How To Measure THD-N in EC

232

02:40:50

This is a normal 2 stage preamp topology. To measure its THD you must build an LCR notch filter first. A notch filter will only reject a single frequency. In this case 1Khz. Here the filter attenuation is -219dB. Once the fundamental is removed all the harmonics and diatorions remain. That is THD+N [Total Harmonic Distortion plus Noise]. After we've determined the THD level ~6.66mV in this case we take its RMS value and get 4.70862mV (0.0066Vx0.707). Now we have to calculate the RMS value of the fundamental frequency. In this case it's 707mV (1Vx0.707). Now we have to divide the distortion/fundamental. So 0.00470862/0.707= 0.00666. Almost there. Now we have to represent this number in percentage. Just multiply it by a hundred. 0.00666x100=0.66%. There you go this amplifier has a rough distortion of 0.66%. To increase the precision you can decrease the time frame by a third (3Khz time frame for 1Khz measurement etc) also design a very narrow notch filter with a very large attenuation of the fundamental. That's it.

published 10 years ago

hurz

10 years ago

I am very happy to see you change your mind! Yes, its possible to measure THD within EC! There is no need to go over RMS because at the end its just a ratio of two values in percent. Importante for this measurement is to slow down EC and to not load the device under test (DUT) with the notch. So put an OpAmp between DUT and notch helps to keep THD correct. I increase the time frame by at least x8 but this depends on the kind of harmonics. Sometimes its not needed because after having x4 it does not futher go down if DUT is having high THD. But a good Amp needs sometimes even oversampling x20 to get a good measurement results! Your DUT here, as you measured, has a THD of less than 0.66% so be fair and say <0.66%. Actually I measured <0.027%. Again, after months of discussions I am very happy you learned something! Please keep it up!!

thebugger

10 years ago

I have a few questions though. I figured this out myself and i may be wrong about somethings. 1. I shouldn't measure the RMS of neither the distortion nor the fundamental right? 2. Should i use the Upk-pk or the Umax?

thebugger

10 years ago

The second question applies to the distortion too.

hurz

10 years ago

Sorry, sure you found out by yourself LOL Anyway, true RMS is extremly complicated to measure for complex wave shapes, so just take the worste case peak values to compare and than write THD is below X%. I think this is faire.

thebugger

10 years ago

Mhm 10x

jose1979

10 years ago

Lol.

davidg

10 years ago

before speaking, try to know what your talking about. ejemm rms

davidg

10 years ago

Jajajajaka.

thebugger

10 years ago

That's very insolent and childish of you. First get on my level then you may speak to me :)

jesus1

10 years ago

which level this you do not even know what he speaks.

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