Interpreting magnitude of HSPICE's FFT-output of mixed signals

AI Thread Summary
The discussion focuses on analyzing the frequency content of transient current draw using HSPICE's FFT output. The user seeks to understand how to interpret the spectral density curve, particularly whether the Y-axis magnitude represents the real amplitude of current at specific frequencies. It is clarified that impedance is not necessary for this analysis; instead, the Power Spectral Density (PSD) of the signal itself should be used. This approach simplifies the calculation of noise voltage on the power rail. Understanding these concepts is crucial for accurate analysis in mixed signal simulations.
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I ran a time-domain simulation in HSPICE & I am trying to analyze the frequency content of the transient current draw...(FYI- I have the power systems "impedance vs. frequency" curve, and my aim is to determine the noise voltage on my power rail given its impedance now)

So I run FFT (current-waveform,t_start, t_stop, NP) using the in-built calculator...

Now how does one interpret the resulting spectral density curve?

Is the magnitude (on Y-axis) the real amplitude of current at that frequency i.e. can I just take that amplitude and multiply it by impedance Z at that frequency to calculate the noise voltage...??

I'd highly appreciate if some one can lend insight into this...thanks a bunch
 
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You don't need impedance at all. Just use the FFT (actually PSD) of the signal itself.
 
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