[SI-LIST] Re: gnd plane kept close to track
- From: Geoff Stokes <gstokes@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 23 Jul 2003 16:55:01 +0100
Hi Jeff
I think my post was a little misleading. I didn't give the conclusion about
what the visible effect would be. The description I have seen before refers
to the wave front being distorted compared to a plane wave front ( a simple
plane wave as found in waveguides with uniform media such as stripline or
coax is described as "transverse electromagnetic", but microstrip is
"quasi-TEM"). The overall effect is seen as dispersion as you say, that is,
a phase distortion of a pulse, or a non-constant group delay in the
frequency domain. I would suggest (not from the text book) that the wave
fronts above and below the line do not actually part company due to the
different velocities, but, because they are strongly coupled (by the surface
current in the metal particularly) the two waves continually decay and
re-appear at a new common location: they continually "trip over" (that
expression was actually in a text book).
The dispersion, as you say, does not cause the pulse to split into two
separate edges. Instead it produces an increasing rounding or other
distortion of the shape as it travels along the line. As you say this might
not normally be a problem. The important point is that the velocity is
determined by the effective dielectric constant, which is less than Er for
the solid alone.
Cheers
Geoff
> -----Original Message-----
> From: Loyer, Jeff [mailto:jeff.loyer@xxxxxxxxx]
> Sent: 23 July 2003 15:44
> To: Geoff Stokes
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [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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