Lots of mechanical devices (like mice and shaft encoders) output quadrature signals. This is just a fancy way of saying that you have two signals and whichever one comes on “first” means something. So in the picture, you can see a mouse encoder wheel moving on a single axis counter clock wise. If it were moving clockwise channel 1 would come on before channel 2.

This is a natural byproduct of how the optical encoder wheels work. They are offset a little from each other, so the slots in the wheels are aligned about like the pulses in the oscilloscope trace shown.

The PAK-VII’s primary purpose is to measure pulse widths, but with very little effort it can also measure quadrature inputs. Want to know more? Read this article.