®
MH89726/728
MT8972 Loop Extender Circuits
Advance Information
Features
•
•
•
•
•
•
Operates from single or dual power supply
MH89726 extends transmission performance
for the MT8972 (DNIC) at 160kbit/s line rate
MH89728 extends transmission performance
for the MT8972 (DNIC) at 80kbit/s line rate
MH89726 and MH89728 are pin for pin
compatible with each other
Compact SIL package
Over 6km loop range on 24 AWG
ISSUE 2
November 1990
Ordering Information
MH89726/728 10 Pin SIL Package
0°C to 70°C
Description
The MH89728 and MH89726 are thick film hybrid
devices which may be used to extend the operating
range of the MT8972 (DNIC). The hybrids combine
signal equalization and amplification for the receive
and transmit paths on each device.
The small size of these hybrids makes them suitable
for line card or terminal use with the MT8972. The
gain of the transmit signal may be adjusted through
the use of an external resistor.
Applications
•
•
•
•
Digital subscriber lines
Digital PABX line cards and telephone sets
High speed, limited distance modem
ISDN U - Interface
TX
GAIN
TX
IN
Pre-
Equalizer
TX
Amplifier
TX
OUT
Phase
Compensator
RX
OUT
Pre-
Canceller
RX
Amplifier
RX
IN
-
RX
IN
+
V
Bias
TEST
V+
V-
Figure 1 - Functional Block Diagram
9-127
MH89726/728
Advance Information
TXGAIN
TXOUT
RXIN +
RXIN -
V-
VBias
V+
TEST
RXOUT
TXIN
1
2
3
4
5
6
7
8
9
10
Figure 2 - Pin Connections
Pin Description
Pin #
1
Pin Name
TX
GAIN
Description
Transmit Gain.
To be connected to Pin 2. An increase in gain can be achieved by
connecting an external resistor R
EXT
between Pin 1 and Pin 2. The resultant gain is
calculated using; A= (R/24) + 2.8, where A=Gain and R= R
EXT
(kΩ).
Transmit Output.
Connect to the termination network.
Negative Receive Signal.
Connect to TX
OUT
, Pin 2.
Positive Receive Signal.
Connected to line transformer.
Negative power supply.
Internal Bias Voltage.
Connect to GND through 0.33 µF for single power supplies.
Connect to GND directly for split power supplies.
Positive power supply.
Test.
Used for production testing. Leave unconnected.
Receive Output.
Connect to L
IN
(Pin 21) of MT8972.
Transmit Input.
Connect to L
OUT
(Pin 1) of MT8972.
2
3
4
5
6
7
8
9
10
TX
OUT
RX
IN-
RX
IN+
V-
V
Bias
V+
TEST
RX
OUT
TX
IN
9-128
Advance Information
Functional Description
The MH89726/728 are 10 pin SIL thick film hybrid
circuits that may be used to extend the loop range
capability of the MT8972 (DNIC). For a detailed
description of the DNIC, refer to the MT8972 data
sheet.
Figure 1 shows a block diagram of the Loop
Extender Circuit (LEC). The LEC takes the line
signal which is transmitted by the DNIC on L
OUT
, pre-
equalizes, and amplifies it before sending it to the
termination network. The gain of the TX amplifier is
adjustable by using an external resistor between
pins 1 and 2. If no external resistor is added, TX
GAIN
and TX
OUT
must be shorted together, and the default
gain condition exists. This gain results in a transmit
signal power level of +10 dBm.
On the receive side, the LEC receives the differential
line signal from the transformer, and performs first-
order pre-cancellation as well as gain and phase
adjustment of the receive signal. The signal is then
output on RX
OUT
which is connected to L
IN
of the
MT8972.
Tables 1 and 2 show typical results obtained by
using the LEC with the MT8972. Actual distance
achieved may vary depending on the characteristics
of the transmission cable being used.
Most
important factors include cable attenuation (dB/km),
bridged taps, and crosstalk interference.
MH89726/728
The LEC is powered typically from ±15 Volts if dual
power supplies are available. Alternately 0 to +30
Volts, or -30 to 0 Volts can be used if only a single
power supply is available.
Applications
Figure 3 shows a typical connection diagram of the
MT8972 and MH89726/728. RX
OUT
and T
XIN
of the
LEC are connected to L
OUT
and L
IN
of the DNIC
respectively. Pins 5, 6 and 7 are the power supply
pins, with the voltages being supplied as indicated in
the table . An external resistor, R
EXT
, may be used
to increase the gain of the transmitter. Without R
EXT
,
TX
GAIN
and TX
OUT
are shorted together resulting in
a transmit power level of +10 dBm.
R
LL
, R
L
, and C
L
comprise the termination network
and are used to match the characteristic impedance
of the transmission line. This provides the 4 wire to 2
wire hybrid conversion necessary for twisted pair
transmission.
Figure 4 shows a typical application of the LEC. On
short loops, jumpers (or switches) could be used to
remove the LEC from the circuit. For longer loops,
the jumpers could be changed to include the LEC in
the signal path.
D1, D2 IN5817
R
ext
Set for different gain requirements
MH89726
or
MH89728
+ 5V
10
22
1
To Power Supplies
MT8972
(DNIC)
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
MH89726
160 kbit/s
R
LL
R
L
C
L
R
ext
R
LL
3.9kΩ, 5%
430Ω, 2%
15nF, 5%
MH89728
80 kbit/s
3kΩ, 5%
430Ω, 2%
12nF, 5%
9
8
7
6
5
4
3
2
1
D1
21
R
L
C
L
1.0µF
100kΩ
11
D2
Power
Supply
Pin 7
(V +)
Pin 6
(V
Bias
)
Pin 5
(V-)
- 15V
- 30V
GND
+ 15V
DUAL
GND
GND
SINGLE
GND*
+ 30V
SINGLE
GND*
*Use 0.33µF capacitor to GND
0.33µF
Figure 3 - Typical Connection Diagram for MT8972 and MH89726/728
9-129
MH89726/728
MH89726 (160 kbit/s)
CABLE
CONFIGURATION
DNIC to DNIC
DNIC with LEC to DNIC with LEC
24 AWG
(8.0 dB/km at 120 kHz)
0.0 to 4.1 km
0.65 to 5.1 km
Advance Information
26 AWG
(11.5 dB/km at 120 kHz)
0.0 to 3.0 km
0.5 to 3.6 km
Table 1 - Typical Transmission Performance at 160 kbit/s
MH89728 (80 kbit/s)
CABLE
CONFIGURATION
DNIC to DNIC
DNIC with LEC to DNIC and LEC
24 AWG
(6.9 dB/km at 60 kHz (Note 1)
0.0 to 5.2 km
0.9 to 6.5 km
26 AWG
(10.0 dB/km at 60 kHz)
0.0 to 3.4 km
0.65 to 4.1 km
Table 2 - Typical Transmission Performance at 80 kbit/s
Note 1: The attenuation of the cable as specified by Bell System Technical Reference PUB 62411.
CB
MT8972
1
L
OUT
RA
+5V
80 kbit/s
RA = 390Ω CA = 22nF CB = 3.3nF
160 kbit/s
RA = 390Ω CA = 22nF CB = 1.5nF
CA
2:1
D1
L
IN
21
D2
LE
IN
LOOP
EXTENDER
and
Network
Termination
1.0µF
LE
OUT1
Twisted Pair
Copper 24-26
Gauge
0.33µF
NOTES:
-D1, D2 IN5817 (Schottky)
-Solid jumpers used to connect the loop extender
-Dotted jumpers connect MT8972 without loop extender
LE
OUT2
Figure 4 - Typical Application of the Loop Extender
9-130
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