High speed noise suppression

27 July 2005 Circuit & System Protection Infrastructure Government and Parastatal (Industry)

Picked up from internal clocks by transmission lines, and radiated by USB cables acting as transmitting antennas, the problems of radiated and conducted noise are well known. These problems are compounded by the usual solution, the ferrite bead, which increasingly distorts the signal waveform as speed rises to USB2.0 (high speed mode) levels.

Murata has performed extensive development in its EMI research centres with material and winding technologies to solve these problems. That solution is high coupling and high common-mode impedance at high frequencies with Murata's wirewound common mode choke coils. Design features include 100 MHz common mode impedance of 67 to 2200 Ω, rated current from 200 to 370 mA, rated voltage 50 V d.c, DWV of 125 V d.c. and 0,3 to 1,2 Ω maximum DC resistance.

As ever, exact choice of part depends on the application and conditions but generally, Murata recommends the DLW21H series for USB lines in PCs, LVDS lines, peripherals, and small items like digital cameras. Especially for USB2.0, it suggests DLM21HN900SQ2 for data lines. Because of 'End of Packet' issues, the company does not recommend impedance values above 90 Ω. For common mode noise suppression in high speed and high-density digital equipment, the DLW21S series is a good choice. Both types are in a standard 0805 format. The DLW31S series performs similar functions in a 1206 package, but includes the higher values.

These highly-coupled choke coils are part of a large range of Murata EMI filters for almost any situation.

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