[SI-LIST] Re: De-emphasis in 3GIO


Chris-

Pre-emphasis and De-emphasis as applied to digital communications
achieve the same thing in slightly different ways.

Since most communications channels exhibit a low-pass characteristic
due to the frequency dependent losses associated with the channel
(read that 'transmission line') the purpose of pre-emphasis is to
emphasize the high frequency components of the transmitted signal
relative to the low frequency components. Conversely you can also
attenuate the low frequency components relative to the hf components
and achieve the same effect. The bottom line is that you ultimately
want a flat composite channel response to assure maximum eye opening
(i.e. to minimize ISI and DDJ)

One possible advantage of using de-emphasis as opposed to pre-emphasis
is that there _may_ be less EMI associated with the de-emphasized signal
as the maximum signal excursion can be less and the resultant signal
has relatively less hf components.

There are certain practical limitations associated with each method.
With pre-emphasis it may be possible to exceed the peak-peak level
specified for your protocol. With de-emphasis the limitation is based
on not achieving the specified minimum level for a particular protocol.

There many ways of achieving the pre/de-emphasis function. Some designs
use an analog approach playing around with the tail current of a
differential pair. Typical digital approaches are based on a FIR filter.
Then there is the difference between 'static' and 'dynamic' pre-emphasis.
In the 'static' variety, each and every bit transmitted gets pre-emphasis
applied. In 'dynamic' pre-emphasis the circuit is smart enough to look
at previous bits and to determine if it is appropriate to pre-emphasize
the current bit or not. Fun stuff.....

-Ray Anderson
Sun Microsystems Inc.

>
>
>Gabriel
>
>As I understand it, and somebody correct me if I am wrong, de-emphasis is
>just another term for the same concept as pre-emphasis.  In most SERDES
>designs, it is called pre-emphasis, however, most communications guys use
>the term de-emphasis due to the fact that the DC gain is, "de-emphasized."
>Technically, it means the same thing.
>
>-Chris Brewster
>
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