spectrumMeasurement for a DC signal

Dear NewScreenName,

NewScreenName said:

Could not find a function which calculates the power in all frequencies, is there a quick way to automate this measurement? Maybe I am missing the correct calculator function?

I may not fully appreciate exactly what you are attempting to compute and hence my suggestion may not be appropriate. 

If I understand, as shown in Figure 1, you have a simulation that produces a DC term of interest and ideally the only component in the spectrum. However, additional frequency components (f1, f2,...fn) are introduced by switching activity. You would like to determine the total power of all these additional frequency components.

If my understanding is correct, there are two possible means to determine the total power exclusive of the DC term. 

1. Assuming your simulation results are clipped so there are no significant transient settling effects and the analysis region encompasses all the significant tonal frequencies associated with your "switching activity", I think the easiest method is in the time domain:

a. Compute the average of your output waveform, say signal "vout":

vout_dc = average("/vout")

b. Subtract the average value of vout from the signal vout and determine its rms value:

v_spurious_rms = rms(("/vout" - vout_dc)

c. The resulting signal power, exclusive of the DC term, is then given by:

v_spurious_rms^2

2. If this is not to your liking, the alternative is to compute the power spectral density (calculator function psd()), and integrate it over the frequency range containing all the frequency spurs. However, this method does require some care when choosing the appropriate simulation parameters (simulation interval for analysis, number of time points, and strobe time) to assure your computed power spectral density is accurate. When done properly, it will provide the same answer as method [1].

Shawn

Figure 1

It would really help to see the DFT of the signal you're trying to compute the THD for, and what arguments you are giving to the thd function - to clarify what you're expecting to get. I have an alternative THD function that I wrote many years ago and I'm trying to understand whether that might be more suitable (it allows you to specify which harmonic is the fundamental and uses the result of a DFT as its input).

Andrew

Andrew Beckett said:

It would really help to see the DFT of the signal you're trying to compute the THD for, and what arguments you are giving to the thd function - to clarify what you're expecting to get. I have an alternative THD function that I wrote many years ago and I'm trying to understand whether that might be more suitable (it allows you to specify which harmonic is the fundamental and uses the result of a DFT as its input).

Andrew

Here you have the DFT and the parameters given, THD computed here is given by considering the 5MHz tone as signal. So what I tried to do was to compute totalharmpower/dcpower, which would be correct only if totalharmpower included all spectrum components a part from dc (which is what I hoped, but I verified isn't).

All the other parameters remain the same, 14 hamonics is chosen to tell the calculator to consider all the spectrum as distorsion components.

Let me know if more information is required.

Thank you.

Andrew Beckett said:

It would really help to see the DFT of the signal you're trying to compute the THD for, and what arguments you are giving to the thd function - to clarify what you're expecting to get. I have an alternative THD function that I wrote many years ago and I'm trying to understand whether that might be more suitable (it allows you to specify which harmonic is the fundamental and uses the result of a DFT as its input).

Andrew

Here you have the DFT and the parameters given, THD computed here is given by considering the 5MHz tone as signal. So what I tried to do was to compute totalharmpower/dcpower, which would be correct only if totalharmpower included all spectrum components a part from dc (which is what I hoped, but I verified isn't).

All the other parameters remain the same, 14 hamonics is chosen to tell the calculator to consider all the spectrum as distorsion components.

Let me know if more information is required.

Thank you.