10 |
SAMLEX AMERICA, INC.
Section 5
| Limiting Electromagnetic Interference
!
CAUTION!
Conducted and radiated noises in this unit are limited as per the applicable National / Inter-
national Standards. In North America, the applicable standard is FCC Part 15(B) for Class “B”
Digital Devices for Residential Installations. The corresponding European standard is EN55022,
Class “B” & EN610000-3-2, 3.
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 communications.
However, this does not guarantee that interference will not occur in a particular installation. 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 switching
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 sup-
ply 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 sup-
ply’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.
