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Why does my scope show a Sine Wave when I expect a Square wave?

Last Updated: Dec 29, 2015 02:15PM PST
A scope user attempting to view a square wave on an oscilloscope might view a sine wave of the same frequency in place of a square wave. This is not a necessarily a sign of a defect with the scope or the generator, however an effect of the constituent higher frequency harmonic components of the square wave itself.

To understand this aspect better, one must develop better understanding of the effects of harmonics on waves.

For example, a 25MHz square wave will be viewed as a 25MHz sine wave on a DS1052E (50 MHz scope), though the actual square wave itself is below the scope's frequency bandwidth ceiling, its harmonic components are already above the scope's 3 decibels (dB) high-frequency cutoff point. Due to this natural cutoff, the square wave’s main contributing harmonic components beyond the 2nd harmonic component (2*25 = 50 MHz) are naturally filtered out. The 3rd and 5th harmonic components however, are at 75 MHz and 125 MHz center frequency respectively. These components are naturally much greater than the square wave’s 25MHz component and way above the scope’s bandwidth ceiling.

For viewing a square wave signal on a scope, it's necessary that the square wave’s 3rd and 4th order harmonic components do not exceed the scope’s bandwidth ceiling

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