Why Should My Signal Fit Within The Screen Of My Tektronix Oscilloscope?

Scaling your signal to fit within the screen will remove signal distortion caused by clipping. Clipping is caused by exceeding the limits of the instrument's input. Tektronix's digital oscilloscopes are designed such that each division is 25 digital levels. For 8-division digital oscilloscopes, there is one division above and below the screen before clipping occurs. For 10-division digital oscilloscopes, the top and bottom of the screen are the limits of input. Any portion of your signal, desired or undesired, may cause clipping. Since this off screen portion of your signal is beyond what is detectable, it is chopped-off (hence the term clipping) and may give you unexpected results.

Demonstration

The effects of clipping are easy to demonstrate.

Test signal: 1 MHz Sine

Input a sine wave into your oscilloscope and scale it to fit on screen.

No clipping (increased time scale)

Adjust the time per division to clearly see the fundamental with a good view of a few harmonics.

FFT of 1MHz sine

The FFT shows little harmonics.

Increased position (half signal off screen)

Now adjust the position so that half of the sine wave is off the top of the screen.

FFT of clipped 1 MHz sine

You will notice an increase in harmonics caused by clipping of the sine wave.