Samlex America SEC-4825BRM - Manual (Page 26)

26 | SAMLEX AMERICA INC. SAMLEX AMERICA INC. | 27
Section 7 | Limiting electro-Magnetic
interference (eMi)
supplies do not employ high frequency switching voltages and will be quieter as com-
pared 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 rectifi er, both rectifi er noise and diode recovery. The AC
input rectifi er / capacitor in the front end of the SMPS (excepting those with Power
Factor correction) generate power supply harmonics due to the non-linear input cur-
rent 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 acceptable levels by internal
fi ltration. The fi ltered 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 fi ltered noise currents are
intentionally leaked to the Earth Ground. This is termed as the “Earth Leakage Cur-
rent”.
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 in the receiver may be caused by the antenna being too close. As with the trans-
mitter 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 an-
tenna. 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 25A immediately before the AC input of the power supply.
• Use additional appropriate DC radio frequency interference (RFI) power line fi lter
rated for minimum 30A 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.
• The DC side Positive and Negative outputs of these power supplies are isolated from
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