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MC68194FJ

Carrier Band Modem (CBM)

ON Semiconductor 

MC68194

SECTION 4

RECEIVER AMPLIFIER/LIMITER WITH CARRIER DETECT

4.1 OVERVIEW

The IEEE 802.4 spec provides that the incoming signal

range for good signal is +10 dB (1.0 mV, 75 Ω) [dBmV] to

+66 dB (1.0 mV, 75 Ω) [dBmV] available at the modem

connector. The IEEE 802.4 further specifies that the modem

will report silence for any signal below +4.0 dB (1.0 mV, 75

Ω) [dBmV]. Therefore, the receiver function must amplify

any signal of +10 dBmV and above to limiting for good data

recovery, and the signal detect must switch within the +4.0

dBmV to +10 dBmV window, that is, it must be “off” for

+4.0 dBmV and below, and be “on” for +10 dBmV and

above. The MC68194 requires a pre–amplifier of about 12

dB in front of the onboard amplifier and carrier detect

function. Clock and data recovery are extracted from the

amplified/limited incoming signal, and the carrier detect is

used to control the clock and data recovery function based

on presence of good signal.

4.2 AMPLIFIER

Figure 4–1 shows a simple block diagram of the receiver

amplifier. Internally, dc feedback is used to bias the

amplifier, and connection points FDBK and FDBK* are

provided to ac bypass the feedback. With both receiver

inputs RXIN and RXIN* available, the device can be wired

either for differential or single–ended operation.

Differential is preferred for low noise.

FDBK

R

R

An external preamplifier with gain of about 12 dB is used

with the onboard amplifier. The pre–amp can drive the CBM

either single–ended or differentially. The onboard amplifier

output signal is used in two ways. One path adds an

additional limiter stage and is used to drive the clock and

data recovery stages. The second path is used to develop

carrier detect.

In the signal window where carrier detect must be active,

the internal amplifier remains in the linear (non–limiting)

range. Its output is fullwave rectified, and the rectified signal

is compared to an onboard threshold that is temperature and

voltage compensated. The rectified signal is also brought out

to an external lead called CARDET. A capacitor can be

added at this pin which combines with the series 125 Ω

resistor to form a low pass filter. This filtering is used to

knock any high frequency noise off of the signal. The output

of the comparator is a series of pulses (when the signal

amplitude is sufficiently large) which are digitally

integrated in the internal squelch signal.

4.3 CARRIER DETECTION THRESHOLD

The carrier detect threshold is internally generated and

compensated for power supply and temperature variation.

The THRESHOLD pin is provided to adjust the threshold

via an external resistor tied to VCC.

GAIN

OR

TO RXMUX

NOR

RXIN*

RXIN

R

R

Vth

125 Ω

CARDET

CARDETS

TO SQUELCH

FDBK*

R = 7.5 kΩ (ALL RESISTORS OF EQUAL VALUE).

CARDET

VCC

THRESHOLD

(A) Receiver Used in Differential Mode

PRE–AMP

FDBK

RT

RXIN*

RXIN

FDBK*

GAIN

R

R

Vth

R

R

125 Ω

R = 7.5 kΩ (ALL RESISTORS OF EQUAL VALUE).

CARDET

THRESHOLD

OR

TO RXMUX

NOR

CARDET

CARDETS

TO SQUELCH

VCC

(B) Receiver Used in Single–Ended Mode

Figure 4–1. Receiver Amplifier With Carrier Detect

http://onsemi.com

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