Switching Power Supplies & Rf Noise - Samlexpower SEC-1235 Installation & Operating Manual

SWITCHING POWER SUPPLIES AND RF NOISE
1. Switched mode power supplies ( SMPS ) employ high frequency
switching and thus, are a source of radio interference, a recipient of radio
interference and a conduit of radio interference. ( Older linear type transformer
based power supplies do not employ high frequency switching voltages and
will be quieter as compared to switching type of supplies ).
2. The primary emission sources originate in the switching devices due
to their fast switching current transitions: harmonics of the switching fre-
quency 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 switching power supplies ( excepting those with power factor
correction ) are notorious for generating 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.
3. Switching power supplies 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.
8.