3rd overtone xtal modeling

[ali kotb]

i'd like to ask how i can model the 3rd overtone crystal osc.
40MHz fundamental

do we need 2 branches parallel to each other (each contain L,c, R) for fundamental and overtone
or 1 branch is enough ??

if 2 branches , can any one help me with equations for L1,L2, C1, C2 ?

thank u

 

[RCinFLA]

You only need to have the parameters for the third overtone but you need to be aware the loop gain of the circuit must be lower then unity for the fundamental frequency so the oscillator does not start up there.

Third overtones crystals are run in series mode. Placing a 1k resistive load across crystal solves the most common issue of oscillator starting up in parallel mode at fundamental resonance.

Computer simulations on crystal oscillators are usually very slow with a long time to achieve startup to steady state since the crystal Q is very high. You can speed sim by placing a small additional resistor with low level AC source in series with crystal at intended oscillation frequency.

 

[ali kotb]

u mean when iam modeling the xtal , i just model it as 1 L, C, R where L and C resonates at 3rd overtone freq not the fundamental.
but if i did this simulator recognize this freq as fundamental not overtone ,

 

[RCinFLA]

See attached.

 

[ali kotb]

i got this schematic , but actually iam trying to make something integrable on chip , the inductor caps here are too large to be integrated on chip ,
actually iam now interested in xtal modeling itself , not the osc.ct
thank u

 

[Diver300]

You would have two separate branches, plus the standing capacitance, C0

The ESR for the fundamental would be a lot less than the ESR for the overtone.

The C1 for the overtone would be around 10 times smaller than the C1 for the fundamental.

 

[ali kotb]

thank u

 

[ali kotb]

hi guys
what about a resonance amplifier to be tuned at the 3rd overtone and suppress the fundamental,it will be placed in feedback loop with the xtal oscillator
can any one advice me of such design
it should be RC resonant amplifier , i need it coil-less