
4. Thereceivermay“hear”thepowersupply.Aslowlymoving,slightly
buzzingcarrierheardin the receiver may be caused by the antenna being
too close.As with the transmitter related noise pick up, a loose coaxial
connectororabroken or a missing ground may aggravate this problem.
Normallythesenoiseswill be below the background or “band” noise.
Increasethe separation between the power supply andthereceiving
antenna.Useanoutdoorantenna.This will reduce the amount of signal
picked up from the power supply and also increase the amount of the
desiredsignal.
5. Theconducted andradiated noises are limitedas per the appli-
cablenational/international standards.InNorthAmerica,the applicable
standard is as per FCC Part 15(B) for Class “B” digital devices. The
Europeanstandardisasper 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
themaximumallowablelevelsbyinternalfiltration.ThefilteredRFnoise
currents ( normally < 5mA ) are bypassed to the chassis of the power
supply.The chassis is, in turn, connected to the earth ground pin of theAC
inputpower cord (for Class 1 units). Thus,the filterednoise 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 require-
mentsareconflicting.
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.Asingle GFCI may
be serving multipleAC 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
supplieshave intentional leakage current asexplainedabove,
it may trip a GFCI feeding multipleAC outlet circuits / recep-
tacles. In such cases, disconnect devices connected to the
otherAC outlet circuits / receptacles served by this GFCI.
9.