sheldon

Rajdeep,
It usually helps to provide context, that is, information about the problem you are trying to solve. From reading through the email, you are trying to simulate a circuit with a 6 MHz clock, and look for the low frequency subharmonics. In addition, you want to avoid aliasing since high frequency harmonics aliasing into the audio band might corrupt the results.
So some suggestions:
1) In this case the DFT [FFT] is not the best choice for converting from time to frequency since it is susceptible to aliasing.
2) Use the Fourier Integral is since it is not susceptible to aliasing
3) To use the Spectre Spectre Fourier Integral in ADE add a fourier component from analogLib to the schematic.
4) You will need to modify the CDF to add support for the normalizing harmonic, that is, to define the harmonic to normalize the results too. Unfortunately, this Spectre Fourier analysis function has not been enabled in ADE.
Let me know and I can provide guidelines for updating the CDF.
5) I would add a window function, Typically I use a Cosine2 VerilogA module to window the data for the Fourier Integral and this approach works well
6) Since your clock is 6MHz and the tones are ~500Hz, you will need to set the transient stop time as ((ratio of clk to tones)/clock frequency or about ~12,000/6MHz
7) I would actually add more frequency resolution since you are windowing the data, for example, 131072/6MHz. The increased frequency resolution will allow you to distinguish between the offset and a tone at 500Hz for example.
8) Note: the calculation in #6 & #7is actually the calculation for the Fourier fundamental frequency. You may need to add some additional time so that the circuit is at steadystate. The Fourier analysis will report on the periodicity.
9) Only keep the number of output tones you actually need. The Fourier Integral is more expensive numerically than the DFT, that is, it can take a long time to run.
10) I am simulating a Class D Audio Amp and I am running a DFT in IC615 with 2^24 points without any issues (with the DFT). Not sure what the issues might be, you might want to talk to your customer support engineer. Best Regards,
Sheldon
