bt8970 Mindspeed Technologies, bt8970 Datasheet - Page 21

• Manufacturer Part Number: bt8970
• Description: Single-chip Hdsl Transceiver
• Manufacturer: Mindspeed Technologies

Bt8970
Single-Chip HDSL Transceiver
programmable for central office and remote terminal applications using the htur_lfsr bit of the Transmitter
Modes Register. The LFSR polynomials for local (HTU-C/LTU) and remote (HTU-R/NTU) unit operations are:
two-level scrambled-ones mode, the LFSR is clocked once-per-symbol; in four-level mode, the LFSR is clocked
twice-per-symbol.
transmitter_off control bit.
mode, the output consists of a single pulse surrounded by zeros.
NOTE:
2.1.2 Variable Gain Digital-to-Analog Converter
A four-level Digital-to-Analog Converter (DAC) is integrated into the Bt8970 to accurately convert the output
of the symbol source to analog form. The normalized values of these four analog levels are: +3, +1, –1 and –3.
Each represents a symbol or quat.
Transmitter Gain Register [tx_gain; 0x29] sets the level.
variations. The Transmitter Calibration Register [tx_calibrate; 0x28] contains a read-only value which nulls this
variation. The value of this register is determined for each Bt8970 device during production testing. Upon
initialization, the Transmitter Gain Register should be loaded based on the transmitter calibration register.
protection), the transmitter gain must be adjusted to include these affects.
2.1.3 Pulse-Shaping Filter
The pulse-shaping filter filters the quats output from the variable-gain DAC. This filter, when combined with
other filtering in the signal path, produces a transmitted signal on the line that meets the power spectral density,
transmitted power, and pulse-shaping requirements, as specified in the Electrical Specifications section of this
datasheet.
2.1.4 Line Driver
The line driver buffers the output of the pulse-shaping filter to drive diverse loads. The output of the line driver
is differential.
100101B
The scrambler is essentially a 23-bit-long Linear Feedback Shift Register (LFSR). The feedback points are
The scrambler operates differently depending on whether a two-level or four-level mode is specified. In
The Transmitter Modes Register can also be used to zero the output of the transmitter using the
The Bt8970 can generate isolated pulses to support the testing of pulse templates. When in the isolated pulse
To provide precise adjustment of the transmitted power, the level of the DAC may be adjusted. The
During the manufacturing of the Bt8970, one source of variation in the transmitter levels is process
If there are other sources of transmit power variation (e.g., a nonstandard hybrid or attenuative lightening
Zero is not a valid 2B1Q level and only occurs in this special mode or when the transmitter is off. The
repetition rate of the pulses is controlled by the meter timer. Any of the four 2B1Q levels may be chosen
via the Transmitter Modes Register’s isolated_pulse[1,0] control bits.
Preliminary Information/Conexant Proprietary and Confidential
remote
local
Conexant
x
x
–
–
23
23
x
x
5 –
–
18
1
1
2.0 Functional Description
2.1 Transmit Section
2-3