Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
1999-06-01
2001-04-10
Marcelo, Melvin (Department: 2663)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C341S150000
Reexamination Certificate
active
06215431
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a droop-free quasi-continuous reconstruction filter interface. More specifically, a reconstruction filter interface is disclosed that resamples the output of a digital-to-analog converter (DAC) and eliminates problems in the output of the DAC such as slewing, glitching, and droop.
2. Description of the Related Art
Asymmetric Digital Subscriber Line (ADSL) technology is used to deliver high-rate digital data over existing phone lines without modification of the phone lines. A modulation scheme called Discrete Multitone (DMT) allows the transmission of high speed digital data. ADSL allows a large bandwidth (1.104 MHz) for transmission in a downstream direction from a central office to a subscriber in a remote location and a lesser bandwidth (138 kHz) for transmission in an upstream direction from of a subscriber in a remote location to a central office. ADSL systems are described further in the ADSL standards ITU-g.992.1 and ITU-g.992.2, which are herein incorporated by reference for all purposes.
FIG. 1A
illustrates the architecture of an ADSL system. A digital network
100
provides data that is to be sent downstream over the ADSL line. The data is delivered to an ADSL transceiver unit in a central office (an ATU-C)
102
. The ATU-C modulates data for downstream transmission using a DMT scheme. ATU-C drives a line
104
with an analog signal. Line
104
is connected to a telephone line
110
that is part of a Public Switched Telephone Network (PSTN)
106
by a splitter
108
. ATU-C also receives data transmitted in the upstream direction on line
104
.
At the receiver end, a splitter
120
splits the signal on line
110
between a message telecommunication service (MTS)
122
(also referred to as plain old telephone service (POTS)) and an ADSL transceiver unit line for a remote unit (an ATU-R)
124
. ATU-R
124
is connected to one or more service modules (SM)
128
by a bus
126
. The ATU-R demodulates data transmitted by the ATU-C in the downstream direction and also transmits data in the upsteam direction.
FIG. 1B
is a block diagram illustrates the transmission path of an ADSL system. The transmission path may be in either the upstream or the downstream direction. An encoder
150
sends digital data to a DAC
152
. The analog output of DAC
152
represent an ADSL modulated signal that is to be transmitted on an ADSL line. However, the DAC is not a suitable line driver because it generally does not have sufficient power output and also because the output of the DAC contains a large amount of spurious, out of band harmonics generated by the zero order hold nature of a realizable analog DAC. A line driver amplifier
153
is used to increase the power drive capability and a reconstruction filter
154
reduces or eliminates the out of band harmonics. The output of the DAC is directed to reconstruction filter
154
and the output of reconstruction filter
154
is directed to line drive amplifier
153
. Reconstruction filter
154
reconstructs the signal from the stairstep output of the DAC for the purpose of driving an ADSL line
156
. Thus, the signal that drives ADSL line
156
is first generated digitally by encoder
150
. DAC
152
converts the digital signal to an analog signal and reconstruction filter
154
drives the line driver, which in turn directs the signal into the line proper.
The performance of reconstruction filter
154
is important because, without reconstruction filter
154
, the output of the ADSL transmitter would include residual harmonic energy over a broad spectrum and would likely violate FCC rules as well as have a poorly defined in band spectrum shape. Furthermore, any high frequency glitching and slew energy would be present on the line, further distorting and corrupting the signal.
FIG. 2
is a block diagram illustrating the basic structure of a typical reconstruction filter. The output of a DAC
200
is input to an amplifier
201
configured in a low pass circuit that includes an input resistor
202
, a feedback resistor
204
and a capacitor
206
. In this low pass configuration, the output of the amplifier at node
210
tends to follow only the low frequency content of the output of the DAC within the design limitations of amplifier
201
, thus achieving reconstruction of the signal.
When the output of the digital-to-analog converter is interfaced directly into the associated reconstruction filter in a continuous time fashion as shown above, several problems arc caused due to the fact that the DAC is inherently discrete time in nature. First, any form of nonlinear settling transient in the DAC such as glitching or slewing distortion is reproduced by the construction filter in the line output. Glitching
320
and slewing
310
are illustrated in FIG.
3
B. Also, the fact that the DAC output is a zero order held waveform results in a frequency dependent droop across the output band, which makes necessary some form of X/sin(X) correction either in the digital processing prior to the DAC or in the analog filter itself. Typically, such correction is costly, both in terms of speed and complexity.
It would be desirable if a system could be designed that could eliminate the problems caused by slewing, glitching and droop that result from applying the output of the digital-to-analog converter to the reconstruction filter.
SUMMARY OF THE INVENTION
Accordingly, a system is disclosed that eliminates problems caused by slewing, glitching, and droop from the digital-to-analog converter output by providing a filter that is quasi-continuous. Such a filter can be simultaneously considered to be both discrete time and continuous time in nature. A filter input is disclosed that resamples the DAC output at a single point in time. Because the point in time is selected to be a time at which the DAC output has settled its desired level, slewing and glitching issues arc eliminated. The resampled DAC output is then input into the continuous time filter by using operational amplifiers in the filter itself to force the charge transfer. If the bandwidth of the filter amplifiers is sufficient, this transfer is nearly instantaneous relative to the resampling rate. Thus, instead of a zero order held signal, the filter is fed a sequence of impulses, each impulse being proportional in weight to the resampled DAC output. The result yields an overall output that is droop-free.
In one embodiment, an ADSL transmission system is disclosed. The system includes a digital DMT generating modulation circuit that generates a DMT modulated digital output. An digital to analog converter converts the DMT modulated digital output into a DMT analog signal. A discrete pulse generating circuit samples the DMT analog signal and the discrete pulse generating circuit outputs a pulsed output signal. A reconstruction filter is driven by the pulsed output signal. The reconstruction filter outputs a line driving signal that is suitable to drive an ADSL communication line so that a line driving signal is produced that reduces or eliminates distortion caused by imperfections in the digital to analog converter.
In another embodiment, a method of driving an ADSL communication line with a DMT signal is disclosed. The method includes creating a DMT modulated digital output and converting the DMT modulated digital output to a DMT analog signal. The DMT analog signal is sampled. A discrete pulse output signal is generated from the sampled DMT analog signal. The discrete pulse output signal is input to a reconstruction filter that outputs a line driving signal that is suitable to drive an ADSL communication line.
In another embodiment, a reconstruction filter is disclosed. An input is configured to receive an output signal from a digital to analog converter. An input sampling circuit is operative to store a sample of the output signal from the digital to analog converter. An input pulse generating switch that generates a pulse, the energy of the pulse being determined by the sample of the output signal from the digital to an
Conroy Cormac S.
Sheng Samuel W.
LSI Logic Corporation
Marcelo Melvin
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