XRT5793IV View Datasheet(PDF) - Exar Corporation
Part Name
Description
MFG CO.
'XRT5793IV' PDF : 16 Pages View PDF
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
VO = Peak output pulse amplitude
RL = Load resistance
Rs
1:n
3
1
Vs
VO
RL
4
2
Figure 5. Equivalent Impedance Schematic
3. Calculate the source resistance, Rs.
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
VO = 3.0V Peak
RL = 120Ω
Req
+
RL
n2
+
120
1.6
+
75Ω
Veq
+
Vo
n
+
3.0
1.265
+
2.37V
ǒ Ǔ ǒ Ǔ Rs + Req
VS
Veq
*
1
+ 75
4.5
2.37
*
1
+ 67.4Ω
2. Calculate equivalent output voltage and load
resistance without the transformer.
Req
+
RL
n2
Veq
+
VO
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
Veq
Figure 6. Equivalent Simplified Schematic
Rev. 2.00
12
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