XR-T5794 Datasheet, PDF (13/20 Pages) Exar Corporation – Quad E-1 Line Interface Unit

XR-T5794
Output Transformer Selection
The 1:1.265 ratio output transformer is recommended for
the XR-T5794 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 V_s, 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
Vo = Peak Output Pulse Amplitude
R_L= Load Resistance
Rs
1:n
3
1
Vs
V_O
R_L
4
2
Figure 6. Equivalent Impedance Schematic
2. Calculate equivalent output voltage and load
resistance without the transformer.
Req
+
R_L
n²
Veq
+
V_o
n
Rs
Vs
Req
V_eq
3. Calculate the source resistance, R_S.
ǒ Ǔ Rs + Req
Vs
Veq
*
1
4. Now calculate the theoretical return loss.
ǒ Ǔ Return Loss + 20 log
Req ) Rs
Req * Rs
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
Vs
Veq
*
1
+ 75
4.5
2.37
*
1
+ 67.4Ω
(Datasheet specifies standard value of 68Ω)
Calculate the theoretical return loss to determine if the
transformer is acceptable.
Figure 7. Equivalent Simplified Schematic
ǒ Ǔ _{Return Loss + 20 log}
75 ) 67.4
75 * 67.4
+ 25.5dB
Rev. 2.01
13