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MC68194FJ

Carrier Band Modem (CBM)

ON Semiconductor 

MC68194

On the reception side, the CBM requires a pre–amplifier

to receive the low level signal from the cable. The signal

available at the “F”–connector can range from +10 dB to +66

dB (1.0 mV, 75 Ω) [dBmV]. The signal required at the CBM

is about 12 dB above this (net gain through the transformer,

pre–amp, and any filtering). The receiver can be used in full

differential or single–ended mode.

A second part of the receiver function is the signal detect

or carrier detect function. The IEEE 802.4 requires that the

receiver detect a signal of +10 dBmV or above (i.e., be

turned “on”) and report Silence for a signal of +4.0 dBmV

RESET

or below (i.e., be turned “off”). Therefore, a 6.0 dB (2:1

voltage ratio) range or window is defined in which the signal

detect must switch. The CBM is optimized for this range

(including the pre–amp gain), although it is trimmed via an

external THRESHOLD.

The remaining external components are associated with

clock recovery. A capacitor and resistor (internal R also

provided) set one–shot timing, and an active filter for a PLL

used in clock and data recovery is required. The active filter

can be implemented via an op amp, or if 5.0 volt operation

is required, an alternate charge pump design can be used.

TXDIS

AMPLIFICATION

AND

WAVESHAPING

SMREQ*

TXSYM2

TXSYM1

TXSYM0

TXCLK

XTAL1

SERIAL DATA COMMANDS

INTERFACE

DECODER

BUFFER

STATION

MANAGEMENT

COMMANDS

TRANSMIT

MODULATOR

CLOCK

GENERATOR

XTAL2

SMIND*

RXSYM2

RXSYM1

RXSYM0

EOTDIS*

SM MODE

OUTPUT

MUX

LOOPBACK

PHYSICAL

MANAGEMENT

RECEIVE

MUX

RECEIVE

DEMODULATOR

BUFFER

JABBER

CONTROL

RECEIVE

AMPLIFIER

AND SQUELCH

RXCLK

BUFFER

ONE SHOT

CARRIER

DETECT

CLOCK

RECOVERY

AND

SYNCHRONIZE

TXOUT

TXOUT*

VCC

JAB–RC

JAB

FDBK*

RXIN

RXIN*

FDBK

THRESHOLD

GAIN

CARDET

RECEIVE

PRE–AMP

F–CONNECTOR

CPW

RPW VCM–C1

D* U* VCX

SET–PW

+5 V

PLL

FILTER

Figure 1–5. Functional Block Diagram

The clock recovery and data decoder is a synchronous

design which provides superior performance minimizing

clock jitter.

Although primarily intended for the IEEE 802.4 carrier

band, the CBM is also an excellent device for point–to–point

data links, fiberoptic modems, and proprietary LANs. The

MC68194 can be used over a wide range of frequencies and

interfaces easily into different kinds of media.

http://onsemi.com

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