Re: [SI-LIST] : pecl termination

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From: D. C. Sessions (si-list@lumbercartel.com)
Date: Wed Jun 06 2001 - 09:18:16 PDT


On Wednesday 06 June 2001 08:07, ruston, matt wrote:
> Binoj, D.C.:
>
> Hi. Binoj, I forgot that you are AC coupled to the xceiver, so what I told
> you before is only partially correct. D.C. is right to say terminate to the
> middle of the I/O range. This will bias the AC coupled signal in the proper
> range. The two bias resistors and the cap should be kept as close as
> possible to the xceiver input pin.
>
> However, you also need to give the PECL driver a DC bias/termination (as I
> stated before and assuming an open-emitter type of output). If not, the
> output will not toggle at all. You will need to put this termination on the
> driver side of the cap at the end of the T-line. This termination can be two
> resistors as before with a thevenin reistance of the Zo of your tline. You
> can put the thevenin voltage at VCC - 2V. There are other ways to terminate
> PECL. Check the Motorola/On Semi databooks.

Not quite. The cap is more or less transparent to waves, so its location isn't
critical. The bias resistors should be line-impedance and close to the receiver.
The driver termination, on the other hand, can NOT be at the far end of the
line due to the caps, so if you have a high-impedance (current-mode) driver
it's best to place the driver termination close to the driver and keep the source
impedance close to line impedance.

> -----Original Message-----
> From: D. C. Sessions [mailto:dcs@lumbercartel.com]
> Sent: Wednesday, June 06, 2001 9:31 AM
> To: si-list
> Subject: Re: [SI-LIST] : pecl termination
>
>
> On Wednesday 06 June 2001 05:25, Binoj G. S. wrote:
>
> > I am using a differential pecl to ttl and ttl to differential
> pecl translator in my design in which the differential pecl o/p is using to
> drive one fo transceivers(hfbr 5903).As the fo transceiver i/p are in a
> different level I used capacitor coupling and biased the differential o\p
> at the fo i/p side.what I did is, biased the lines to middle of fo i/p
> (1.96v) with a thevenin termination which provides a 50 ohm impedance.
> > But what I read is that the termination voltage should be vcc-2,i.e.
> 1.3v(3.3-2).How can I satisfy both of this condition or I am doing something
> false?The fo I/p levels are as follows
> > vil min-1.49v vil max-1.825 vih min-2.135 vihmax-2.42
>
> The termination to VCC-2 is in the context of open-source drivers
> (ECL open emitters, NMOS open sources, or PMOS open drains.)
> In your case, you want to terminate to VCC-1.32 volts which is
> the center of the common-mode range.
>
> --
> | I'm old enough that I don't have to pretend to be grown up.|
> +----------- D. C. Sessions <dcs@lumbercartel.com> ----------+
>
>
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