19-03-2014, 04:57 PM
POWER SUPPLY LAYOUT – Layout considerations for Line-Power Supplies
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Harmful noise from Power Supplies
Power supplies, especially switching-mode power supplies, are a serious source of electric
and magnetic interference fields. Noise (EMI, electromagnetic interference) appears in both
conducted and radiated form. While conducted EMI is mostly found in the low frequency
range of several kHz to 30 MHz, the radiated EMI is mostly found in the frequency range
from 30 MHz to 10 GHz. Electrical noise is transmitted into a system through the following
ways: galvanic (direct electrical contact), electrostatic coupling, electromagnetic induction,
or radio frequency interference (RFI).
Unfortunately, the ground symbol used in schematics generates the illusion that an unvary-
ing reference ground actually exists. In reality, there are only signal sources and their re-
turns. A reliable PCB design requires first of all careful layout. Applying proper board-layout
techniques at design time prevents from “unexplainable” or “random” system failures later.
One of the most common symptoms of an inappropriate electronic design or layout is a
drastically reduced system performance (i.e. range wide reduction and “phantom tele-
grams” in case of radio receivers).
POWER SUPPLY LAYOUT
The most important requirement for the RF receiver modules to perform a wide range is
a very stable and low noise supply voltage output. As important as the choice of power
supply type and components is the design of the PCB to which these items are attached
with regard to control both radiated and conducted emissions. The following recommen-
dations apply also for designing the layout for additional electronics like digital circuits.
Since parasitic impedances increase with frequency, a simple PCB signal trace might
become a complex path (antenna!), rather than just a low resistance with respect to DC
measurements. Due to these parasitic impedances – both capacitive and inductive in
nature, the layout of the PCB is very critical for the entire system.