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17.
Benefits of the Shielding Approach
Emissions reduction can be achieved using shielding source reduction techniques, such as decoupling, or
PCB layout and grounding, or a combination of the two. Once a shield is in place, any revisions to product
circuitry have no effect on emissions levels. If a circuit level approach is used to control the emissions, a
change in circuitry can bring a new unknown to the emissions performance.
In addition, perfecting shielding designs now will provide a jump on future requirements for emissions
reduction. With the proliferation of wireless communications, the emissions levels will need to be reduced.
We feel it is inevitable that the FCC will tighten up the Part 15 regulations.
18.
Alternate EMI Reduction Methods
Although shielding is the brute-force method of reducing emission levels, other methods are available, such
as:
•
PCB layout modification using ground layers adjacent to high speed layers
•
Capacitive or filter decoupling
•
Redistribution of module interconnects
•
Clock pulling
18.1 Clock Pulling
Clock pulling is effective when the emission sources are narrowband (Figure33). To implement
clock pulling, a method must be devised for the modem to tell the host it is having difficulty
receiving. Devising such a method is admittedly very difficult. The host provides pulling of its
internal emission source, which is identified as a potential problem.
If this source is the cause of the interference, the pulling or slight shift of the source frequency
moves the harmonic energy out of the receive channel. This is an inexpensive way of solving the
problem, as no special shielding or decoupling is required.
The limitations of the clock pulling method are:
•
Computing devices have many more than one source
•
Each source must be identified and controlled. This identification is at times difficult.
•
The host and modem must communicate the problem at hand to attempt to correct it.
e
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m
A
d15 Modem Integration and Application Developers Guide version draft 0.53
~
Fs
Fss
Figure 33 – Clock pulling
Fs = Int erference source fundament al frequency
Fss = Shif ted source fundamental freq.
Fh = I nterference source harmonic
Fhs = Interference source harmonic shifted
Fc = Channel frequency.
Fh
Fhs
Fc
Freq
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