Crystals and XOs are generally cost-effective unless the output requirements are unusual or stringent. Crystals provide a frequency output when an electrical signal is applied. The output is a single-ended sine wave typically ranging from 32 kHz to 50 MHz. Each output frequency requires a different resonator cut and an oscillator circuit to operate. XOs integrate the crystal with the oscillator circuit in a standalone package. XOs output either a LVCMOS single-ended or differential square wave. Differential signaling is used in high-speed, jitter sensitive applications. Some XOs provide multiple frequencies via I2C or pin control. XOs are generally cost effective unless the application requires a variety of clock frequencies. VXCOs are XOs with a varactor diode that allows applied voltage to change or skew the output frequency.