Measure harmonics with analog oscilloscope.

  • Thread starter Ozone Hole near South Pol
  • Start date
O

Ozone Hole near South Pol

Guest
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
 
Ozone Hole near South Pole wrote:
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
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.
 
Ozone Hole near South Pole wrote:
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
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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