14 | SAMLEX AMERICA INC.
SAMLEX AMERICA INC. | 15
SeCtIOn 7 |
limiting electro-Magnetic
Interference (eMI)
!
cAUtion!
This unit generates, uses and can radiate radio frequency energy and, if not installed and
used in accordance with the instructions, may cause harmful interference to radio communi-
cations. However, this does not guarantee that interference will not occur in a particular in-
stallation. If this equipment does cause harmful interference to radio or television reception,
which can be determined by turning the equipment off and on, the user is encouraged to try
to correct the interference by one or more of the measures recommended in the following
paragraphs.
Un-intentionAL RF noiSe GeneRAteD BY SWitcHeD
MoDe PoWeR SUPPLieS (SMPS)
Switched Mode Power Supplies (SMPS) employ high frequency switching (25 KHz in this
unit) and thus, are a source of radio interference, a recipient of radio interference and a
conduit of radio interference. (older Linear Type, low frequency 50 / 60 Hz transformer
based power supplies do not employ high frequency switching voltages and will be
quieter as compared to SMPS).
The primary emission sources originate in the switching devices due to their fast switch-
ing current transitions: harmonics of the switching frequency and broadband noise
created by under-damped oscillations in the switching circuit. The secondary source is
from the bridge rectifier, both rectifier noise and diode recovery. The AC input rectifier
/ capacitor in the front end of the SMPS (excepting those with Power factor correction)
generate power supply harmonics due to the non-linear input current waveform. The
noise is both conducted and radiated through the input power cord and the DC output
wiring to the radio.
FiLtRAtion oF conDUcteD noiSe
The conducted rf noise from this SMPS unit 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 (for Class 1 units). Thus, the filtered
noise currents are intentionally leaked to the Earth Ground. This is termed as the “Earth
Leakage Current”.
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 sup-
ply 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
