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On Air Distorition

  • Thread starter janedoahlovesbrentlee
  • Start date

We differ.

But, give it a try. Dredge out the ol' PI, and actually measure the point at which you can see the sine wave change. Then go back, use a 2KHz or so squarewave, and note the point of overload at which you can see the waveform differ from the input.

I suggest you'll see it at or before on the square wave, and that you'll be around 1% THD on the sinewave.
 
littlejohn said:
We differ.

But, give it a try. Dredge out the ol' PI, and actually measure the point at which you can see the sine wave change. Then go back, use a 2KHz or so squarewave, and note the point of overload at which you can see the waveform differ from the input.

I suggest you'll see it at or before on the square wave, and that you'll be around 1% THD on the sinewave.
Click to expand...


We DO differ! But perhaps mostly in perspective. I'm sure we get similar results when the goal is headroom management.

Though I can understand how you can use a square wave with a meter on a measured system's output, watching for where its level stops while increasing input, I just question how one can see (on a scope) a flat-topped waveform become flatter. :)

With a clipped sine wave, a glance at the shape alone can tell you how deep you are into overload clipping, and exactly where that point is. Whereas with a square wave you actually have to measure before and after 'height' on a scope (or compare input-versus-output level on a meter), IOW, you must be able to reference to the original signal; the exact clipping point is otherwise ambiguous and not self-evident.

This is not a requirement with a sine wave.
If the overload point is not obvious enough, just drive it harder! It becomes even clearer where the clipping occurs, because it starts looking like.... A square wave! ;D [FWIW, a triangle wave would make the clip point even more noticeable visually, since the peak is more defined than a sine wave.]

But a square wave? Concerned mothers want to know, how can one expect a profoundly clipped signal to visually identify the clipping point? ;)

And what about the case of those using ears to do this procedure, how do you HEAR this phenomenon? A low to mid-frequency sine wave will have lots of new, quite audible harmonics filling the spectrum when it barely touches clipping, and the effect can be heard distinctly even by the untrained ear. Not so with a square wave, where a large portion of its energy is already in its harmonics, obscuring any change to its audible character as it clips.


Littlejohn, I'm sure you have had success with your method, otherwise you wouldn't be participating in this dialogue. More power to you!

When one is comfortable with their particular favorite quick-and-dirty technique, they recommend it. That's where I'm coming from.

All the Best, and Kind Regards,
David


PS: Don't you just love how going digital meant we didn't have to pay attention to such things anymore? ;)
 
It won't get flatter. It will, however, tilt or ring or both. And, in that a decent square wave contains significant energy out to at least the 10th harmonic, it will let you know if the system has a frequency dependant overload. e.g. it dies at +2 at 14KHz, but at +10 at 1KHz. I fine tune transmitters with a thermometer and a monitor aet for incidental AM, too :):)
As to the digital, now we really do need a swept analyzer to see what's going on with the carriers. And, I sort of miss the graceful fade to noise when there's a good inversion around. These things have a totaly binary solutuion set: It's there just fine, or it ain't there at all. However, I note that some work better than others as far as coverange goes, for about the same power. We have a complete new set of parameters, problems, and fixes to learn. First thing we've found, so far, is on the high level injection - the addition of a fine matcher in the waster load line makes it a lot easier to hit under the mask when you're setting one up. All of the loads I've seen are reactive to some degree, but the matcher lets you tune a lot of it out, and it really helps the intermod products. For low level generation in high powered amplifiers, I suspect pre-distortion, expecially active predistortion, is going to be the salvation.
 
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