Crystal oscillators are loosely divided by the frequency they work at and whether or not they are fundamental oscillators—get the job done on the frequency marked over the can or at even multiples of that (overtone oscillators). As you could see in the graph underneath, crystals have two resonant details. https://webcastlist.com/story20132791/how-much-you-need-to-expect-you-ll-pay-for-a-good-bulk-crystal-oscillator