In most microphone (MIC) circuits, it is common to see a series inductor and a parallel capacitor (typically around 27pF to ground). But what is the purpose of this configuration?
Purpose of the Capacitor and Inductor
The main function of these components is to suppress RF interference. The series inductor (or more commonly, a ferrite bead) blocks high-frequency signals, while the parallel capacitor provides a low-impedance path to ground for those frequencies, effectively filtering out unwanted radio frequency noise.
How Are the Values Chosen?
The values of these components are typically selected based on experience rather than strict theoretical calculations, because real-world conditions often differ from ideal simulations. While the capacitor value is often fixed (such as 27pF), the inductance or ferrite bead value can vary depending on the application and target RF frequency range.
TDMA Noise and Frequency Band Considerations
This filtering configuration helps reduce TDMA noise, which is often caused by RF interference in GSM/DCS/PCS systems. Different inductance values may be selected to target different frequency bands. However, even for dual-band or quad-band devices, variations in inductor selection may still exist due to different layout or noise suppression strategies.
Component Roles Summary
- Series Inductor: Blocks RF signals; more effective for lower-frequency interference.
- Ferrite Bead: Suppresses high-frequency noise; absorbs and dissipates RF energy as heat.
- Parallel Capacitor: Provides a low-impedance path for high-frequency noise to ground; usually fixed at a few picofarads.
The combination of series inductors or ferrite beads and parallel capacitors is a simple but effective method to ensure cleaner audio signals in microphone circuits, especially in RF-noisy environments. Design choices should be made with both theoretical understanding and practical experience in mind.
