The filter configuration in the figure below is commonly used for SMD oscillator devices. It consists of a ferrite bead and 3 capacitors. The combination of the capacitors, parasitic resistance from the printed circuit board, and the resistive component of the ferrite bead will behave as a low pass filter and help suppress high frequency noise. The selection criteria for the capacitor values (C1, C2 and C3) was based on performance, values readily available and cost. Other values can be used to improve the rejection level of the low pass filter with an increase in cost and supplier lead times. The filter was simulated with three different Murata ferrite beads. Refer to the following table. The primary difference between them is the impedance at 100MHz, rated current and DC resistance. The conditions for the simulation were taken using nominal voltage and temperature on Cadence Spectre.
Back to top