I agree.  My only concern may be how you attach the twisted pair from PCB to PCB.
Will there be a consistency issue from each unit manufactured to the next?

____________
Brian Guralnick


----- Original Message -----
From: "Richard Sumner" <[EMAIL PROTECTED]>
To: "Protel EDA Forum" <[EMAIL PROTECTED]>
Sent: Thursday, December 06, 2001 3:58 PM
Subject: Re: [PEDA] 50 ohm differential impedance connections and traces


| Scott,
|
| Maintaining signal integrity is a matter of degree. Any discontinuity in
| impedance will produce a reflections and changes in the received
| signal.  But, if you are using ordinary pecl, 1-2 ns rise times, what you
| want to do is easy. With high speed pecl, 200-300 ps rise times, you
| will  have to be more careful, but extreme measures are usually not
| required. Discontinuities in the controlled impedance signal path that are
| short compared to the signal rise time will have very small effects on the
| signal.
|
| First, keep the pull down resistors on the pecl output close to the driving
| chip.  Do not place a Thevenin equivalent termination on the other board
| (you don't want dc power supply currents between the two boards).  The best
| is a simple parallel termination (no connection to ground) at the receiving
| end.
|
| If you can't start the twisted pair at the signal source (close to the
| chip) then run two 50 ohm controlled impedance lines to where the twisted
| pair will start. For ordinary fr4, a line (on an outside layer) about twice
| as wide as the layer spacing (outside layer to next layer down, which
| should be either a ground (best) or power plane, not a  wiring layer) will
| give you about 50 ohms.  The two lines spaced 3-4 line widths apart
| (uniform spacing) will give you an approximately 100 ohm differential line.
| Connect to the next board with a short piece of twisted pair. Typical
| twisted pair has an impedance about 100-120 ohms. One inch is only about
| 130 ps delay.
|
| There must be a ground connection between the boards, but this does not
| need to be extremely close to the twisted pair signal path.  You can also
| use two pieces of 50 ohm coax, which will connect the grounds. Unless you
| are very sensitive to emi, twisted pair will be fine.
|
| Continue on the next board with controlled impedance if necessary.
| Terminate with a 100 ohm resistor between the 2 lines of the differential
| pair. This should be very close (or a little beyond) the receiver.  Do not
| terminate with resistors to ground or to a power supply.
|
| If you can launch the twisted pair close to the driver, and land close to
| the receiver, you can dispense with the controlled impedance lines on the
| pcb, and use a longer piece of twisted pair. This will improve the received
| signal, by eliminating the discontinuities in the total path (where the
| path changes between pcb and twisted pair). You can transmit differential
| pecl (or ecl) over several feet of twisted pair with very little signal
| degradation.
|
| I hope this is clear!  (and helpful)
|
| Richard
|
|
| >A couple of questions.
| >
| >Does anyone have a good way of doing board  to board connection (only
| >about an inch) for a differential 50 ohm connection?
| >
| >Can I make two parallel connections with 100 ohm twisted pair?
| >
| >We have done some calcs for a FR-4, 1 oz copper, double sided, 1.6mm
| >thick, and we get traces 140thou, 8thou spacing over a gound plane for a
| >50 ohm differential impedance. Can anyone confirm our result? 140 thou is
| >getting up there!
| >
| >The reason for the questions is that we need to take a differential PECL
| >signal from one board to another and maintain the signal integrity.
| >
| >Scott Ellis
| >Manager
| >Novatex Research - Excellence in Electronic Research & Development
| >[EMAIL PROTECTED]
| >41 Yule Road, Merewether, Newcastle, NSW 2291, Australia
| >Ph 0412 988408   Fax 02 49636058
| >
|
| Cheesecote Mountain CAMAC
| 24 Halley Drive; Pomona, NY 10970
| 845 364 0211, www.cmcamac.com
|
|

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