[sdiy] Harmonic effect of rectification

[Richie Burnett]

There are two common types of rectification.  Half-wave and full-wave
rectification.  Both introduce DC and even order distortion components.

You can think of both types of rectification either as a wave-shaping function
or alternatively as a modulation process.

Thinking of rectification as an amplitude modulation process might not seem so
intuitive, but it might give you a better insight into what is going on in the
frequency domain.

For example if you put a sinewave through a half-wave rectifier the positive
half-cycle is multiplied by +1 and the negative half-cycle is multiplied by 0. 
This is equivalent to multiplying a sinewave with a squarewave that exists
between 0 and +1.  This is amplitude modulation (or ring-modulation where the
square-wave input has a DC bias.)

For the example of a full-wave rectifier the positive half-cycle is multiplied
by +1, and the negative half-cycle is multiplied by -1 in order to flip it about
the x-axis and make it become positive.  This is equivalent to multiplying a
sinewave by a squarewave that exists between -1 and +1.  It's the same ring-mod
process as above but this time there's no DC bias applied to the squarewave.

Thinking of sinewave rectification as amplitude modulation by a squarewave might
help you to figure out where all those harmonics come from.

I hope this helps,

-Richie,

> On 20 Apr 2012, at 11:56, Neil Johnson wrote:
>
>> Hi Tom,
>>
>> (to list this time!)
>>
>>> What's the harmonic effect of rectification? Has this been studied anywhere?
>>
>> http://www.rfcafe.com/references/electrical/periodic-series.htm
>>
>> Just apply the Fourier transform and some undergrad maths.
>>
>> Neil
>
> I can apply the fourier transform to the output for a particular case and find
> out what it did to X or Y wave, but that doesn't help me know what will happen
> when I feed in Z wave. Given that sometimes it just changes the overall
> frequency and leaves the harmonic structure alone, sometimes leaves the
> frequency alone and changes the harmonic structure, and sometimes changes both,
> it doesn't seem straightforward to predict what will happen.
>
> But hang on a minute...I think I've got it...Rectification is an example of a
> waveshaping function, and the required theory is all worked out for waveshaping.
> Ok, sorry to have bothered you.
>
> Tom
>
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