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Closing the loop on an oscillator and plotting the time vs frequency graph of the output signal

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Hi everyone, in a task we are asked to design and build an LC oscillator with a frequency of 8MHz. I designed it on MWO to the point where I could simulate an open-loop circuit. My struggle now is to make it a closed-loop circuit and simulate it such to plot a time vs frequency graph (which should be a sin wave of course) and in addition, plot the power vs frequency graph. Please assist me. 

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Message 1 of 5

I cannot read all your values and I cannot tell what transistor model you are using, so I cannot actually run your project.  Based on the limited amount I can see, I have the following comments:


  1. Please look at some of our oscillator examples and note how the OSCAPROBE element is set up.  It appears to me that you have set Fstart = Fend = 0.  These parameters define the frequency range where the simulator searches for oscillation, so you are effectively telling the program to look for oscillation at DC, which is impossible.  You should also move the OSCAPROBE so it is between the BJT and the resonator - it should not be connected to the output.
  2. You have a voltmeter in the lower right that is measuring nothing, and it is part of the loop.  Remove that voltmeter and short the two wires together to close the loop.
  3. Again, as indicated in all of our examples...  It appears that you have a 50 Ohm resistor to ground as the oscillator output.  In order for oscillator simulation work, you must remove the resistor/ground and replace them with a PORT.
  4. Your bias resistor appears to be 380 Ohms.  While that may be sufficient to act as an RF choke, I recommend adding a fairly large inductor in the collector bias line to make sure no RF escapes the loop that way.
  5. You should delete (or disable) the OSCNOISE element (and any related measurements) until you have the behavior you desire without considering noise.  This just complicates the analysis and adds extra areas where problems can arise.

If you are still unable to get the simulation running properly, please ZIP your project and post it so I can work on your actual circuit.

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Message 2 of 5

I realized it was silly to have placed a voltmeter across nothing only after sending the image. I am not sure where would be the ideal position you recommend for the OSCPROBE, could you please elaborate. Also, the circuit with the ports in its current position (see attached) works perfectly as an open-loop circuit. If I remove the ports and place a short, where then would the ports need to be placed in order to simulate, or could they be left out? 

I've tried to make sense of the examples, it's just confusing me a lot more. See attached.

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Message 3 of 5
Accepted by topic author GdduToit

Please see attached project...



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Message 4 of 5

Thank you for your help. This is exactly what i needed.

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Message 5 of 5