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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 noises are limited as per the applicable
national / international standards. In North America, the applicable standard is
as per FCC Part 15(B) for Class "B" digital devices. The European standard is
as per EN55022, Class "B" & EN610000-3-2, 3. Thus, the RF interference is
limited but not entirely eliminated.
6.
The conducted RF noise from these power supplies is limited to the
maximum allowable levels by internal filtration. The filtered RF noise currents
(normally < 5mA ) 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 filtered noise currents are intentionally leaked to
the earth ground. This is termed as the "Earth Leakage Current". For safety
against electric shock, this earth leakage current is also required to be limited.
It will be seen that these two requirements are conflicting.
NOTE: In some cases, to prevent electric shock hazard due to abnormal
leakage current (like in marinas, spas, hot tubs, wet spaces etc.), the AC
outlet circuits / receptacles in these areas are served through a GFCI
(Ground Fault Circuit Interrupter ). This GFCI is normally set to trip when it
senses an earth leakage current > 5 mA. A single GFCI may be serving multiple
AC outlet circuits / receptacles and therefore, will be sensing the sum of all the
leakage currents of the devices connected to these. As the switching power
supplies have intentional leakage current as explained above, it may trip a
GFCI feeding multiple AC outlet circuits / receptacles. In such cases, discon-
nect devices connected to the other AC outlet circuits / receptacles served by
this GFCI.
9.
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