Designer’s Guide Community

‹ Previous topic | Next topic ›

Pages: 1

Anyone ever heard about double reflection? (Read 1817 times)

henrytqy

Junior Member

Offline

Posts: 28

Anyone ever heard about double reflection?
Oct 18^th, 2012, 6:41pm

Hi dear all,

Recently, I was desgning a broadband driver. One of the paper said "In broadband applications, the buffer must drive an on chip back termination resistor of about 75 ohm in addition to an off chip load of 50 ohm in order to reduce the double reflections"

Anyone know this conception? Why 75/50 can reduce it?

Thanks~~~

IP Logged

aaron_do

Senior Fellow

Offline

Posts: 1398

Re: Anyone ever heard about double reflection?
Reply #1 - Oct 18^th, 2012, 6:53pm

Hi,

it sounds like double reflection simply means reflection at the load, and re-reflection at the source. So without the 75-ohm resistor, the buffer would have an output imepdance close to zero.

What is the impedance of the transmission line? In theory, if the impedance is 50-ohm, then the load reflection will be minimized. In that case I can't see why you need an additional 75-ohm resistor. If Z0 is 75-ohm however, then the source re-reflection will be minimized.

Whatever it is it sounds fishy. Just my thoughts.

regards,
Aaron

there is no energy in matter other than that received from the environment - Nikola Tesla

IP Logged

loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: Anyone ever heard about double reflection? Reply #2 - Oct 20^th, 2012, 8:17pm If you got a transmission line with some impedance (37-50-75-200 or whatever) the impedance at each end of the line should match that transmission line. That's the total termination impedance. Combination of the driver circuitry and the termination circuitry.
Back to top	Jerry Twomey www.effectiveelectrons.com Read My Electronic Design Column Here Contract IC-PCB-System Design - Analog, Mixed Signal, RF & Medical IP Logged

Pages: 1

‹ Previous topic | Next topic ›