Re: [SI-LIST] : PECL oscillator termination

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Thu Nov 09 2000 - 12:50:00 PST


Tae-Kwang Jeon wrote:

> Hello everyone,
>
> Here's a question on 3.3V PECL osc. termination.
>
> I know that a Thevenin termination is used on the diff'l outputs of the osc.
> But, what i'm concern is the case of AC-coupling to a 50ohm termination.
> The qestions occurred to me are;
> #1 Is there any recommended value of the coupling cap?

0.1uF is OK

>
> #2 How to size the resistors to DC-bias the PECL outputs? Is there also
> any recommended range of size?

330 Ohm typical.

>
> #3 In the case of #2, the resistors are supposed to be grounded, then what
> if I make them in series?
>

In series with what? WIth the praces of the diffpair?
The resistors are there to provide bias. Make them in series with the pair - and
you divide your signal between your resistor and the trx-line. The series
termination does just that, but then, if you need AC-coupling, wher's your bias?
YOu will open can of worms, when everything is simple - connect 330 Ohm to GND,
run your traces, put the caps in series (where you like; some people have
different preferences - beginning, end) and terminate - either for diff. mode
(diff. impedance resistor between traces in the end) of for common mode (one
trace impedance resistor to GND from each trace in the end).

Mike.

>
> Any comments would be appreciated.
>
> Regards,
> TK
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> TaeKwang Jeon, LSI Logic Corp.
> 1525 McCarthy Blvd Phone: (408) 433-6818
> MS G-715 Fax : (408) 433-2840
> Milpitas, CA 95035 Email: tkjeon@lsil.com
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
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