Hello . Can someone help me to design a collpits oscillator who generate a sine wave with 12.5 MHz frequency? .

 

Why does it have to be a Colpitts oscillator?

 

Are there simpler oscillators that achieve the same result?

 

What are your design goals -

1) Absolute accuracy
2) Spectral performance
3) Amplitude
4) Load description, what are you driving with it
5) Drift
6) Power available
7) Any mechanical constraints, like baord space, volume of circuit
8) Ambient T range

Regards, Dana.

 

It should be a simple project . It has no practical purpose . It's only objective is toate output sine wave with a frequency of 12.5 Mhz , the amplitude of the sine wave is not really so important .
I want to implement the circuit on a test board and to see the output signal on the oscilloscope.
Thank you for your quick response !

 

If the only purpose is to look at it with an oscilloscope, why does it have to be 12.5 MHz?

Bob

 

Here, try this:



Works in simulation anyway.

Bob

 

Thank you a lot! It has no practical use , It is an assignment that i have to complete..
I have tried to create such an oscillator circuit and i've watched some tutorials on the internet but i couldn't understand exactly what was going on . Also our professor didn'd give us a lot of information ,he said to look it up online..
I have also simulated the circuit , but in my simulation the output has a frequency of 12.2 Mhz.
I am trying to understand how the circuit works and how you have come up with those values.
Can you provide the step by step solution for calculating the values?
I would greatly appreciate it!

 

You should have told us this was homework, that is what I was trying to get at with my question. You are basically cheating. If I had known it was homework, I would not have given you the solution that way.

As for how the values were calculated, in this instance, I did it by trial and error. If you calculate the frequency (do you know how to do that?) it comes to 16.6 MHz. Do you know why it might not oscillate at the calculated frequency? Would it be that far off if the frequency was 100 KHz? What do you think the professor is trying to teach you with this exercise?

I guarantee that if you build it, if it even works, the frequency will be different. Do you know why?

Bob

 

I know how to calculate the frequency and i know the result is 16.6 Mhz, but i do not know why the output is 12.5 Mhz .
I have also tried solving this by trial and error but i could not get near the 12.5 Mhz value .
Can you please explain how the output value correlates to the resonance frequency of the tank circuit?
That is what I do not understand..
I have posted the question here because i have done a lot of simulations but they did not produce the appropriate output.

 

Try changing the caps to 100 nF and then calculate an inductor to oscillate at 100 KHz. What is the simulated frequency now? Why is this case different than the 12.5 MHz one?

Bob

 

I have calculated the inductance and the result is 50.7uH ,the output frequency is 95.6 khz . In this configuration the output frequency is much closer to the resonance frequency

 

Do you have any ideas why is that the case? What could cause such an effect?

 

I don't know, do you have some links that explain this behaviour?

 

Did you ever hear about transistor parasitic components in particular, about transistor parasitic capacities?

 

Now that you have mentioned it, i remembered about the Miller effect, is that the cause ?

 

Yep. The transistor has small capacitances, a few picofarads, that will effect the tank circuit when its capacitance is also on the picofarad region, but not so much with the 100 nanofarad caps. My simulation, using LTSPICE, had the 100 KHz oscillator right on frequency. Are you using the same transistor as on my circuit?

Bob

 

Yes, i am using 2n3904.
Thanks for your help, i understood why there is such a difference at high frequencies, can can you point me to some documentation on solving the circuit mathematically? ,So that i don't have to solve this by trial and error anymore

 

What kind of amplifier have you drawn in your circuit?,i can't tell If it is common emitter,common base , or common collector (this are the ones that i know)

 

It is the standard Colpits oscillator.

I beleve it is considered common base since the feedback path is going from collector to emitter.

Also note that the base is held at 1/2 the supply voltage.

Bob