Measure harmonics with analog oscilloscope.

Jan 09, 2004 2 Replies
O

Hi,



I am playing with some switch mode power supply circuit and would like to inspect its influence to other equipment. I only have an old fashion analog ocilloscope and want to measure the first few harmonics (50, 150, 250 Hz probably). The only solution I can think of so far is to build a few band pass filter and measure the magnitude one after another one.



Do you know if there is any smarter solution than this? I have got a feeling that I may take advantage of the X-Y input. Any idea whether I am on the right track?



Ozonehole


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R

One cannot see distortion of a sinewave if it is less than about 3%, and X-Y does not help. Use of filters, on the other hand, allows one to get an idea as to how much of a given signal (band) is present. Essentially, one is making one's own test gear; feeding a DVM or scope is almost not relevant.

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M

This is typically done with notch filters. A "well designed/constructed" bridge-T can get rid of the fundamental well enough that you have a prayer of looking at what's left. At this frequency, this is not an easy task as the inductive component can introduce it's own harmonics. For gross harmonic components, you can sometimes do a FFT on an approximation to the waveform and get close enough to make decisions.

For accurate numbers, you really want an audio spectrum analyzer or a digital scope with FFT. mike

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