DS21Q58 E1 Quad Transceiver
49 of 74
21.3 Transmit Waveshaping and Line Driving
The DS21Q58 uses a set of laser-trimmed delay lines and a precision digital-to-analog converter (DAC) to create
the waveforms that are transmitted onto the E1 line. The waveforms meet the ITU G.703 specifications
(
Figure 21-4
). The user selects which waveform is to be generated by properly programming the L2/L1/L0 bits in
the line interface control register (LICR). The DS21Q58 can be set up in a number of various configurations
depending on the application (
Table 21-2
).
Table 21-2. Line Build-Out Select in LICR
L2
L1
L0
APPLICATION
TRANSFORMER
RETURN LOSS
*
Rt ( )
**
0
0
0
0
1
1
0
0
1
1
0
0
0
1
0
1
0
1
75 normal
120 normal
75 with protection resistors
120 with protection resistors
75 with high return loss
120 with high return loss
1:2 step-up
1:2 step-up
1:2 step-up
1:2 step-up
1:2 step-up
1:2 step-up
N.M.
N.M.
N.M.
N.M.
21dB
21dB
0
0
2.5
2.5
6.2
11.6
*
N.M. = Not meaningful (return loss value too low for significance)
**
Refer to
Application Note 336: Transparent Operation on T1, E1 Framers and Transceivers
for details on E1 line interface design.
Because of the nature of the transmitter’s design, very little jitter (less than 0.005UI
P-P
broadband from 10Hz to
100kHz) is added to the jitter present on TCLK (or source used for transmit clock). Also, the waveform created is
independent of the duty cycle of TCLK. The device’s transmitter couples to the E1-transmit shielded twisted pair or
coax through a 1:2 step-up transformer, as shown in
Figure 21-2
. For the devices to create the proper waveforms,
the transformer used must meet the specifications listed in
Table 21-3
. The line driver in the device contains a
current limiter that prevents more than 50mA (RMS) from being sourced in a 1 load.
Table 21-3. Transformer Specifications
SPECIFICATION
Turns Ratio
Primary Inductance
Leakage Inductance
Intertwining Capacitance
DC Resistance
RECOMMENDED VALUE
1:1 (receive) and 1:2 (transmit) ±3%
600 H minimum
1.0 H maximum
40pF maximum
1.2 maximum
Figure 21-2. External Analog Connections (Basic Configuration)
RTIP
RRING
TTIP
TRING
E1 TRANSMIT
LINE
DS21Q58
0.47 F
(
NONPOLARIZED
)
DVDD
DVSS
0.1 F
RVDD
RVSS
0.1 F
TVDD
TVSS
0.1 F
+3.3V
Rr
0.1 F
0.01 F
2.048MHz
MCLK
1 : 1
2 : 1
Rr
1/4
E1 RECEIVE
LINE