| bogdan_borko |
How to measure distortion in circuit maker 2000?
How to do Fourier analysis and measure harmonics in dB? |
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| bogdan_borko |
I hope you understood what i`m asking for...
I meant "Harmonic distortion".
thanks |
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| Steven |
Select Analog Mode in Simulation, then go to Analyses Setup (F8), enable Transient/Fourier analysis and click on that button. In Transient and Fourier Analysis Setup enable Fourier (check box). At this moment I have the default timing selected (5 cycles, 50 points per cycle). Use a sine generator in the circuit diagram as input, e.g. at 1 kHz. Probe the output, or any point in the circuit.
Default the fourier analysis shows a linear frequency scale as the X-axis and volts or amps on the Y-axis. Right click on the Y-axis to get a pop up where you can select Primary Y-axis with Magnitude in Decibels. Then adjust the scaling, normally in the left pane, to e.g. 20dB per division and adjust the offset to get the harmonics in the graph. Distortion is calculated from the ratio of the harmonics and the fundamental. |
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| bogdan_borko |
thanks a LOT!
What is a good result in these measurements? |
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| bogdan_borko |
this is not good...?
could you comment?
it is a push pull mosfet class a power follower I`m simulating... it has no feedback, classic source follower... |
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| Steven |
| Your harmonics are more than 60 dB below the fundamental, so distortion is approximately 0.1%, not bad for an open loop design. |
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| bogdan_borko |
| how did you calculated 0.1%? |
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| sreten |
| quote: | Originally posted by bogdan_borko
how did you calculated 0.1%? |
Hi,
0.1% is a thousanth = 10 to the power - 3 x 20 = - 60dB.
From the graph 2nd and 3rd dominate so I'd say ~ 0.2%.
:)/sreten. |
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| bogdan_borko |
thanks a lot guys!
I`ve searched the forum but haven`t found much about
what harmonic affects the sound the most (I just now the 2nd harmonic is "good") and what harmonic should we look at when we design the amplifier...
ANy link will be helpfull.
thanx! |
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| sreten |
Hi,
wieght the harmonics thus for degradation :
(harmonic number)squared / 4.
so 2nd = same, 3rd = x 9/4, 4th = x 4, 5th = x 25/4, 6th = x 9, etc
For 1khz 20 khz = x 100 or + 20dB, higher harmonics cannot be ignored......
:)/sreten. |
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| bogdan_borko |
| I don`t think I understand last post...:xeye: |
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| sreten |
Hi,
THD is simply the linear sum of the harmonics + noise compared to the signal.
So one amplifier could have 5% THD consisting mainly of 2nd harmonic.
Another could have 5% spread evenly across the harmonics up to 20K.
The second will sound very much worse than the first.
The formula I gave is a way of weighting harmonics to more accurately
reflect their impairment of sound quality. It shows why a valve amplifier
with 3% distortion could sound nicer than a transistor amplifier with 0.5%.
Weighting is not popular as it always increases the quoted distortion figure.
Weighting is popular when it improves the quoted figures,
for example A-weighted rumble figures for turntables.
:)/sreten. |
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| Steven |
Just to make the calculations a bit more clear:
In the shown graph the fundamental has a level of 30dB, the 2nd harmonic is -40dB, 3rd also -40dB, 4th is -56dB, 5th also -56dB, etc.
If we normalise the fundamental to 0dB, then the 2nd harmonic is 70dB below the fundamental, so at -70dB, 3rd at -70dB, 4th at -86dB, 5th at -86dB, 6th ~ 9th at -100dB.
For calculation of the THD it is easy to go back to real values instead of dBs. The fundamental is 0dB = 1, the 2nd harmonic is -70dB = 10^(-70/20) = 3.16E-4, 3rd also 3.16E-4, 4th is -86dB = 10^(-86/20) = 5.01E-5, 5th also, 6th ~9th harmonic is -100dB = 1E-5.
Total harmonic distortion (THD) is the square root taken from the sum of the harmonics squared, divided by the fundamental.
THD = SQRT((3.16E-4)^2) + (3.16E-4)^2 + (5.01E-5)^2 + (5.01E-5)^2 + (1E-5)^2 + ...)/1 = 4.53E-4 = 0.045%
Weighted harmonic distortion (WTHD) takes into account that higher harmonics are more annoying than lower oder harmonics. Shorter proposed in 1950 the n^2/4 rule, with n is the number of the harmonic. So 2nd harmonic is multiplied by 1, the 3rd by 9/4, et cetera. After weighing of the harmonics, the WTHD is calculated in the same manner as the THD. In the above case the WTHD is 9.13E-4 = 0.091%. This is not much worse than the unweighted THD because higher harmonics are very low in this example. If e.g. the 9th ahrmonic would have had the same value as the 2nd, then the WTHD would have been 0.65% already. |
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| bogdan_borko |
thank you steven.
I know that 2th harmonic sounds "good", 3rd is "sharp"
and 5th is "bad" becouse is apsolutly out of tune...
but I`d like to know what I must have in mind about harmonics when I design an amplifier stage, what level of what harmonic is unacceptable, what is the "best" level difference between 2nd and 3rd harmonic.... |
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| jcx |
Don't read Cheever's thesis - he abuses too many concepts and will leave your understanding of the subject in negative territory - you will have to "unlearn" almost anything you think you've understood from Cheever
please Don't recomend Cheever to anyone - especially someone without experience and background |
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| sreten |
| quote: | Originally posted by jcx
Don't read Cheever's thesis - he abuses too many concepts and will leave your understanding of the subject in negative territory - you will have to "unlearn" almost anything you think you've understood from Cheever
please Don't recomend Cheever to anyone - especially someone without experience and background |
Hi,
I found the first chapter a good overview.
The second chapter - fairly pointless.
:)/sreten. |
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