SAMLEX AMERICA, INC.
| 11
Section 5
| Limiting Electromagnetic Interference
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.
ADDITIONAL GUIDELINES FOR REDUCING RF NOISE
• Use additional appropriate AC Radio Frequency Interference (RFI) Power Line Filter
rated for minimum 10A immediately before the AC input of the power supply. For
example, consider suitability of Model # 10VN1 from TE Connectivity www.te.com
or similar. Filtered, Ferrite Coated Cord Set is another choice. 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 attenua-
tion 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. For example, consider suitability of EMC Cord
“IMU-14” from EMC Eupen www.emceupen.com or similar.
• Use additional appropriate DC radio frequency interference (RFI) power line filter
rated for minimum 30 A immediately after the DC output of the power supply. For
example, consider suitability of 30A filter Model # “FD10EE030” from Curtis Industries
www.curtisind.com or similar.
• Twist the Positive and Negative wires from the output of the power supply to the radio.
• The DC side Positive and Negative outputs of these power supplies are isolated from
the chassis. As explained earlier, the noise currents are filtered to the chassis of the unit
and the chassis is connected to the Earth Ground through the Earth Ground Pin of the
AC power outlet receptacle. Avoid connecting (referencing) the DC Negative output
terminal of the power supply to the Earth Ground.
• Connect a ¼” wavelength of wire on the Negative terminal of the power supply. Con-
nect one end of the wire to the Negative terminal and leave the other end free. The
wavelength corresponds to the wavelength of the interfering frequency. (May not be
practical for long wave lengths).
[Formula: Wave length (Meters) = 300 / frequency in MHz]
COMBINED FILTERED NOISE CURRENTS FROM MULTIPLE SMPS ON A
BRANCH CIRCUIT MAY TRIP GROUND FAULT CIRUIT INTERRUPTER (GFCI)
During malfunction or an accident, the metal chassis of a device may get energized to
unsafe voltage due to internal high voltage section coming in contact with the chassis.
If a person standing on Earth touches this energized chassis, a leakage current
