it does not lose much energy quickly (or gain much energy), i.e. the crystal has enormous inertia. Whatever you do, it takes an almighty effort to force an amplitude change. I don't see how you can get around that.
In my opinion, you can only speed up things in the sense that you avoid using suboptimal methods which themselves would slow down the process of the crystal reaching its steady-state. So, using the amplitude regulation schemes like in Vittoz's paper may be a way of speeding things.
Best regards,
Vivek