For AT crystals used in analog multiplier XO and PLL-XO, spurious modes have different temperature coefficients compared to that of the carrier frequency’s. There may not be problems at 25°C, but when the temperature or voltage changes, the spurious mode may overlap with the carrier frequency, causing the frequency to “dip”; this is called an “activity dip.” Since SAWs do not have spurious modes, activity dips and its associated problems, they allow continuous oscillation with low jitter and excellent phase noise characteristics.