XRT5793IV Exar, XRT5793IV Datasheet - Page 12

• Manufacturer Part Number: XRT5793IV
• Description: Peripheral Drivers & Components - PCIs Quad E1 LIU
• Manufacturer: Exar

Specifications of XRT5793IV

• Rohs: yes
• Mounting Style: SMD/SMT
• Package / Case: TQFP-80

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