Variations in the length of the twisted pair can easily be kept less than 
1/4", about 30 ps. If this range causes concern, you probably want to use a 
rigid connection.

Richard

At 07:18 PM 12/6/2001 Thursday -0500, you wrote:
>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
>|
>|

Cheesecote Mountain CAMAC
24 Halley Drive; Pomona, NY 10970
845 364 0211, www.cmcamac.com

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