The crystal or element must be _____ for higher frequencies.

• A. Thicker
• B. Thinner
• C. Wider
• D. Heavier

Answer: (B)

For higher frequencies in many crystal-based resonators, the controlling dimension is the thickness. In a thickness-mode piezoelectric crystal, the resonant frequency is roughly proportional to the acoustic velocity divided by twice the thickness (f ≈ v/2t). This means that when you make the crystal thinner, the waves complete their round trip across the thickness faster, so the frequency goes up. Conversely, increasing the thickness lowers the frequency. Dimensions like width or mass mainly affect other aspects of the device (such as mode spacing or coupling) and do not directly drive higher frequencies in this common configuration. So, to reach higher frequencies, the crystal must be thinner.