RE: Shielding

Grasso, Charles ([email protected])
Wed, 2 Jul 1997 08:17:59 -0600

Interesting topic.

I used to use guardbands years ago when I first started in the EMC
buisness. After a while, and a little
research, I have stopped using them as a primary EME protection tool. I
will still use them as a crosstalk
mitigation tool for circuits with high impedance inputs, for example.

The concept of guardbands, on the surface, looks beneficial but the
implementation is difficult and still controversial.
For example, I downloaded some "EMI design rules" that emphatically
state guard bands should only be grounded
at one end!!

There was also a paper at the Santa Clara EMC Symposium that indicated
that the inductance of the ground
plane was reduced with guard bands. In my opinion, the measured
reduction was an artifact of the test set up
& not real (Comments any one).

I concur with others on this discussion area:
1. Tightly coupled traces to planes is MORE beneficial for EME
reduction than guardbanding.

2. Guardbands are difficult to implement and consume board space.

3. Guardbands could be used as a crosstalk mitigation tool (depending
on the design).

Charles Grasso
EMC Engineer

>From: Peterson, James F (FL51)[SMTP:[email protected]]
>Sent: Wednesday, July 02, 1997 6:00 AM
>To: 'Signal Integrity'; 'Sean Murray'
>Subject: RE: Shielding
>I don't think you will have digital signals crosstalking into your
>analog section with your configuration.
>But you could have analog signals crosstalking into other analog
>circuits. It would depend on the slew rates, signal swing, and how
>sensitive the circuits are. The analog ground plane will reduce
>crosstalk but it won't get rid of it. What kind of signal to noise ratio
>can you tolerate?
>Good luck,
>>From: Sean Murray[SMTP:[email protected]]
>>Sent: Tuesday, July 01, 1997 10:26 AM
>>To: 'Signal Integrity'
>>Subject: Shielding
>>I'm a little confused on the subject of shielding (guard traces). I'm
>>currently designing a board for a customer, in which the analog & digital
>>traces are on seperate layers. Both layers are shielded by their respective
>>planes (i.e. Digital by Dgnd; Analog by Agnd). My question is; do the
>>analog traces still need to have guard traces to protect against any noise
>>that might be coming from other analog traces on that layer? (and likewise
>>for Digital). The customer has stated that he wants the traces shielded,
>>but when asked why he could not give me a scientific answer. I was under
>>the understanding that you only needed to shield the traces if they were
>>very high speed and were not utilizing proper ground shielding (planes)
>>Any help?
>>Sean Murray
>>M&M Specialties, Inc.