RE: [DC] How to minimiZe PCB trace propagation time

K.Balasubramanian

The equation for propagation velocity is:

        v = c /sqrt(Er)

Where: v = velocity in meters per second
         c = speed of light in a vacuum ~ 3 x 10^8 m/s
         sqrt(Er) = square root of your dielectric constant.

This means that the only two things affect the velocity of your signal.  The
first is the dielectric constant, or permittivity, of the material.  The
second factor that can impact is whether or not the trace is microstrip or
stripline.  For example with a permittivity of 4.1, a microstrip will have a
propagation velocity of:
        135.2 ps/in

And a stripline signal will have a velocity of:
        171.4 ps/in

The reason that the microstrip will propagate faster is that half of its
dielectric environment is composed of air, with a permittivity of ~1, thus
making it faster.

The long and short is that if your signal is already on the surface of the
board, then there is little if anything you can do except change the
permittivity.  If the signal is a stripline, consider moving it to the
surface.  Another option would be to adjust the microcontroller code to
compensate. Finally, you could run a piece of wire over the board so that
its dielectric material consists mainly of air.  Bear in mind that I didn't
say that any or all of these are practical.  Those darn laws of physics...

Hope this helps,
Devon

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Devon J. Post - C.I.D.                           Mayo Foundation
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Voice:  (507) 538-5479                           Sn 2-134C
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Internet:  http://www.mayo.edu/sppdg/sppdg_home_page.html
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-----Original Message-----
From: Balasubramanian Krishnan [mailto:[log in to unmask]]
Sent: 31 August, 2006 8:21 AM
To: [log in to unmask]
Subject: [DC] How to minimise PCB trace propagation time

Dear Experts,
      In one of our design we see that signal round trip delay (clock 'high
to low' transition and data arrival) is beyond the timing requirement of
the microcontroller used. The microcontroller expects the data to arrive
within 14 nanoseconds after the clock pulse is activated but the data is
reaching only after 16 nanoseconds delay and hence the device fails. No
over shoots or undershoots are noticed in the clock signal, the clock
frequency is 24 MHz with rise time of about 4  to 5 nano seconds. The PCB
trace length cannot be reduced. Costly PCB material (for low relative
permittivity) is not acceptable being a cost effective product.
      How to handle this issue? Your expert guidance will be of great use.


Sincerely

K.Balasubramanian
Project Leader - Hardware.

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