
ISAC-SX
PEB 3086
Description of Functional Blocks
Data Sheet
60
2003-01-30
3.3.7
For both, receive and transmit direction a 1:1 transformer is used to connect the ISAC-
SX transceiver to the 4 wire S/T interface. Typical transformer characteristics can be
found in the chapter on electrical characteristics. The connections of the line
transformers is shown in
Figure 28
.
S/T Interface Circuitry
Figure 28
Connection of Line Transformers and Power Supply to the ISAC-SX
For the transmit direction an external transformer is required to provide isolation and
pulse shape according to the ITU-T recommendations.
3.3.7.1
The ITU-T I.430 specification for both transmitter and receiver impedances in TEs results
in a conflict with respect to external S-protection circuitry requirements:
– To avoid destruction or malfunction of the S-device it is desirable to drain off even
small overvoltages reliably.
– To meet the 96 kHz impedance test specified for transmitters and receivers (for TEs
only, ITU-T I.430 sections 8.5.1.2a and 8.6.1.1) the protection circuit must be
dimensioned such that voltages below 1.2 V (ITU-T I.430 amplitude) x transformer
ratio are not affected.
This requirement results from the fact that this test is also to be performed with no supply
voltage being connected to the TE. Therefore the second reference point for
overvoltages
V
DD
, is tied to GND. Then, if the amplitude of the 96 kHz test signal is
greater than the combined forward voltages of the diodes, a current exceeding the
specified one may pass the protection circuit.
The following recommendations aim at achieving the highest possible device protection
against overvoltages while still fulfilling the 96 kHz impedance tests.
External Protection Circuitry
21150_05
Protection
Circuit
Protection
Circuit
1:1
1:1
Transmit
Pair
Receive
Pair
SX1
SX2
SR1
SR2
GND
VDD
VSS
3.3 V
10 μF