[SI-LIST] Re: LVDS Routing
- From: wolfgang.maichen@xxxxxxxxxxxx
- To: Prashanth Rajappan <prashanth_rajappan@xxxxxxxxxxx>
- Date: Wed, 6 Apr 2011 10:30:47 -0700
Hello Prasanth,
you do not say what your signal parameters are ("low frequency" can mean
very different things to different people!).
As long as the necked-down section is shorter than approx. 1/10 to 1/4 of
the shortest wavelength of interest, its impact will be minimal. Note that
this shortest wavelength (and associated maximum frequency) is not
determined by your clock frequency or bit rate, instead it depends on your
signal rise time. So even a 1 MHz clock can potentially have a bandwidth in
the GHz region if the rise time is sufficiently fast:
BW = 0.33 / T_rise
If your line is a data signal that gets clocked (strobed) far away from the
edge then even strong ringing may not be important if it settles out before
the strobe. But if it is a clock signal though then typically more caution
is necessary - the system may run at a slow clock rate but if the receiver
logic is fast then you could still get double-clocking caused by strong
reflection.
If you want to see in detail how the neckdown affects your signal, it is
pretty easy to set up a simple first-order spice simulation where you
stitch together two sections of loss-less transmission lines - with
characteristic impedances calculated from the 14min and 8mil line width and
respective line spacings.
Regards,
Wolfgang
From: Prashanth Rajappan <prashanth_rajappan@xxxxxxxxxxx>
To: si-list@xxxxxxxxxxxxx
Date: 04/06/2011 03:58 AM
Subject: [SI-LIST] LVDS Routing
Sent by: si-list-bounce@xxxxxxxxxxxxx
Hi ,
For the LVDS pair (with low frequency signal) in my board has target
impedance is 100 Ohms. The trace width required (microstrip - 2Layer board
stack) is 14mils according to the calcuation.
However from the pads of the lvds transmitter max trace width that can be
taken out is 8 mill without any DRC issue. Will there be any problem if i
neck the trace to 8 mils to take out route from pads w.r.t perfomance?Can
someone suggest a possible soultion.
thanks in advance !!!
Prasanth
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