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Spectrum assistant for FFT - How is the signal freq decided?

jdp721

Hi,

How does the Spectrum assistant tool for FFT in ViVA decide on the signal freq for computing the SNR, etc. values?

Like, the input transient wave might have multiple frequency peaks, and I might be interested to get SNR considering the power of only a particular signal freq.

Thanks.

  • Andrew Beckett

    This is described in the Spectrum Assistant Application Note. I expect it's in the documentation too, but I didn't check that (actually from a quick scan, I think it's a bit vague about how the signal frequency is identified).

    Andrew.

  • jdp721
    Thank you Andrew for your reply.
    The info in this application note is almost same as that in Cadence Help, and nothing is stated as such about how the signal frequency is identified ! (as you said).
    Is there any other function in Cadence calculator to find SNR considering the power of only a specified signal freq., when the input transient wave might have multiple frequency peaks?
  • Andrew Beckett

    Actually it does say in the app note how it determines it:

    For calculation of P_Signal (total signal power), P_Noise, and P_distortion, we consider bins (from FFT, range is from startBin to endBin) from Max(signalBin, startBin) to endBin.

    For the range mentioned in “Signals bin” (say this array is arr), get the array index where magnitude is maximum (frequency that has maximum y value). Around this frequency (say freq) we have signal from freq - signalBin to freq + signalBin.

    P_Signal (Total Signal Power) = arr[i] * arr[i] ; i is from (freq - signalBin) to (freq + signalBin).

  • jdp721

    Sorry, I overlooked that part.

    Thanks Andrew.

    So it seems that the chosen fundamental (signal) is the frequency bin that has maximum y value (but, the defined range from Max(signalBin, startBin) to endBin is somewhat vague!).

  • Andrew Beckett
    I agree - this is badly worded. I suggest you contact customer support; this really should be clarified to make it more precise.

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