VSC8201DL データシートの表示（PDF） - Vitesse Semiconductor

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VSC8201DL

SimpliPHY® VSC8201 PCB Design and Layout Guide Single Port 10/100/1000BASE-Tand 1000BASE-X PHY

Vitesse Semiconductor 

Characteristic impedance of the stripline is calculated using the equation:

VSC8201DL

Zo

=

--6---0--- ⋅ ln

εr

0---.--6---7---π----4(--0--⋅-.--8b---w------+-----t--)

Ω

b where is the distance between the planes above and below the stripline. The above formula is used when the stripline is equidistant

from the above and below planes.

The propagation delay of the strip line is:

tpd = (85) ⋅ εr ps/in

L If the length of the line such that:

L(in)

<

-----------------R----i--s--e----–-----t--i--m-----e---(--p---s---)-----------------

P

r

opaga

tion

–

delay





i---np---c-s--h-

×

1--

6

Then the line must be considered a transmission line and proper termination techniques be employed.

Assuming a dielectric constant of 4.4 for FR-4, we get tpd =141ps for a microstrip line. If the rise time of the signal is 0.7ns then a trace

with length L greater the 0.827 inch should be considered a transmission line.

The 2 most common termination techniques are as follows:

• Place a resistor equal to the characteristic impedance of the line in series with and close to the signal source. This is called series

termination or source termination.

• Place a resistor equal to the characteristic impedance of the line between ground and the end of the signal trace. This is called end

termination.

The common value for the characteristic impedance of a transmission line is 50 Ω .

Rev. 1.6.3 - 9/29/04

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