Samlex Europe SEC-1212G, Руководство пользователя, Страница: 7 / 13

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 frequency and broadband noise created by
under-damped oscillations in the switching circuit. The secondary source is from the bridge
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.
4 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 these noises 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.
5 The conducted and radiated RF noise from these power supplies is limited by internal
supply. The chassis is, in turn, connected to the earth ground pin of the ac input power cord.
Thus, the chassis of the power supply is connected to the earth ground of the 2 0 V
distribution system. The net RF noise generated by these power supplies complies with
the limits laid down in the European Standards for emissions and immunity.
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