ali kotb wrote on Feb 24th, 2012, 11:54am:iam new to this site , hope i get the best of it
iam trying to make a crsytal oscillator as reference for PLL
the crystal has fundamental 40Mhz which is not suitable to work at for certain specs in PLL, it should work at 3rd overtone (120MHz)
i need help in modeling the xtal plz
knowing that co (shunt cap)=5pF
cload=15PF
i got the values at 120MHz : Lm=3.52m , cm=0.2f
actually it is not oscillating because at high freq impedence of sunted cap c0 is too small (shorting the series Lm , cm , Rm)
the solution is to use another Lo to be shunted to co to remove that effect but the value of Lo would be 0.35u which is not integrated on chip , what can i do ? help me plz
can any one post me a practical on chip schematic for such oscillator, thnx
With a little research you can find the equivalent LC model for a crystal.
Essentially, it is nothing more than a tuned tank, but with a very high Q
(roughly 10E5 if I recall.)
Now comes the interesting parts:
In a simulation environment, the Q is so
high that it will not simulate easily and quickly.
So you may want to adjust the resistance to represent a
value of Q = 100 (give or take)
That will make the initial simulation work easier to run.
Now, one other thing with the device is starting the oscillation
process for test and simulations.
Use an initial condition of a DC current in the inductor
that is the electrical equivalent of the crystal.
The start of oscillation process in a crystal is very slow and
takes a long time to simulate (you need to introduce noise
into the model in order to get the oscillator to start.)
The IC of current in the inductor easily gets you around this.
Try it, you will like it.