ADM202E/ADM1181A
RADIATED EMISSIONS
RADIATED NOISE
Radiated emissions are measured at frequencies in excess of
30 MHz. RS-232 outputs designed for operation at high baud
rates while driving cables can radiate high frequency EM energy.
The reasons already discussed that cause conducted emissions
can also be responsible for radiated emissions. Fast RS-232 out-
put transitions can radiate interference, especially when lightly
loaded and driving unshielded cables. Charge pump devices are
also prone to radiating noise due to the high frequency oscillator
and high voltages being switched by the charge pump. The
move towards smaller capacitors in order to conserve board
space has resulted in higher frequency oscillators being em-
ployed in the charge pump design. This has resulted in higher
levels of emission, both conducted and radiated.
The RS-232 outputs on the ADM202E products feature a con-
trolled slew rate in order to minimize the level of radiated emis-
sions, yet are fast enough to support data rates up to 230 kBaud.
Figure 21 shows a plot of radiated emissions vs. frequency. This
shows that the levels of emissions are well within specifications
without the need for any additional shielding or filtering compo-
nents. The ADM202E was operated at maximum baud rates
and configured as in a typical RS-232 interface.
Testing for radiated emissions was carried out in a shielded
anechonic chamber.
dBµV
DUT
ADJUSTABLE
ANTENNA
TO
RECEIVER
TURNTABLE
Figure 20. Radiated Emissions Test Setup
80
70
60
50
40
30
20
10
0
START 30.0 MHz
STOP 200.0 MHz
LIMIT
Figure 21. ADM202E Radiated Emissions Plot
–10–
REV. 0
AD [ ANALOG DEVICES ]
