[SI-LIST] Re: Power planes

  • From: Ray Anderson <Raymond.Anderson@xxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Thu, 24 Jul 2003 09:27:36 -0700 (PDT)

>This is quite a general question.
>If I place the GND plane too close to the track wouldn't it affect the 
of signal on track ?
>Possible reason is the capacitance b/w track and GND will increase which will 
cause the delay in changing signal(rising edges) as it will need more time to 
charge the increased capacitor value.Geoff just told us that placement of GND 
affects the velocity of Microstrip and Not Strip Line. How far is it true ?

The distance between a signal trace and a reference plane in a
stack-up does not effect the velocity of the signal propagating 
on the signal trace. It can effect the impedance of the signal
trace. The velocity of propagation is dependent only on the effective
dielectric constant (Er) or electric permittivity of the substrate material.

Yes, there will be more distributed capacitance between the signal
trace and the plane for small spacings, but this does not effect
the velocity of propagation as the PUL inductance of the signal
trace will decrease as well.

some relevant equations:
        Velocity = C/sqrt(Er)    <= velocity is a function of Er
                (where C is speed of light in this equation)
        Velocity = 11.8/sqrt(Er) inches/nSec  <= numeric version of above
        Velocity = 1/sqrt(L*C)  hence:
           L = (delay^2)/C  and C = (delay^2)/L
-Ray Anderson
Sun Microsystems

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