sheldon
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MWJ1975,
The simple answer is that this problem is why Shooting Newton and Harmonic Balance with autonomous-mode are provided.
If you don't have access, then you might want to consider the folllowing
1) Use the freq function to make sure the oscillator has settled to steady state. As I remember the frequency returns the frequency of the last period, you need to reach sinusoidal steady-state before performing a DFT. If you don't the settling will contaminate the spectrum 2) There is no reason to assume that the frequency is the same at 50ns and 300ns, you need to check the settling before choosing the window to perform the analysis. 3) Your time window of 50ns and period of ~995.02488ps are not conducive to useful results. You will need to either define the correct sample window based on the oscillator frequency or use a window function 4) The resolution of your FFT is 20MHz, yet you need 5MHz resolution. 1.005GHz --> means you need tones every 5MHz or the DFT will will be corrupted by spectral leakage 5) Did you strobe your waveform to produce the samples you need for 2^15 FFT frequencies, the tool will interpolate values if the simulator does not provide the data required for the DFT. Interpolation error degrades accuracy
What you want to do can be done but you need to be more thorough in your simulation setup in order to achieve accurate results.
Sheldon
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