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Motherboard/Modem PCB Layout Guidelines

Floyd Kling 



Tip/Ring trace Layout Guidelines for Motherboards in Embedded applications:

Trace Routing (IMPORTANT):
Regarding the Tip/Ring (T/R) traces: 

All suggested spacing's are MINIMUM... i.e. METAL TO METAL, in all directions - 360 degrees

1.  Min trace width of T and R....  .010" 
2.  Min distance from metal of trace to ANYTHING outside of the DAA.... 0.120"(3mm) or greater...
3.  Min distance between Tip and Ring traces (within DAA area)...  0.035" or greater
Conducted and Radiated Emissions:
Radiated Emissions
Your final product, with modem inside, will need to be tested for Part 15(RFI emissions)  There will be two types of emissions tested... radiated and conducted.
We suggest the following to be placed in your mother board layout to minimize possible radiated emissions that can creep out the T/R lines.
In your artwork, place a .001uf/3kv  from Tip to gnd and a .001uf/3kv from Ring to ground. Ideally, tie them to your metal front panel.  If that is not possible, find the strongest ground available.  This will help minimize radiation that may creep out the T/R leads.  The grounds should be as STRONG as possible... these caps should be placed as close to the RJ-11 as possible.
Note:  Add these caps in the artwork, but do not load them.  When you go to your Pt.15 testing, you will then have the provisions, if needed, to add these caps.  If you don't need them to pass Pt.15, then do not load them in production.... this is only a 'ace in the hole' that most designs do not need.
If you need to load these caps, and you still have excessive radiation coming from the T/R please give me a call to discuss this... YOU NORMALLY DO NOT NEED TO GO HIGHER THAN .001uf, but please contact me first.
Here are two physical versions of a .001uf/3KV caps.
SMT version:
     Manufacture: NOVACAP
     Part Number:  LS1808N102M302NT.
Through Hole version:
 - rather large but you still might be interested.
    Manufacture:  CERAMITE
    Part Number:  30TSD10.

These particular caps are suggestions, not mandatory.  If you have other sources, they would be fine also, just ensure the voltage is at least 3KV.

Why 3KV min when the FCC Pt.68 Tests to 1500v PEAK? - Actually you could get by with 1KV or 2KV caps, but to keep a good margin of error, 3KV is recommended... 3KV are physically about the same size and cost as 2KV.

Conducted Emissions:

Conducted emissions are RARELY caused by the modem. Overwhelmingly, excessive conducted emissions are a result of the hosts power supply.
Make sure your host power supply and power traces and planes are substantive and well bypassed.  You should add copious amounts of HF (High frequency) bypass caps <to a strong ground> on the power traces that feed the modem. 

If necessary, you may consider an inductor in series with the power supply trace to the modem and add bypasses (to ground) on the modem side. 

There are usually three fundamental reasons that cause excessive EMI/RFI... here they are in order of priority.

  1. Grounding
  4. and if that does not help... try more grounding (get the point?)

    IMPORTANT - Do NOT put grounding plane (or Power Plane... or ANY PLANE) in the areas of the DAA including traces and components that are in the DAA area.... that is a BARRIER AREA .... Components that must go into the DAA and also require a ground, make the component straddle the barrier... AGAIN, THIS IS IMPORTANT.

Still having emissions problems?

If the radiation is propagating out the RJ-11 cable and all else has failed, try adding a common mode filter in series with Tip and Ring.  The filter may be added on your modem/mother board or actually in the RJ-11 cable itself. 

If you select the cable method, you will need to ship the special cable<with filter> with every product.

Here a link to a PCB mountable Common Mode Filter which consists of 2 separate Inductors that are to go one(1)each in series with Tip and and one in series with Ring:

Hope this helps...



Copyright © 1997-2011 Floyd Kling 
- All Rights Reserved -