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Why the FFT simulation of VCO output is like this?

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fft simulation

Dear everyone,

I have simulated a simple LC VCO by Hspice, which oscillation frequency is 2.5G and I perform a fft simulation like:

.fft v(vco) start=100n stop=150n

but the probe-out spectrum is strange because the phase-noise is about -5dBc/Hz@1Mhz, this value is different from the result got out of SpectreRF of cadence(it is about -120dBc/Hz@1Mhz).

Could anyone tell me why?
 

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