Bel Fuse Inc. 198 Van Vorst Street, Jersey City, NJ 07302 Tel 201-432-0463 Fax 201-432-9542 www.belfuse.com
HIGH FREQUENCY MAGNETICS
1999 Bel Fuse Inc. Specifications subject to change without notice. 06.99
TM9919A
T136
Application Note for Miniature Pulse Transformer
2795N Transformer Application Note
The 2795N transformer provides two transformation ratios to match both 120
and 75
line impedances with
Lucent 5 V transceivers. The ratio of the two transformer impedance ratios is within 0.07% of the ratio of the two
line impedances. This virtually perfect match allows the transformer to work with a single terminating network
design for both line impedances. The transformer can also be used for 100
lines with a change in resistor values.
For 2795N data sheet refer to document TM9918.
Suggested External Line Termination Network for Lucent 5 V Line Transceivers
A suggested line termination network consisting of the 2795N transformer, a capacitor, and resistors arranged to
provide the proper voltage gain and impedance matching to the characteristic impedance of the transmission line
is shown below. The diagram shows the appropriate external components for a receive line interface and a transmit
line interface for one channel. The network is designed to operate with both E1 cable impedances (75
and 120
)
with no change in components. For DS1 applications (100
), only the resistor values need to be changed. The
same transformer can be used for all three cable impedances in both receive and transmit operation. Figure 1
below shows the proper connections for each cable impedance. The components and values are listed in Table 1
(see next page).
Figure 1. Connections for each cable impedance
Transmit Data
Z
O
RT
RT
1
5
4
6
2
3
120
100
75
(Line)
Device
(1 Channel)
RTIP
RRING
TTIP
TRING
RR
CC
2795N
Transformer
Receive Data
Z
O
RR
3
1
4
(Line)
RS
75
120
100
5
2
6
T1
T2
2795N
Transformer
CORPORATE
Bel Fuse Inc.
198 Van Vorst Street
Jersey City, NJ 07302
Tel
201-432-0463
Fax
201-432-9542
www.belfuse.com
FAR EAST
Bel Fuse Ltd.
8F / 8 Luk Hop Street
San Po Kong
Kowloon, Hong Kong
Tel
852-2328-5515
Fax 852-2352-3706
www.belfuse.com
EUROPE
Bel Fuse Europe Ltd.
Preston Technology Management Centre
Marsh Lane, Suite F23, Preston
Lancashire, PR1 8UD, U.K.
Tel
44-1772-556601
Fax 44-1772-888366
www.belfuse.com
1999 Bel Fuse Inc. Specifications subject to change without notice. 06.99
TM9919A
T137
HIGH FREQUENCY MAGNETICS
Application Note for Miniature Pulse Transformer
Symbol
Name
Value
Z
O
Characteristic impedance
75
(E1)
120
(E1)
100
(DS1)
CC
Center tap capacitor
0.1 F
0.1 F
0.1 F
RR
Receive series resistor
36.5
36.5
30.1
RS
Receive shunt resistor
64.9
64.9
53.6
RT
Transmit series resistor
25.5
25.5
4.6
T1, T2
Bel part number
2795N
2795N
2795N
Table 1. Line termination components and values
2795N Transformer Application Note
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