Page "Distortion" Paragraph 10

# A high-pass filter ( green trace ) will distort the shape of a square wave by reducing its low frequency components.

This is the cause of the " droop " seen on the top of the pulses.

This " pulse distortion " can be very significant when a train of pulses must pass through an AC-coupled ( high-pass filtered ) amplifier.

As the sine wave contains only one frequency, its shape is unaltered.