Calculation for THD for a low pass filter in OrCad

[subbuindia]

Hi Friends..

We are designing filter in OrCad. For that, we need THD calculation. Please, suggest how to measure total harmonic distortion in OrCad tool. Thanks in advance.

 

[SLK001]

1) Measure total power (POW_TOT).
2) Measure tone power (POW_TON).

THD = (POW_TOT - POW_TON) / POW_TON x 100

 

[subbuindia]

1) Measure total power (POW_TOT).
2) Measure tone power (POW_TON).

THD = (POW_TOT - POW_TON) / POW_TON x 100

Thanks for your reply. But, what is tone power.? How can I measure it in OrCad.?

 

[FvM]

It's power of fundamental wave, see also https://en.wikipedia.org/wiki/Total_harmonic_distortion

If distortion is low, you probably get more accurate result by performing fourier analysis and summing harmonics. THD is already calculated with .FOUR analysis.

 

[Audioguru]

You did not say of the filter is a highpass that increases THD or is a lowpass that decreases THD.

The THD of an amplifier depends on how much negative feedback it has. If its gain is very high then it will not have much negative feedback then its THD will be high.
The datasheets do not say and many sim programs do not know that the common low power LM324 and LM358 opamps have crossover distortion.

 

[Ata_sa16]

Apply sine wave in the input with a desired amplitude (depends on your design ) and if you have low pass filter set the input frequency to the -3dB of your low pass filter.



Then you have to do a long transient simulation.

After that, take an FFT from vout sine wave with MATLAB or other software.

find the amplitude of 10 harmonics.

then calculate THD:

 

[subbuindia]

Apply sine wave in the input with a desired amplitude (depends on your design ) and if you have low pass filter set the input frequency to the -3dB of your low pass filter.

View attachment 132208

Then you have to do a long transient simulation.

After that, take an FFT from vout sine wave with MATLAB or other software.

find the amplitude of 10 harmonics.

then calculate THD:

View attachment 132209

Thank you sir. Can't we calculate %THD directly without any calculations (like FFT of Vout spikes, V1, V2..) in OrCad.? If not, please clarify how to calculate these FFT of Vout in MATLAB, V1, V2, ..... Sir, please attach any PDF or LINK which has the step by step procedure to do the %THD in OrCad for LPF or HPF or APF or NF or BPF.

 

[KlausST]

Hi,

post#2 gives THD+noise (this is possible without FFT)

post#6 gives pure THD. For pure THD you need to specify upper overtone limit. (here you need frequency analysis like FFT)

Klaus

 

[Ata_sa16]

In TRAN setup you have to activate FOURIER and to specify the fundamental frequency of the signal you want to analyze as well as the corresponding node voltage. After completion of the analysis the result (THD in %) is written in the *.out-file.