[SI-LIST] PREVENTING EMISSIONS IN USB SYSTEM

USB People,

Since my last mail I have read a little about USB. I will be operating =
USB 1.1 at full speed. Some questions:-

1. Filtering of Differential data lines

I have seen a number of recommendations for filtering of the signalling =
lines. These include:-

(a) Chip based solutions such as microsemi LX7201 which basically give =
termination resistances plus shunt capacitance of 47 pF to ground for =
each line in a single chip. This chip also has ESD protection on board.
(b)  Ferrite beads such as Murata BLM
(c) Common mode chokes such as Murata PLP3216 or TOKONT2520.

I have no practical experience of the use of these with USB, can anyone =
suggest what they have found works well in practice?

2. Routing from Board to connectors

Whats the best way to do this from a 'best EMC' performance? I was =
thinking about a board mounted connector stuck out through the wall of =
the enclosure with solid grounding from connector to metal casing? If I =
can not do this whats the best way to get from a connector on the case =
to a board some distance away inside the box?

3. USB power and ground

My idea for filtering of the USB power is to  filter the 5 volt USB line =
using a T network of two high current SMD chokes such as the meggitt =
3615 series 100nH 3.5 ampere part. SRF is not as high as I might like =
(600 MHz). The shunt capacitance in the T network would be say 470 pF. =
Does this seem sensible?

I am not sure what I will do regarding the ground input? I will be using =
a complete continuous ground plane. I am wondering if it is a good idea =
to place a similar T network as that on the VCC line on the ground line? =


4. External cables.

The spec suggests that the USB diff signalling likes about 90 ohms Zo Tx =
line. Twisted pair is suggested as the way to go. I would like to use a =
screened cable but I think I read somwhere that there is some rule about =
termination of screens with USB one end only maybe ? Can anyone shed any =
light on this one?



Thanks in advance



Steve Rogers B.Eng (Hons) C.Eng IEE
RF Design Engineer

Micromill Electronics Limited
Leydene House
Waterberry Drive=20
Waterlooville Hampshire
PO7 7XX

Tel: +44 (0) 23 9236 6600
Fax: +44 (0) 23 9236 6673

Registered No. 1456922 (England). =20
Registered Office Brook Road
Wimborne, Dorset BH21 2BJ

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