mc68194 ON Semiconductor, mc68194 Datasheet - Page 15

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

6.1 OVERVIEW
generated by the clock recovery logic are used by the data
recovery logic. The MC68194 recovers the data from the
encoded receive signal by opening sampling windows
around the 1/4 and 3/4 bit time positions and looking for
edges in the received signal (refer to Figure 6−1 for the
encoded data representations). A data ONE has transitions
only at the 0 and 1/2 bit time positions. A data ZERO has
transitions at the 0, 1/4, 1/2, and 3/4 bit time positions. A
NON−DATA symbol has transitions at the 0, 1/4, and 1/2 bit
time positions (ND1) or at the 0, 1/2, and 3/4 bit time
positions (ND2). NON−DATA symbols should always
occur in pairs; each pair is made up of one of each type of
NON−DATA encoded symbols as shown in Figure 6−2
(ND1 followed by ND2).
UP*
DN*
K
The RXOUT signal from the receive amplifier and clocks
R
ƒ
PULLDOWN
= (RC s +1) / C s
0 0.25
Figure 6−1. Encoded Data Representation
Figure 5−5A. Active Filter Using Op Amp
BIT TIME
MPSH81
0.5
V
R
ONE
0.75
CC
PULLDOWN
Figure 5−5B. Charge Pump/Filter
R
1
R
FILTER
1
ZERO
UP*
R
C
2
3
2
C
R
LOWPASS
R
−
+
2
A
C
7
4
V
VCX
C
DD
ND1
6
MPS2369
MPSH81
R
LOWPASS
SECTION 6 − DATA RECOVERY
ND2
C
LOWPASS
http://onsemi.com
VCX
DN
15
5.3.4 Loop Characteristics
If an active filter as shown with an op amp is used, the
general PLL loop transfer function now becomes:
Its characteristic equation is set to the form:
Relating to the standard form (s
solving:
where
If a change pump loop filter is used, the general PLL loop
transfer function alternately becomes:
Its characteristics equation is set to the form:
Relating to the standard form (s2 + 2ξω
solving:
decoded by keeping track of the 1/4 and 3/4 bit time position
transitions. The ONEs, ZEROs, and NON−DATA pairs are
then reported on the RXSYMX pins as described in the serial
interface discussion. Two other conditions can also be
reported while receiving in MAC mode — BAD SIGNAL
and SILENCE. BAD SIGNAL is reported when a ND1
symbol is not followed immediately by a ND2 symbol or
when a ND2 symbol is received and not immediately
preceded by a ND1 symbol.
1. When the amplitude of the received signal is not large
2. When in internal loopback mode and SILENCE is being
3. During the PLL training period at the beginning of a
G(s) H(s) = K
C.E. = 1 + G(s) H(s) = 0
G(s) H(s) = K
C.E. = 1 + (Gs) (Hs) = 0
ω
ONEs, ZEROs, and NON−DATA pairs can be easily
SILENCE is reported when one of four conditions occurs:
enough to trigger the on−chip carrier detect circuit.
Reporting SILENCE when the carrier detect signal is not
asserted prevents the chip from responding to low level
noise.
requested on the TXSYMX pins, SILENCE will be
reported on the RXSYMX pins. An internal digital
carrier detect is used during loopback and this signal is
negated when SILENCE is requested on the request
channel.
transmission. When an incoming signal first triggers the
n
2
ω
ω
ξ = damping factor.
= (K
n
n
2
= s
= s
= Natural frequency
= (K
p
2
2
K
= K
= K
+ (K
+ (K
v
p
) / C N 2ξω
K
p
p
p
p
p
[(R C s + 1) / C s] (K
p
v
K
[(R2 C s+1) / R1 C s] (K
K
) / R1 C N 2ξω
K
K
ƒ
ƒ
v
v
K
K
R2) s / (R1 N) + K
R) s / (N) + (K
o
o
K
K
n
n
n
= (K
2
p
n
+ 2ξω
K
= (K
p
v
v
K
R) / N
/ s) (1 / N)
v
p
p
) / (C N)
n
n
v
K
K
s + ω
s + ω
/s) (1 / N)
v
v
) / (R1 C N)
R2) / R1 N
n
n
2) and
2
) and