crystal circuit

[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?????

 

[MikeMl]

I guessing (since I am not familiar with either of the simulators you are using) that the crystal model X1 is a parametric model, and it is waiting for you to specify the crystal frequency (as a parameter) to the underlying crystal simulation model. It is like a resistor model. You don't have a different models for hundreds of standard value resistors; instead, you have a single resistor model, and you pass it a parameter which specifies the resistance.

 

[dr.mohandes]

MikeMl you're right. in proteus there is just one model for quartz crystal and i specify its frequency but i see zero in oscope. but i now the output of my circuit is a sine wave in the frequency of 40 MHz
thanks for your attention

 

[dr.mohandes]

MikeMl you're right. in proteus there is just one model for quartz crystal and i specify its frequency but i see zero in scope. but i now the output of my circuit is a sine wave in the frequency of 40 MHz
thanks for your attention

 

[RCinFLA]

Because quartz crystal resonators are such high Q the simulation startup can take a very long time. A real crystal oscillator may take 300 usec to 700 usec to come to steady state. If you have you simulation scope set to see nsecs based on oscillator frequency you will have a long wait.

Just make sure you let the simulation run for 700 usec in simulation represented time before you give up. It eats up a lot of memory and may take 10-15 minutes of computer run time depending on the speed of your machine.

 

[dr.mohandes]

dear RCinFLA
in proteus can i set my simulation run? the scope in proteus can,t show times lower than 0.5 usec.
in multisim the scope can regulate for lower times until 700 nsec. but the output singnal is just 7 Mhz. i have changed the frequency of the crystal (for example 32.768) but the output doesn't change just it shows 7Mhz.

 

[RCinFLA]

Your problem is the loop gain is higher for the LC components then the crystal so the oscillator is running at the LC combination and the crystal is just acting as a capacitor. (If you replace the crystal with a 6 pF cap you will get close to the 7 to 8 MHz oscillation you are now seeing).

LC oscillator, having a much lower Q will start up much quicker then a crystal oscillator in simulation.

Your inductors are very unrealistic for the 40 MHz frequency. They are appropriate for chokes at 40 MHz.

Multisim allows you to manually set the compution points in time. Sometimes this works better then allowing it to automatically select what it thinks is the necessary time increments between calculations. I usually always set compution increments manually when simulating oscillators. This is not your problem at present, it is the inductor values.

A 32 kHz crystal can take several seconds to startup in the real circuit so simulation is a very long time.

A 40 MHz crystal falls into the frequency zone that it could be fundament mode or third overtone mode crystal. I assume you are expecting it to be a fundamental mode. You need to give us some more info on the crystal spec. Is is fundamental mode for 40 MHz? Is is cut for series mode operation or parallel mode? If parallel mode what is its expected load capacitance for spec'd frequency?

If all you have is the component in software stock library look at the model. What is Co, Cm, Lm, and Rs in the model?

Attached is a colpitt osc for third overtone 40 MHz crystal. If crystal is truely a fundamental you can just eliminate (short out) the warp coil and eliminate resistor in parallel with crystal and run it as parallel mode. Circuit in this modified config will present about 10 pF of parallel load capacitance on crystal run as fundamental mode requiring 10 pF load on crystal to achieve stamped crystal frequency.