mc68194 ON Semiconductor, mc68194 Datasheet - Page 12

• Manufacturer Part Number: mc68194
• Description: Carrier Band Modem Cbm
• Manufacturer: ON Semiconductor

4.1 OVERVIEW
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
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
Differential is preferred for low noise.
The IEEE 802.4 spec provides that the incoming signal
Figure 4−1 shows a simple block diagram of the receiver
for
PRE−AMP
differential
FDBK*
FDBK
RXIN
FDBK*
FDBK
RXIN*
RXIN
RXIN*
or
RECEIVER AMPLIFIER/LIMITER WITH CARRIER DETECT
R
T
single−ended
Figure 4−1. Receiver Amplifier With Carrier Detect
R = 7.5 kΩ (ALL RESISTORS OF EQUAL VALUE).
R = 7.5 kΩ (ALL RESISTORS OF EQUAL VALUE).
R
R
R
R
(B) Receiver Used in Single−Ended Mode
(A) Receiver Used in Differential Mode
operation.
R
R
R
R
http://onsemi.com
SECTION 4
GAIN
12
GAIN
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.
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
compensated for power supply and temperature variation.
The THRESHOLD pin is provided to adjust the threshold
via an external resistor tied to V
An external preamplifier with gain of about 12 dB is used
In the signal window where carrier detect must be active,
The carrier detect threshold is internally generated and
CARDET
CARDET
125 Ω
125 Ω
V
V
th
th
THRESHOLD
THRESHOLD
CC
V
V
CC
OR
NOR
CARDET
CARDETS
CC
OR
NOR
CARDET
CARDETS
.
TO RXMUX
TO SQUELCH
TO RXMUX
TO SQUELCH