參數(shù)資料
型號(hào)： DS21Q50N
英文描述： Telecommunication IC
中文描述： 通信集成電路
文件頁數(shù)： 68/99頁
文件大小： 325K
代理商： DS21Q50N
DS21Q50
Page 68 of 99
19.1.1 TERMINATION
The DS21Q50 is designed to be fully software-selectable for 75 ohm and 120 ohm termination without the need
to change any external resistors. The user can configure the DS21Q50 for 75 or 120 ohm receive termination by
setting the IRTSEL (CCR5.4) bit. When using the internal termination feature, the external termination resistance
should be120 ohms (typically two 60 ohm resistors). Setting IRTSEL = 1 will cause the DS21Q50 to internally
apply parallel resistance to the external resistors in order to adjust the termination to 75 ohms. See Figure 19-2
for details.
19.2 TRANSMIT WAVESHAPING AND LINE DRIVING
The DS21Q50 uses a set of laser–trimmed delay lines along with 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. See Figure 19-3. The user will select which waveform is to be generated by properly programming
the L2/L1/L0 bits in the Line Interface Control Register (LICR). The DS21Q50 can set up in a number of
various configurations depending on the application. See Table 19-1.
Table 19-1 LINE BUILD OUT SELECT IN LICR
L
2
1
0
0
0
0
75 ohm normal
1:2 step–up
0
0
1
120 ohm normal
1:2 step–up
0
1
0
75 ohm w/ protection resistors
1:2 step–up
0
1
1
120 ohm w/ protection
resistors
1
0
0
75 ohm w/ high return loss
1:2 step–up
1
0
1
120 ohm w/ high return loss
1:2 step–up
* NM = Not Meaningful (Return Loss value too low for significance)
** See separate application note for details on E1 line interface design
Due to the nature of the design of the transmitter in the DS21Q50, very little jitter (less then 0.005 UIpp
broadband from 10 Hz to 100 kHz) 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 transmitter in the device couples to
the E1 transmit shielded twisted pair or COAX via a 1:2 step up transformer as shown in Figure 19-1. In order
for the devices to create the proper waveforms, the transformer used must meet the specifications listed in
Table
19-2
. The line driver in the device contains a current limiter that will prevent more than 50 mA (rms) from
being sourced in a 1 ohm load.
L
L
APPLICATION
TRANSFORMER RETURN
LOSS
*
NM
NM
NM
NM
RT
**
0 ohms
0 ohms
2.5 ohms
2.5 ohms
1:2 step–up
21dB
21dB
6.2 ohms
11.6 ohms
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