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

In summary, the speaker ran a time-domain simulation in HSPICE and is now trying to analyze the frequency content of the transient current draw. They have the power systems "impedance vs. frequency" curve and their goal is to determine the noise voltage on the power rail. They ran FFT using the in-built calculator and are now asking for insight on how to interpret the resulting spectral density curve. They are wondering if they can simply take the magnitude on the Y-axis and multiply it by the impedance at that frequency to calculate the noise voltage. However, the expert summarizer suggests that there is no need for impedance and that they can just use the FFT (specifically PSD) of the signal itself.
  • #1
prasfft
1
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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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  • #2
You don't need impedance at all. Just use the FFT (actually PSD) of the signal itself.
 

1. What does the FFT output in HSPICE represent?

The FFT (Fast Fourier Transform) output in HSPICE represents the frequency domain representation of the mixed signals. It shows the amplitude and phase of each frequency component present in the mixed signals.

2. How do I interpret the magnitude in the FFT output?

The magnitude in the FFT output represents the strength or amplitude of each frequency component. The higher the magnitude, the stronger the presence of that particular frequency in the mixed signals.

3. What is the significance of the phase in the FFT output?

The phase in the FFT output shows the relative timing or delay of each frequency component in the mixed signals. This can be useful in identifying the cause of any signal interference or distortion.

4. How can I use the FFT output to improve my circuit design?

The FFT output can help identify any unwanted frequency components in the mixed signals, which can then be eliminated or reduced through circuit design modifications. It can also be used to optimize the frequency response of the circuit.

5. Can the FFT output be exported for further analysis?

Yes, the FFT output in HSPICE can be exported in various file formats for further analysis in other software tools. This can be useful for more in-depth analysis and troubleshooting of circuit performance.

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