Total Harmonic Distortion

i want to calculate THD in PSPICE.....but in Option Total no of Harmonics.
as i increase nos. the THD will increase.
so what will be the parameters which decides Nos(Total no of Harmonics).
Thanks!!!

Hi,

so what will be the parameters which decides Nos

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this value usually is decided by the designer of the device.
(is it you?)



Klaus

The total number of harmonics is determined by the high frequency cutoff of the amplifier.
If the amplifier is linear then the level of the harmonics reduces anyway as their frequency is higher.

You can take a squarewave that has extremely high distortion because it has high level harmonics at many frequencies and feed it into a 4th-order (or higher order) Butterworth lowpass filter. The output will be a sinewave with very low distortion if the cutoff frequency is close to the fundamental frequency so that the 3rd harmonic is reduced a lot.

Audioguru said:

if the cutoff frequency is close to the fundamental frequency so that the 3rd harmonic is reduced a lot.

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why specifically 3rd harmonic...
can u please elaborate.

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actually i am using class D amplifier and want to calculate THD so as to adjust LC values of butterworth filter to match Design needs.
but i am a bit confused.
actually i need Q=0.7,R_load=4 ohm, Phase Delay <10 Degrees. to amplify a 400 kHz SINE Wave.
Thanks!!!

root2hell said:

..........................................
actually i am using class D amplifier and want to calculate THD so as to adjust LC values of butterworth filter to match Design needs.
.......................

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The value of the filter should have little effect on the THD of the signal, which is determined by the linearity of the PWM analog to pulse-width conversion. The filter is there to reduce the PWM switching frequency component in the signal (which will appear as a peak in the FFT of the output signal at the switching frequency).

root2hell said:

why specifically 3rd harmonic...
can u please elaborate.

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Most audio power amplifiers are symmetrical so do not produce even harmonics (2nd, 4th, 6th, 8th etc) but instead produce odd harmonics (3rd, 5th, 7th, 9th etc). Then the strongest harmonic is the 3rd harmonic that you want to reduce to a very low level with a lowpass filter. But you want the filter to pass the fundamental frequency so the slope of the filter must be sharp.

actually i am using class D amplifier and want to calculate THD so as to adjust LC values of butterworth filter to match Design needs.

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The LC filter must pass the highest audio frequency (usually 20kHz) but attenuate the switching frequency. The amplified modulating signal (audio) must have low distortion.

but i am a bit confused.
actually i need Q=0.7,R_load=4 ohm, Phase Delay <10 Degrees. to amplify a 400 kHz SINE Wave.
Thanks!!!

Click to expand...

Is your class-D amplifier an audio amplifier? If it is then you do not amplify the 400kHz switching frequency. The audio signal that modulates the 400kHz carrier frequency with PWM must be amplified so that it produces up to full modulation.

actually i dont want to build an audio amplifier.
i need to amplify a 400KHz signal.
i.e. Fs=400KHz, so i choose Ft >> 3Fs (Ft=6MHz).
and i am getting a phase delay of 30Degree and unable to calculate THD.

If you are worried about lowpass phase shift, you could try a bandpass.

Carrier frequency residuals of a class-D amplifier aren't counted as THD, because they aren't harmonics. Or are you designing a class-E amplifier? This would make more sense for a 400 kHz single frequency sine signal.

The numbers of your "specification" need still explanation. What's the meaning of a Q or a phase shift for your application? Is the 400 kHz signal modulated?

from this Q i mean to say i don't need over damping.
and i want output with minimal delay.
and i have to amplify video signal.

and i want output with minimal delay

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In case of doubt use a linear amplifier.

Class-D is used for an audio power amplifier so that the output devices do not get too hot and so that the supply current is lower than a linear amplifier.
The frequency of the PWM is about 400kHz (20 times higher than the highest audio frequency of 20kHz). The PWM is rectangular pulses, not a sinewave.

Old fashioned standard definition video has a bandwidth up to about 5MHz, not 400kHz. Modern high definition video goes up to much higher frequencies.
I have never seen a class-D video amplifier because the highest video frequency might be 50MHz then the PWM must be 1GHz. Nobody needs a power amplifier for video anyway because a video monitor is not 4 ohms.

I think you are mixed up.
Early in this thread you said you are making a class-D audio amplifier to amplify 400kHz (which is not audio) with a 4 ohms load.
Now you say you are amplifying a video signal.