Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
4. C6 and C8 should be selected to maintain the lower frequency roll-off of the microphone inputs as
follows:
C6 = 30nF ´ ½GAINMic1½ (and C6 > 1000mF / |R7|)
C8 = 30nF ´ ½GAINMic2½ (and C8 > 1000mF / |R9|)
Digital ground plane
Digital Ground
1
2
3
28
27
26
C10
C11
+
Clock Output
4
5
6
7
8
25
24
23
22
21
VSS(D)
L1
VDD(D)
VSS(A)
Digital +ve Supply
Analogue Ground
CMX880
provision for wire link
VSS(A)
VBIAS
C12
C13
9
20
19
+
10
L2
VDD(A)
Analogue '+ve Supply
C14
11
12
13
14
18
17
16
15
Analogue ground plane
Figure 3 Power Supply Connections and De-coupling
L1 100nH (see note 5)
C10 10nF
C11 10mF
L2 100nH (see note 5)
C12 10nF
C13 10mF
C14 100nF
Resistors ±5%, capacitors ±20% unless otherwise stated.
Notes
5. The inductors L1 and L2 can be omitted but this may degrade system performance.
PCB Layout Guidelines and Power Supply Decoupling
It is important to protect the analogue pins from extraneous baseband noise and to minimise the impedance
between the CMX880 and the supply and bias de-coupling capacitors. The de-coupling capacitors C10, C11,
C12 and C13 should be as close as possible to the CMX880, particularly C10 and C12. It is therefore
recommended that the printed circuit board is laid out with separate ground planes for the VSS(A) and VSS(D) in
the area of the CMX880, with provision to make a link between them close to the CMX880.
VBIAS is used as an internal reference for detecting and generating the various analogue signals. It must be
carefully decoupled, to ensure its integrity, so apart from the decoupling capacitor shown, no other loads are
allowed. If VBIAS needs to be used to set the FM discriminator mid-point reference, it must be buffered with a
high input impedance buffer.
The single ended microphone inputs and audio output must be AC coupled as shown, so their return paths can
be connected to VSS(A) without introducing DC offsets. Further buffering of the audio output is advised.
If the CMX880 is being used for APCO 25 based applications, the crystal, X1, should be replace with a
temperature compensated clock module with better than 10ppm accuracy. The internal clock generating circuit
can be placed in power-save mode if the clock is provided externally.
ã 2001 Consumer Microcircuits Limited
8
D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
