LIMITING ELECTROMAGNETIC INTERFERENCE (EMI)
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 cir-
cuit. 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. Filters are used to
limit the noise to acceptable level.
3. Switching power supplies are also recipients of radio interference. The nor-
mal operation of the power supply can be disturbed due to RF noise getting
coupled into the power supply. Thus, the power supply may generate exces-
sive RF noise and lose output voltage regulation due to excessive transmitter
energy being coupled through the AC / DC lines to the power supply’s regu-
lator 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 an-
tenna. This will reduce the amount of signal picked up from the power supply
and also increase the amount of the desired signal.
13.
