Part Number: CM2722
Function: EMI Suppression for Firewire & USB 2.0 Applications
Manufacturer: Steward ( www.steward.com )
EMI Suppression for Firewire & USB 2.0 Applications Overview A primary problem for the Firewire IEEE1394 and USB2.0 as with the USB1.1 port designer is common mode noise conducted through the power and signal lines from various sources. EMI susceptibility on data/power lines is also a major concern. The use of a common mode choke in this application will provide a complete low cost, effective, modular, filter solution. Steward’s CM2722 (for single and dual ports), CM3822 (for tri ports), and CM5022 (for quad ports) series of components have the lowest normal mode impedance in the industry and suppress a wide range of digital common mode noise. Normal mode impedance is net circuit impedance i.e. the impedance seen by the differential signal. Discussion Firewire and USB2.0 is a serial data / power port providing a broad range of data and current loads. The initial USB industry specification called for a serial port which operates in a hybrid full duplex mode and a power port that operates up to 1 amp of current. This design has created several layers of EMI problems that must be corrected. Also, USB component performance requirements have been stretched beyond initial specifications which has caused field failures of 1 amp and even 3 amp rated ferrite chip beads. Firewire and USB2.0 have increased the expected design loading to a minimum of 4.0 amps to insure failure free operation. The source of most USB EMI problems is the two outside conductors in the connector for the power port. The primary problem is conducted common mode noise on the power line and the best EMI solution is a common mode choke.
Common mode chokes have not traditionally been considered complete power and data line solutions because: 1. PROBLEM: Traditional common mode choke geometry generates excessive normal mode impedance which causes failure in the transmission signal integrity due to excessive net differential impedance in line. This is exacerbated with Firewire and USB2.0 both of which are clocking at greater than 480 Mb/s, compared to USB1.1 which was operating @ 12Mb/s max. STEWARD SOLUTION SOLUTION: Steward’s unique product geometry minimizes net circuit Impedance at 500 MHz. The low normal mode impedance will allow for video signals to pass through Steward’s common mode chokes without any perturbation to the picture quality. Steward’s high performance common mode chokes have the lowest normal mode impedance in the industry today. 2. PROBLEM: Traditional common mode chokes exhibit excessive amounts of crosstalk between data lines. STEWARD SOLUTION: Steward’s unique ferrite material used in all Steward multi-line common mode chokes has high volume resistivity (ohms per cm) which eliminates electrical crosstalk. [ … ]