ADM206E/ADM207E/ADM208E/ADM211E/ADM213E
Conducted emissions are measured by monitoring the line
power supply. The equipment used consists of a LISN (Line
Impedance Stabilizing Network) which essentially presents a
fixed impedance at RF, and a spectrum analyzer. The spectrum
analyzer scans for emissions up to 30 MHz and a plot for the
ADM211E is shown in Figure 32.
V
CC
S1
S3
V+ = 2V
CC
C1
S2
GND
S4
V
CC
C3
RADIATED EMISSIONS
INTERNAL
OSCILLATOR
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 which cause conducted emissions
can also be responsible for radiated emissions. Fast RS-232
output 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 fre-
quency oscillator and high voltages being switched by the charge
pump. The move towards smaller capacitors in order to con-
serve board space has resulted in higher frequency oscillators
being employed in the charge pump design. This has resulted in
higher levels of emission, both conducted and radiated.
The RS-232 outputs on the ADM2xxE 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.
RADIATED NOISE
Figure 30. Charge Pump Voltage Doubler
ø
1
DUT
ø
2
SWITCHING GLITCHES
TURNTABLE
ADJUSTABLE
ANTENNA
TO
RECEIVER
Figure 33. Radiated Emissions Test Setup
Figure 31. Switching Glitches
80
70
60
50
dB V
Figure 34 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 ADM2xxE was operated at maximum baud rates
and configured as in a typical RS-232 interface.
LIMIT
Testing for radiated emissions was carried out in a shielded
anechoic chamber.
80
70
60
50
dB V
40
30
20
10
0
40
30
20
10
0
START 30.0 MHz
STOP 200.0 MHz
LIMIT
0.3
0.6
3
6
1
LOG FREQUENCY – MHz
18
30
Figure 32. Conducted Emissions Plot
Figure 34. Radiated Emissions Plot
REV. B
–13–
AD [ ANALOG DEVICES ]
