xrt5897 Exar Corporation, xrt5897 Datasheet - Page 18

• Manufacturer Part Number: xrt5897
• Description: Xrt5897 -seven-channel E1 Line Interface
• Manufacturer: Exar Corporation

Output Transformer Selection
A 1:2 ratio transformer is recommended for both 75W and 120W operation because the transmitter, when equipped with
this device, meets both the ITU G.703 output pulse amplitude requirement and, the ETSI return loss specification.
Although a center--tapped output transformer is not required, choosing a part with a center-tapped secondary allows the
use of the same type of unit at the receiver input.
A theoretical justification for the 1:2 ratio transformer follows:
Where:
Figure 9 may be converted to a single--ended model:
Where:
XRT5897
Vs
Rs
R3 = R4 = 9.1W (Differential line driver external source resistance from Figure 1)
R
n = 2 (Transformer turns ratio)
Vo = Transmitter peak output voltage (Measured across R
VS = ÷Vs
RS
RS
Rev. 1.11
L
pos
pos
int
ext
= 75W or 120W (Transmitter load resistance)
= RS
= Vs
= R3 + R4
= Rs
pos
neg
pos
neg
VS
VS
÷ + ÷Vs
pos
neg
V
+ Rs
= 1.25V typical (Differential line driver peak output voltage swing)
= 0.8W typical (Differential line driver internal source resistance)
S
Figure 9. Single-ended Line Driver Model
Figure 8. Transmitter Line Driver Model
neg
neg
÷
RS
RS
RS
pos
neg
int
TRING
TTIP
R4
RS
R3
ext
1:n
1:n
18
V
O
V
L
O
= 75W or R
R
L
R
L
L
= 120W)