[SI-LIST] Re: gnd plane kept close to track

Hi Geoff,
I've heard talk before of the phenomena you describe - "different waves =
traveling at different velocities" - on microstrip.  I would expect this =
to manifest itself as a degraded risetime (perhaps 2 distinct edges?).  =
I have not seen this in any measurements (TDR).  The signal appears to =
propagate as a single entity at a velocity called for by a dielectric =
constant somewhere between that of air and the dielectric.  Do you have =
any examples of the effect you describe?  My current thinking is that =
this is a misconception.

The only similar phenomena I've measured (at least, I think this is what =
I was seeing) is the dispersion effect - VNA indicates the velocity =
varying as a function of frequency.  But, it's effect was very slight, =
measured up to 6GHz.

Jeff Loyer



-----Original Message-----
From: Geoff Stokes [mailto:gstokes@xxxxxxxxx]
Sent: Wednesday, July 23, 2003 4:46 AM
To: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Re: gnd plane kept close to track


Karan
=20
In microstrip the electric field is shared between the solid dielectric =
and
the air above.  The solid part is between the line and the ground plane.
So, there is a wave travelling above the line at the velocity we find =
for
air ( 0.3 m/ns), while there is another wave travelling in the solid at =
a
slower velocity.  The received signal is, then basically messed up but =
for
many purposes it's good enough and looks as though the velocity was
somewhere in between.  The closer the ground plane, the more the field
concentrates in the solid dielectric, and the air part is relatively
reduced, so the effective velocity is reduced.  The MECL System Design
Handbook gives the equations.

-----Original Message-----
From: karan bagga [mailto:kbagga31@xxxxxxxxx]
Sent: 23 July 2003 12:28
To: Geoff Stokes
Subject: Re: [SI-LIST] Re: gnd plane kept close to track


I go it it geoff..
=20
There is one doubt from your mail.
=20
How does velocity gets affected in microstrip i.e. it increases or =
decreases
?
&=20
why this affect is not seen for strip lines ?
=20
Karan

Geoff Stokes <gstokes@xxxxxxxxx> wrote:

Karan

If the unwanted noise is mainly from other circuits and equipment, the
signal integrity needs improvement. The ground plane improves signal
integrity by controlling waveform shape and crosstalk. Here is an =
extract
from the "MECL SYSTEM DESIGN HANDBOOK" 4th edition (Motorola, 1983):

&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
Chapter 3: Printed Circuit Board Connections
Any signal path on a circuit board may be considered a form of =
transmission
line. If the line propagation delay is short with respect to the rise =
time
of the signal, any reflections are masked during the rise time and are =
not
seen as overshoot or ringing..... .......However as edge speeds increase
with faster forms of logic, the line lengths must be shorter to retain
signal integrity.
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&

The text goes on to discuss reflection problems, overshoot, ringing,
crosstalk, transmission line equations for specific board design
applications etc. If you want to know about transmission lines and
reflections, see Chapter 1 of "Microwave Circuit Design" by Vendelin, =
Pavio
and Rohde (Wiley,1990) or a good book on Electricity and Magnetism. =
(This
relates partly to some of Sainath's questions).

Karan, you mention EMI which has two aspects, emission and =
susceptibility.
They usually come to the same thing because of the well-known theorem of =
the
reciprocity of an isotropic passive network. For example if there is
crosstalk coupling from line A to line B, measured as a power ratio, =
there
is the same amount of crosstalk from line B to line A. If one end of =
line A
is terminal 1 and an end of line B is terminal 3, a signal applied to
terminal 1 will appear as an attenuated output at the other terminals,
including for example terminal 3. Also the same signal applied to =
terminal
3 will give the same attenuated output at terminal 1. In y- and
s-parameters, that's y31 =3D y13 or S31 =3D S13. Emission and =
sensitivity can
be caused by conduction or radiation paths and are classified that way =
in
EMC work. The voltage and current relationships are described in the =
same
book by Vendelin etc.

For high frequency applications, a track next to one or two ground =
planes is
viewed as a transmission line: microstrip for one plane, stripline for =
two.
The important properties of the transmission line are characteristic
impedance (Zo), phase velocity, loss (includ! ing radiation), uniformity
and,
sometimes, dispersion. From the text books or suitable design tools, the
ground plane spacing and trace width are designed to give a desired Zo.
When the ground plane is brought closer to a track, the equations I've
mentioned above show that the Zo drops. The other parameters don't =
change
much. The velocity is changed a little for microstrip but not for
stripline. To compensate the Zo drop we can make the trace more narrow.
The reduced separation from the plane and the reduced width produce less
crosstalk coupling by radiation or ground current coupling. If the
crosstalk reduction is sufficient, it solves the original problem. This
applies to all connections including signals, power, ground returns and
control lines.

This was only a start, but hope it helps....

Cheers
Geoff

> -----Original Message-----
> From: karan bagga [mailto:kbagga31@xxxxxxxxx]
> Sent: 23 July 2003 10:56> To: kbagga31@xxxxxxxxx; =
si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: gnd plane kept close to track
>=20
>=20
> please tell me if i have stated something wrong.
> i am starting with this.
>=20
> Karan
> karan bagga wrote:
> It is recommended that ground plane be placed very close to=20
> the Routing Plane
> as it reduces the noise.
> What kind of noise it reduces ?=20
> What is the exact problem it solves ?
>=20
> I know of one thing i.e. it reduces the Loop Current and thus=20
> reduces EMI.
> Has it any thing to do with Reflection , Delay and anyother=20
> kind of Noise ?
>=20
>=20
> Regards
> Karan
>=20
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