[Moved] problem with crystal circuit. help!!!!

[dr.mohandes]

hi
i want to simulate the circuit has shown below in Proteus but it doesn't show any thing in oscilloscope. i think the problem is with crystal. i think it doesn't work.
i have simulated it by multisim. i see a sine wave on oscilloscope but not in frequency i want(40MHz). in multisim when i change the frequency of crystal the output doesn't change. it look like the crystal is not activated. how can i activate it???
what about proteus?????

**broken link removed**

 

[bigdogguru]

Re: problem with crystal circuit. help!!!!

Concerning Proteus:

Have you right clicked on the crystal and set the required frequency with the device properties?

Also, have you started the simulation before attempting to analyze the signal with the scope?


BigDog

 

[dr.mohandes]

yes i have set the frequency of the oscillator on 40.68 Mhz before but in scope it showes just zero. the output of this circuit most be a 40.68MHz sine wave. the simulation file is attached.

 

[jiripolivka]

Not all crystals may work in your circuit. Please remove the crystal and replace C11 with it. It should oscillate and by adjusting the emitter capacitance you can select the best output for your 3rd overtone.

 

[E-design]

To get crystal oscillators to simulate can be tricky. This is due to the lengthy start-up time determined by the high Q of the crystal. It may take as much as 30 to 100mS to start, and you need to set your minimum time step at least 10 times smaller than 1/freq which explains the long simulation times. There are some tricks like injecting a small signal at the crystal frequency into the circuit via a high impedance to get it going and then removing it once oscillations are steady. This can be done via a manual or time operated switch.
Search the Labcenter forum for more crystal oscillator start-up tips. http://support.labcenter.co.uk/forums/viewforum.php?f=9

 

[RCinFLA]

Assuming you finally get past the simulation issues, the inductors within the circuit are very non-realistic for 40 MHz operation. 10 uH to 15 uH physical inductors will be very difficult to achieve at 40 MHz as just the parasitics, either of the coils themselves or stray capacitance of the circuit layout, may cause the coils to be near or past self resonance at 40 MHz.

 

[dr.mohandes]

dear friends
do you know any other simulation software could simulate these kind of ciruits which use high frequency crystals upper than 40 MHZ ??? its important for me to set the frequency of crystal in any value i want???

 

[E-design]

You can create a model for almost any crystal you want. You can either get the parameters from the crystal manufacturer or tweak existing models to suit your needs.