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Design >> RF Design >> Anyone ever heard about double reflection?
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Message started by henrytqy on Oct 18^th, 2012, 6:41pm

Title: Anyone ever heard about double reflection?
Post by henrytqy on 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~~~

Title: Re: Anyone ever heard about double reflection?
Post by aaron_do on 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

Title: Re: Anyone ever heard about double reflection?
Post by loose-electron on 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.