14 | SAMLEX AMERICA INC.
SECTION 5
| Limiting Electro-Magnetic
Interference (EMI)
5.3 FILTRATION OF CONDUCTED NOISE
The conducted RF noise from these SMPS units is limited to the maximum allowable levels by
internal filtration. The filtered RF noise currents (< few hundred micro Amps) are bypassed
to the chassis of the power supply. The chassis is, in turn, connected to the Earth Ground pin
of the AC input power cord. Thus, the filtered noise currents are intentionally leaked to the
Earth Ground. This is termed as the “Earth Leakage Current”.
5.4 EXCESSIVE RF OUTPUT INTERFERENCE BY SMPS DUE TO
INCOMING RF INTERFERENCE WHEN POWERING RADIO TX / RX
SMPS are also recipients of radio interference. The normal operation of the power supply
can be disturbed due to RF noise getting coupled into the power supply. Thus, the power
supply may generate excessive RF noise and lose output voltage regulation due to excessive
transmitter energy being coupled through the AC / DC lines to the power supply’s regulator
feedback path. This may be due to antenna being too close or due to the antenna or feed
system not radiating properly. First check the antenna system SWR. Then, if necessary,
relocate either the antenna or the power supply farther apart. The receiver may “hear” the
power supply. A slowly moving, slightly buzzing carrier heard in the receiver may be caused
by the antenna being too close. As with the transmitter related noise pick up, a loose coaxial
connector or a broken or a missing ground may aggravate this problem. Normally, this noise
will be below the background or “band” noise. Increase the separation between the power
supply and the receiving antenna. Use an outdoor antenna. This will reduce the amount of
signal picked up from the power supply and also increase the amount of the desired signal.
5.5 ADDITIONAL GUIDELINES FOR REDUCING RF NOISE
• Use additional appropriate AC Radio Frequency Interference (RFI) Power Line Filter
immediately before the AC input of the power supply (should be rated for the AC
input voltage and input current of the power supply). Filtered, Ferrite Coated Cord
Set for AC input is another choice (rated for AC input voltage and input current of the
power supply). These cord sets, with integral line interference filters, reduce Common
and Differential Mode Interferences over a wide frequency range. Because they are
shielded, they are also effective against radiated interferences. In addition to the
built-in filter networks, the cable conductors are coated with an RF absorbing ferrite
compound. This provides additional attenuation at high frequencies that is lacking in
most regular LC filters. The RF absorption of the ferrite-coated cable avoids resonances
at high frequencies, reducing the conducted and radiated RF noises even further.
• Use additional appropriate DC radio frequency interference (RFI) power line filter
immediately after the DC output of the power supply.
• Twist the Positive and Negative wires from the output of the power supply to the radio.
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