V.22 bis Modem with AT Commands
CMX866
1.4
External Components
C1, C2
R1
X1
22pF
100kΩ
11.0592MHz
C3, C4, C6
C5, C7
L1, L2
100nF
10uF
100nH
(optional)
Resistors
±5%,
capacitors
±20%
unless otherwise stated.
Figure 2a Recommended External Components for Typical Application
This device is capable of detecting and decoding small amplitude signals. To achieve this DV
DD,
AV
DD
and V
BIAS
should be decoupled and the receive path protected from extraneous in-band signals. It is
recommended that the printed circuit board is laid out with both AV
SS
and DV
SS
ground planes in the
CMX866 area, as shown in Figure 2b, with provision to make a link between them close to the CMX866.
To provide a low impedance connection to ground, the decoupling capacitors (C3 – C7) must be mounted
as close to the CMX866 as possible and connected directly to their respective ground plane. This will be
achieved more easily by using surface mounted capacitors.
V
BIAS
is used as an internal reference for detecting and generating the various analogue signals. It must
be carefully decoupled, to ensure its integrity. Apart from the decoupling capacitor shown (C3), no other
loads are allowed. If V
BIAS
needs to be used to set external analogue levels, it must be buffered with a
high input impedance buffer.
The DV
SS
connections to the Xtal oscillator capacitors C1 and C2 should also be of low impedance and
preferably be part of the DV
SS
ground plane to ensure reliable start up of the oscillator.
©
2004 CML Microsystems Plc
6
D/866/4
CMLMICRO [ CML MICROCIRCUITS ]
