CMX881 查看數據表（PDF） - CML Microsystems Plc

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CMX881

Baseband Processor for PMR and Trunked Radios

CML Microsystems Plc 

PMR Signalling Processor

CMX881

signal (e.g. RSSI) these components are not required. The values of C9 and R10 set the attack

time, C9 and R9 set the decay time. D1 can be any suitable small signal diode. R10 should be a

high enough value so as not to distort the signal source.

1.4.1 PCB Layout Guidelines and Power Supply Decoupling

VSS(D)

provision for

wire link

VSS(A)

VBIAS

C15

1

28

2

27

3

26

4

25

5

24

6 CM X881 23

7

22

8

21

9

20

10

19

11

18

12

17

13

16

14

15

C11

VDD(D)

VSS(A)

C13

VDD(A)

Digital Ground

Digit al

ground plane

C12

+

Clock Output

L1

Digital +ve Supply

Analogue Ground

C14

+

L2

Analogue +ve Supply

Analogue

ground plane

Figure 3 Power Supply Connections and De-coupling

C11 10nF

C12 10µF

C13 10nF

C14 10µF

C15 100nF

L1 100nH

L2 100nH

See note 7

See note 7

Resistors ±5%, capacitors and inductors ±20% unless otherwise stated.

Notes:

7. The inductors L1 and L2 can be omitted but this may degrade system performance.

It is important to protect the analogue pins from extraneous inband noise and to minimise the impedance

between the CMX881 and the supply and bias de-coupling capacitors. The de-coupling capacitors C11,

C12, C13 and C14 should be as close as possible to the CMX881, particularly C11 and C13. 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 CMX881, with provision to make a link between them close to the

CMX881.

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 should be connected. If VBIAS needs to be used to set the discriminator mid-point reference, it

must be buffered with a high input impedance buffer.

The single ended microphone input(s) 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.

The crystal X1 can be replaced with an external clock source if required/desired. The internal clock

generating circuit can be placed in power-save mode if the clock is provided externally.

 2004 CML Microsystems Plc

11

D/881/7

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