We have about 10 Agilent 33120A frequency-generator / synthesizers in our lab, and a few other similar synthesizers. They feature a convenient digital frequency and amplitude programming knob that's based on optical encoders. A pair of left-right pushbuttons sets which decade you're adjusting with the knob, and over or underflows operate on the next higher decade. There's a little indent near the edge of the knob so you can spin it with your fingertip. You can also use up-down pushbuttons or directly punch in numbers, but the spinning knob is very convenient. Typically such encoders have 16 or 32 ticks per revolution. A very nice design feature.
The optical shaft encoders were originally made by HP, but now of course they're made by Avago. Sometimes they call these a digital potentiometer or a "Panel Mount Optical Rotary Encoder". For example, the HRPG-AD16 product line, about $34 each at Mouser.
Since they use an optical 2-bit quadrature grey-code (as opposed to using a pair of switch contacts), they're supposed to be extremely reliable.
But recently I've encountered two instruments with identical tuning-knob failures, one in an elegant 33250A 80MHz model that's on my bench, and the other in one of our standard 33120A 15MHz instruments.
In both cases the knob seems 'stuck" and not to work as you turn it, except every now and then you'll get one or two increments or decrements. Totally useless.
I'm wondering if anyone else has encountered this failure?