The Designer's Guide Community
Welcome, Guest. Please Login or Register. Please follow the Forum guidelines.
Jun 3rd, 2023, 2:00pm
Pages: 1
Send Topic Print
DFT Assistance (Read 1258 times)
New Member

Posts: 8

DFT Assistance
Nov 17th, 2017, 6:22pm

I am trying to simulate the DFT response in Cadence. I have constructed an LC Cross-Coupled oscillator operating at approximately 1GHz. I used the 'frequency' function in the calculator to obtain the value above and I am getting a free running frequency of 1.005GHz. I would like to analyze the harmonics of the signal (at least of to the 4th harmonic).

I attempted to use the DFT function but I am getting an error when I try to plot my response. The first thing I did was run a transient analysis for 300ns. I then clipped the waveform from 50ns to 100ns. Setting the 'From' and 'To' of the DFT function is where my main confusion arises.

From the initial values set, I am assuming this is the start and stop time. I would like to examine 1GHz and 4GHz, so my logic was to start at 0.2n to 2n. Which I was assuming would include frequencies from 500MHz to 5GHz. I chose a sample value of 2^15 and left all the other options to the default value. I am not able to get any sort of response.

I am having trouble locating the documentation for the DFT function, so I last resorted to this. Could anyone provide any tips or relevant information? Thank you.
Back to top
View Profile   IP Logged
Community Fellow

Posts: 751

Re: DFT Assistance
Reply #1 - Dec 7th, 2017, 9:21pm

  The simple answer is that this problem is why Shooting Newton and
Harmonic Balance with autonomous-mode are provided.

If you don't have access, then you might want to consider the folllowing

1) Use the freq function to make sure the oscillator has settled to steady
   state. As I remember the frequency returns the frequency of the last
   period, you need to reach sinusoidal steady-state before performing
   a DFT. If you don't the settling will contaminate the spectrum
2) There is no reason to assume that the frequency is the same at
   50ns and 300ns, you need to check the settling before choosing the
   window to perform the analysis.
3) Your time window of 50ns and period of ~995.02488ps are not
   conducive to useful results. You will need to either define the
   correct sample window based on the oscillator frequency or
   use a window function
4) The resolution of your FFT is 20MHz, yet you need 5MHz resolution.
   1.005GHz --> means you need tones every 5MHz or the DFT will
   will be corrupted by spectral leakage
5) Did you strobe your waveform to produce the samples you need
   for 2^15 FFT frequencies, the tool will interpolate values if the
   simulator does not provide the data required for the DFT. Interpolation
   error degrades accuracy

What you want to do can be done but you need to be more thorough
in your simulation setup in order to achieve accurate results.

Back to top
View Profile   IP Logged
Pages: 1
Send Topic Print
Copyright 2002-2023 Designer’s Guide Consulting, Inc. Designer’s Guide® is a registered trademark of Designer’s Guide Consulting, Inc. All rights reserved. Send comments or questions to Consider submitting a paper or model.