Could some electronics guru here please help me undestand the following.

I created the SPICE simulation for a common collector Colpitts oscillator. The target oscillation frequency is

500.0 MHz, and I am using the HFA3134 RF/microwave NPN transistor, with a Ft of 8 GHz. When the raw transient analysis data is plotted and the resolution adjusted to clearly see individual oscillations, the waveform looks approximately like a sine wave - 'approximately' as the top peak of each oscillation looks a bit distorted. Using the cursor feature of the plotting tool, the oscillation frequency is estimated as 507 MHz. So far so good. The transient analysis sampling interval is 25 picoseconds.I have creted a aimple C program to compute the Discrete Fourier Transform of a given time dependent signal, based on sample code in Numerical Recipes in C. To test the DFT code, I created a 450 MHz sample signal, and from the generated power spectrum of this test signal, the main peak is at 453 MHz. This gives me confidence that the DFT code is working fine.

Now the interesting part. When the transient analysis data from the above Colpitts oscillator(target oscillation frequency 400 MHz) the highest signal peak(excluding the

0 frequency DC value) is at 1.16 GHz, which is approximately double the signal frequency obtained by plotting the transient analysis data - please see above.So does this mean that most of the oscillator signal energy is concentrated at double the design oscillation frequency ? All ideas/suggestions are welcome. Thanks in advance.