XR-T5793 Datasheet, PDF (12/16 Pages) Exar Corporation – Quad E1 Line Interface Unit

XR-T5793
Output Transformer Selection
The 1:1.265 ratio output transformer is recommended for
the XR-T5793 because this ratio gives the best possible
transmitter output return loss for 120Ω balanced E1
service. However, other transformers may provide an
adequate return loss for many applications. The two
characteristics that determine series build-out resistor
requirements are:
D Driver output impedance is less than 5Ω.
D Vs, which is the driver open circuit output voltage, is
4.5V peak.
The following method may be used to determine
transformer suitability for a given use.
1. List the application requirements.
Transformer ratio = 1:n
V_O = Peak output pulse amplitude
R_L = Load resistance
Rs
1:n
3
1
Vs
V_O
R_L
4
2
Figure 5. Equivalent Impedance Schematic
3. Calculate the source resistance, Rs.
ǒ Ǔ R_S + Req
V_S
V_eq
*
1
4. Now calculate the theoretical return loss.
ǒ Ǔ Return Loss + 20 log
Req ) R_S
Req * R_S
The calculation given below uses the recommended
1:1.265 ratio transformer as an example:
Transformer Ratio = 1:1.265
V_O = 3.0V Peak
R_L = 120Ω
Req
+
R_L
n²
+
120
1.6
+
75Ω
Veq
+
V_o
n
+
3.0
1.265
+
2.37V
ǒ Ǔ ǒ Ǔ Rs + Req
V_S
Veq
*
1
+ 75
4.5
2.37
*
1
+ 67.4Ω
2. Calculate equivalent output voltage and load
resistance without the transformer.
R_eq
+
R_L
n²
V_eq
+
V_O
n
(Datasheet specifies standard value of 68Ω)
Calculate the theoretical return loss to determine if the
transformer is acceptable.
Rs
ǒ Ǔ _{Return Loss + 20 log}
75 ) 67.4
75 * 67.4
+ 25.5dB
Vs
Req
V_eq
Figure 6. Equivalent Simplified Schematic
Rev. 2.00
12