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

SWITCHING POWER SUPPLIES AND RF NOISE
1. Switched mode power supplies ( SMPS ) employ high frequency switch-
ing and thus, are a source of radio interference, a recipient of radio interfer-
ence 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 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 trans-
mitter 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 buzz-
ing 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 connec-
tor 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.
8.