RE: [SI-LIST] : Other materials for GHz application

Mellitz, Richard ([email protected])
Tue, 2 Dec 1997 09:13:15 -0500

I guess this begs the infamous question "why?". Are you taking about
microwave RF applications, digital, etc.? How long are your etch runs?
e.g. How big a system are we taking about? If your running 700 MHz
digital that sort if implies that your edges are faster. How much
faster? What type of digital technology are you willing to use? Most
of the problem boils down to how much dispersion can your design
tolerate and signal fidelity/performance trade offs you need to make.
Yup, a whole bunch of work! :-)

You can get a good feel for what FR4 will do with a good TDR instrument.
Most can easily inject a 50 ps signal down a line. Look what happens.
There are too effects to notice. The first is the slope change of the
"flat" step region due to skin effect, but may be altered by a
non-homogeneous dielectric or geometry anomalies. The second effect is
to observe what happens to the reflected rise time (assuming you are
driving an open line.) Most of what you will see is dispersion. You
will likely be able to sustain 150 ps into an open for line ~ 12". Now
ask yourself the question how are loads going to effect this edge.
Maybe the loads are a bigger part of the picture than the media.

The bottom line is that your question is not really so simple. A rule of
thumb answer is OK for planning, but you will likely be able to push (or
understand) these limits with a little work and analysis.

Rich Mellitz, NCR

>From: [email protected][SMTP:[email protected]]
>Sent: Tuesday, December 02, 1997 9:55 AM
>To: [email protected]
>Subject: [SI-LIST] : Other materials for GHz application
>Does anyone have information on the following:
>1. Why and What is the limitation of using FR4 up to 500-700 MHz?
>2. Beyond this clock freq, what should be recommended for the PCB and why?
>3. Under what circumstances, we should migrate MCM and why?
>Thanks in advance.
>Best Regards.
>Cheah Soo Lan
>SINGAPORE 118 230
>EMAIL: [email protected]