simulation of crystal oscillator problems

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guyrib

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Hi !

I need help with simulation that i did in ltspice.
the first simulation (file that called simul ) i use inductor instead a crystal and the simulation works good.
now i want to put the crystal and a resistor that give me dc feedback (1Mohm usually)
i tried this in file xtal1 for 8MHz but it doesent work properly.

thanks.
 

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What do you mean?

The plot is exactly what I'd expect to see when a crystal is energised: it'll ring for a fairly long time and die away exponentially, just like any underdamped third order system.
 
The 1k ohm resistor in parallel with the crystal represents too much damping. I don't quite understand where the gain comes from in your simulation, but it seems to work when there is just an inductor.

You could try without either resistor. Obviously the series 30 ohm should be a link for no damping.
 
Your parallel resistor value is too low, and is lowering the Q of the circuit. Try 1Megohm, or even higher.
 
You need an amplifier, there's nothing wrong with the simulator, it's your circuit that's wrong.
 
Hero999 said:
You need an amplifier, there's nothing wrong with the simulator, it's your circuit that's wrong.
Click to expand...

Actually, his circuit is bizarre, but will oscillate if the conditions are right. The Behavioral Current source switches polarity based on the sign of the voltage at node 1, which creates a "negative resistance", which in turn pumps the HiQ tuned circuit. By paralleling the low value resistor, the Q is lowered, and the current source cannot supply enough energy to sustain the oscillations.
 
Last edited:
Yeah, I had concluded that the behavioral current source was some sort of amplifier. Otherwise, the LC circuit could not have produced the waveform he posted.
 
I missed the behavioural current source but what type of component does it simulate?

Can you build your own behavioural current source in real life?
 
Tunnel diodes exhibit negative resistance; transistors or an opamp can be configured as a "Gyrator", which is a form of negative resistance device. Actually a conventional Colpits or Clapp crystal oscillator can be thought of a Gyrator, because one end of the crystal is grounded, and the crystal is not in the feedback path around an amplifier, like it would be in a Pierce circuit.

(Example circuit source)
 

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