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Why do I get only odd harmonics appearing? This not only happens at saturation of the transfer curve, but also in the linear portion of the curve.

I make an input signal and the power of this will saturate the amplifier.

So the details of the amplifier is that I have a a transfer curve for power in and power out. I also have a transfer curve for power in and phase deviation. Then I make a phasor that amplifies each point on the signal to do AM and PM of the amplifier.

This is basically how I do it

My input signal, x(t), is the sum of two sine waves

For each point on the transfer curve above I apply Amplitude and Phase modulation by using this array

modulating_phasor[] = (power_out / power_in) * exp(i*phase_deviation)

Then I look at my input signal and lookup where its value matches the input power of the transfer curve and use that location in the modulating_phasor array

output_signal[] = x(t) * modulating_phasor[location]

This makes the output signal complex array so I take the absolute value and for parts that should be negative, I multiply that location on by -1 by finding the correct location on input signal.

Now this is my output and no matter what my input level on my signal is, I always get odd harmonics!

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enter image description here

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  • \$\begingroup\$ You are leaving out the details of the amplifier -- the one that's leads to the note "Amplifier Saturat..." (presumably "Saturation") in the upper right plot on your set of four. The amplifier is clearly distorting with a function that is odd-symmetric, which leads to odd harmonics. \$\endgroup\$
    – TimWescott
    May 23, 2019 at 15:14
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    \$\begingroup\$ She is asking why only odd harmonics, not even harmonics also. Any symmetrical wave will only produce odd harmonics, but why? Symmetrical defined in this context as one period can be reversed in time and still be the same (or inverted). \$\endgroup\$
    – Mattman944
    May 23, 2019 at 15:19
  • \$\begingroup\$ So the details of the amplifier is that I have a a transfer curve for power in and power out. I also have a transfer curve for power in and phase deviation. Then I make a phasor that amplifies each point on the signal to do AM and PM of the amplifier. \$\endgroup\$ May 23, 2019 at 15:19
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    \$\begingroup\$ Try adding a DC offset to the amplifier input (I assume its response goes to 0Hz)...then watch those even harmonics appear. \$\endgroup\$
    – glen_geek
    May 23, 2019 at 15:35
  • \$\begingroup\$ Looks to me like your amplifier is saturating, which makes its output look a lot more like a square wave, which contains only odd harmonics (and quite a lot of them). \$\endgroup\$
    – Hearth
    May 23, 2019 at 15:40

1 Answer 1

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The even harmonics, like 2f , 4f , are synchronous (to) the zero crossing of each half-cycle of alternating-polarity and thus (with) perfect 50% duty cycle (it) has no even harmonics, as they cancel out.

Whereas 49% or 51% duty per cycle adds even harmonics that are 1% of the input or approx -20dB in power density. ( not exactly as I left out some details).

With this wisdom you can measure a square wave symmetry on a spectrum analyzer for slicing a sine wave with offset far easier than using a DSO then make a conversion table or null adjust even harmonics.

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  • \$\begingroup\$ Another lack-lustre inexperienced downvoted afraid to comment \$\endgroup\$ Jul 15, 2019 at 21:52
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    \$\begingroup\$ I downvoted, and I'm not afraid to comment on it. The answer is barely written in English, and you put in a quote and don't say where it's from. I'm not as experienced as you seem to be, but I am experienced. If a student of mine handed me this post, I'd hand it back to him or her and ask the student to take another crack at it. Correct your grammar, write in sentences, attribute your quote, and I'll remove my downvote. \$\endgroup\$ Jul 16, 2019 at 14:28
  • \$\begingroup\$ @ScottSeidman Are you familiar with what I said, or is the grammar, iOS auto-incorrect with my fat finger over your head? \$\endgroup\$ Jul 16, 2019 at 22:11
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    \$\begingroup\$ @ScottSeidman does it read OK now? (+1) TY for your brave comment (lol). \$\endgroup\$ Jul 17, 2019 at 10:39
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    \$\begingroup\$ I took down my -1. I was borderline on it in the first place, but by the time I thought about removing it, I couldn't until there was an edit. \$\endgroup\$ Jul 17, 2019 at 12:45

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