USBDFxxW5

Application information

3/11

Figure 2.

Application example

2.1

EMI filtering

Current FCC regulations requires that class B computing devices meet specified maximum

levels for both radiated and conducted EMI.

●

Radiated EMI covers the frequency range from 30 MHz to 1 GHz.

●

Conducted EMI covers the 450 kHz to 30 MHz range.

For the types of devices utilizing the USB the most difficult test to pass is usually the

radiated EMI test. For this reason the USBDF device aims to minimize radiated EMI.

The differential signal (D+ and D-) of the USB does not contribute significantly to radiated or

conducted EMI because the magnetic field of the two conductors exactly cancels each

other.

The inside of the PC environment is very noisy and designers must minimise noise coupling

from the different sources. D+ and D- must not be routed near high speed lines (clocks...).

Induced common mode noise can be minimised by running pairs of USB signals parallel to

each other and running grounded guard trace on each side of the signal pair from the USB

controller to the USBDF device.

If possible, locate the USBDF device physically near the USB connectors. Distance between

the USB controller and the USB connector must be minimized.

control, and must be placed between the USB Controller and the series termination

practicable.

The USBDFxxW5 ensure a filtering protection against electroMagnetic and radio-frequency

Interference thanks to its low-pass filter structure. This filter is characterized by the following

parameters :

●

cut-off frequency

●

Insertion loss

●

high frequency rejection

Figure 4. shows the attenuation curve for frequencies up to 3 GHz.

D+

Peripheral

transceiver

D-

CABLE

Host/Hub USB

port transceiver

D- (2)

D+ (1)

D+ (2)

D- (1)

(1) for a low-speed port

(2) for a full-speed port

+Vbus

Upstream port

Downstream port

USBDF

xxW5

USBUF

xxW6