Ok, you could try the simplest form, two sine waves in quadrature (90 degrees phase shift). That draws a circle, and if you speed that up gently, you'll see the circle break up with erratic rings accompanied by overall distortion of shape, and hear the scanners start to scream. Do at your own risk. That will show a direct upper frequency, but it's still highly dependent on scan angle, and is harder to derive an equivalent point rate from than the trapezoids I described earlier. Those would have a much slower top speed too.