XRT5894 Datasheet, PDF (16/19 Pages) Exar Corporation – Four-Channel E1 Line Interface (3.3V or 5.0V)

XRT5894
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:
VS_pos
VS_neg
RS_pos TTIP
R3
TRING
RS_neg
R4
1:n
V_O
R_L
Figure 8. Transmitter Line Driver Model
Where:
Vs_pos = Vs_neg = 1.25V typical (Differential line driver peak output voltage swing)
Rs_pos = Rs_neg = 0.8W typical (Differential line driver internal source resistance)
R3 = R4 = 9.1W (Differential line driver external source resistance from Figure 1)
R_L = 75W or 120W (Transmitter load resistance)
n = 2 (Transformer turns ratio)
Vo = Transmitter peak output voltage (Measured across R_L = 75W or R_L = 120W )
Figure 9 may be converted to a single--ended model:
RS_int
RS_ext
V_S
1:n
V_O
R_L
Figure 9. Single-ended Line Driver Model
Where:
VS = ÷Vs_pos÷ + ÷Vs_neg÷
RS_int = RS_pos + Rs_neg
RS_ext = R3 + R4
Rev. 1.10
16