ADM202E/ADM1181A

–10–

REV. 0

RADIATED EMISSIONS

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.

TURNTABLE

DUT

RADIATED NOISE

ADJUSTABLE

ANTENNA

TO

RECEIVER

Figure 20. Radiated Emissions Test Setup

START 30.0 MHz

STOP 200.0 MHz

LIMIT

80

70

60

50

40

30

20

10

0

Figure 21. ADM202E Radiated Emissions Plot