Linearity

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maxpou said:
Thank you for answers!
but what's CRO?
Thank you!
Maxpou

A CRO is an oscilloscope (Cathode Ray Oscilloscope).

The triangle waveform generator is a good idea. And it is easy to build a fairly good one. (Do a search at national.com for "tri-wave generator".)

To check linearity, you would want to make sure that vout/vin is constant, for all expected values of vin.

Checking linearity with a triangle waveform for vin is easy: If vout is also a triangle shape, the system is linear versus input voltage. But if the rise or fall portion of the output waveform is curved, or worse, then the system is not linear.

You would probably want to check that that is also true when vin is changing, for all expected rates of change of vin. The triangle generator would also handle that, if it had variable frequency as well as amplitude.
 
Onvinyl said:
Hi,
interesting topic! What is the ballpark of THD-N when the triangle-wave is visible deformend?

Rüdiger


That's a difficult one, as a perfect triangle is a sine wave + 30% 2nd harmonic. Plus 4th, plus 6th, plus....;) .

Just as a perfect square wave is a sine wave + 30% 3rd harmonic. Plus 5th, + 7th + ...

That is why a tringle is a very crude way to guesstimate linearity.

Jan Didden
 
Is the triangle trace any better than the classical XY trace of I/P - O/P sinewave. Loss of linearity is immediately apparent by curvature of the (ideally) straight line at midband, or lack of ellipticity elsewhere (and shows phase to boot!)
This is how I try to detect slew rate limiting. Does that make sense?

Brian.
 
A "bow" shape (red curve), due to low frequency corner. Nothing in common with non-linearity.
 

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a THD analyzer is best. if distortion is high enough to become visible on a scope, it's definitely far above the audibility threshold.
depending on the sharpness of the scope trace, distortion doesn't become visible until it's about 1 or 2 percent. you can approximate a medium sensitivity THD analyzer by using a dual trace scope, using the ch1&ch2 add function, with ch2 inverted (it's now subtracting) and monitoring the input signal to ch1, and monitoring the output on ch2, and adjust for the smallest waveform.
 
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maxpou said:
Thank You Guys for answers,
But I am not an EE who wants to test a lot of circuits only my designs, I concluded that the least expensive solution is to buy a analyzer THD (HP334A) on ebay or i have another choice?
Thank you! Maxpou

That should work fine. I got a recently-calibrated and cosmetically-very-good one from ebay for about $150, a few years ago. The minimum full-scale distortion is 0.1%, which puts the smallest scale-markings down to 0.002%, which should be good for measurements at least down to 0.001%. Note that the HP334A is "manually" operated. i.e. You have to twiddle the frequency-tuning and null knobs until you get the lowest reading. It usually takes me about one to two minutes, to make a THD measurement.

On the other hand, I had a Tektronix DA4084 plug-in for a 7603 oscilloscope mainframe, for a while. It was much nicer, since it was fully automatic, and also had a digital display.
 
gootee said:


A CRO is an oscilloscope (Cathode Ray Oscilloscope).

The triangle waveform generator is a good idea. And it is easy to build a fairly good one. (Do a search at national.com for "tri-wave generator".)

To check linearity, you would want to make sure that vout/vin is constant, for all expected values of vin.

Checking linearity with a triangle waveform for vin is easy: If vout is also a triangle shape, the system is linear versus input voltage. But if the rise or fall portion of the output waveform is curved, or worse, then the system is not linear.

You would probably want to check that that is also true when vin is changing, for all expected rates of change of vin. The triangle generator would also handle that, if it had variable frequency as well as amplitude.

Hi Gootee,
i'm very disappointed because i have an audio generator with triangle waveform and scope! Maxpou
 
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