Integrated analog adaptive equalizer

Details Integrated analog adaptive equalizer Integrated analog adaptive equalizer

1998-12-21

2003-04-01

Pham, Chi (Department: 2631)

Pulse or digital communications

Equalizers

Automatic

C333S02800T, C708S322000

Reexamination Certificate

active

06542540

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to communications, and in particular to an adaptive analog equalizer.
Due to the widespread popularity of the World Wide Web, internet traffic is at an all time high and rapidly increasing. The resulting congestion is taking its toll on users and telephone companies alike. Users are often frustrated by the length of time it takes to download complex graphics and videos. For example, a ten megabyte video clip which is the equivalent of a four minute movie, takes approximately ninety-three minutes using a 14.4 kilobyte modem and forty-six minutes using a 28.8 kilobit modem.
In addition, lengthy data transmissions are tying up telephone company switches that were designed to handle brief telephone calls. Broadband modems, and in particular asymmetric digital subscriber line (ADSL) modems are an emerging technology that promises to dramatically increase the ability to transfer data over conventional telephone lines. Significantly, ADSL modems allow data transfers at rates over two hundred times faster than current modems, and over ninety times faster than ISDN lines.
ADSL was originally conceived of as a technology for delivering interactive multimedia services, such as video on demand over existing telephone networks. However, it is internet access that is currently driving the demand for ADSL. Unlike other high speed data transmission technologies such as ISDN, ADSL requires no massive rewiring or other changes to a telephone company's local exchange or central office. Notably, ADSL modems use the existing telephone infrastructure, including the so-called “last mile” of the network, which is the leg from the central office to a subscriber site (e.g., a home or office) that uses a twisted pair of copper lines. Although it is often referred to as the “last mile”, the leg from the central office to the subscriber site is typically about 12,000-18,000 feet long.
The bandwidth of a conventional copper twisted pair telephone line is approximately 1 MHz. However, conventional analog signals that carry voice over these lines operate in a 4 kHz bandwidth. Advantageously, ADSL takes advantage of the remaining portion of the 1 MHz bandwidth. Specifically, ADSL technology effectively subdivides the 1 MHz bandwidth of the copper twisted pair line into three information channels: i) a high speed downstream channel, ii) a medium speed duplex (upstream/downstream) channel, and iii) a conventional voice channel. Downstream refers to transmissions from the telephone network to the ADSL modem located at a subscriber site, while upstream is the route from the subscriber site to the telephone network. This multichannel approach enables subscribers to access the internet, order a video for viewing and send a facsimile or talk on the telephone all at the same time.
FIG. 1
illustrates a communication system
10
that employs ADSL technology. The system
10
includes a subscriber site
12
, which includes a phone
14
, a facsimile machine
16
and a personal computer or computer network
18
. The subscriber site
12
receives a twisted pair of copper telephone lines
20
that connect the subscriber site with a telephone central office
22
. The run length of the telephone line
20
between the subscriber site and the central office
22
is typically 12,000 feet and it could reach 18,000 feet. A POTS splitter
24
located at the subscriber site
12
is connected to the telephone line
20
and couples the telephone line to an ADSL modem
26
and to the phone
14
and facsimile machine
16
.
Central office
22
includes a POTS splitter
30
that is operatively connected to an ADSL modem rack
32
and to a public telephone switch
34
. As known, the public telephone switch
34
communicates over a public switch telephone network
36
. The ADSL modem rack
32
also communicates over the public switch telephone network and is connected via an internet backbone
38
to various devices including a video server
40
, a video conferencing server
42
and a World Wide Web server
44
.
FIG. 2
is a functional block diagram illustration of the ADSL modem
26
and the POTs splitter
24
. The modem
26
includes a hybrid circuit
50
that couples a transmit circuit
52
and a receive circuit
54
to the telephone line.
The transmit circuit
52
includes a digital signal processor (DSP)
56
that provides a digitized transmit signal on a line
58
to a digital-to-analog converter (DAC)
60
. The resultant analog signal is input to a low pass filter (LPF)
62
and a filtered transmit signal is provided on a line
64
to the hybrid circuit
50
.
The receive circuit
54
receives a signal on a line
66
and includes a high pass filter
68
, a programmable gain amplifier
70
, a low pass filter
72
, an analog-to-digital converter (ADC)
74
and a DSP
76
which together process the signal on the line
66
in a known manner.
The POTs splitter
24
includes a high pass filter
78
and a LPF
80
. The LPF
80
has a cut-off frequency set at approximately 4 kHz in order to allow the voice band signal to pass onto the line
28
. The HPF
78
filters signals that are transmitted and received by the modem. Therefore, the cut-off frequency of the HPF
78
can be set at no higher than about 30 kHz to ensure that signals from the transmit circuit
52
pass relatively unattenuated through the POTS splitter. In addition, the hybrid
50
is typically used to terminate the HPF
78
in this embodiment.
Attenuation caused by the twisted pair is not constant over frequency spectrum. That is, the telephone line attenuates high frequency components within the received signal spectrum more than lower frequency components. To compensate for signal losses due to the cable/wire, a programmable gain amplifier (PGA) is typically placed in front of the analog-to-digital converter (ADC). The function of the PGA is to amplify the received signal and to increase/maximize the dynamic range of ADC. However, the PGA gain is flat over the frequency band. Therefore, after amplification, the low frequency components will still have a much higher amplitude than the high frequency components. As a result, the dynamic range of ADC is often limited by the low frequency signals. This leads to a situation where the dynamic range of ADC needs to be greater in order to achieve required signal-to-noise ratio (SNR) for system performance.
The amount of signal gain to be provided is further complicated by the fact that signal attenuation increases with the length of the copper wire. Since the distance between the subscriber site and the central office varies considerably (e.g., generally between 12 and 18 kilo-feet), modems at different subscriber sites will see various levels of high frequency signal attenuation. Moreover, signal attenuation is also a function of temperature and copper conditions that are not easily controlled. Hence, modems may experience different degrees of copper loss over time.
Therefore, there is a need for an adaptive equalizer that compensates for the attenuation of the high frequency components, while leaving the lower frequency components relatively unchanged.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an adaptive equalizer to compensate for signal attenuation at high frequencies in the receive path of a broadband communications device.
Briefly, according to the present invention, a broadband communications system includes a receive circuit path and a hybrid circuit. The hybrid circuit provides a receive signal to the receive circuit path that comprises an adaptive equalizer circuit, which conditions the received signal and provides a compensated received signal that is processed by the receive path circuit. In a preferred embodiment, the broadband communications system includes a modem.
The adaptive equalizer is an adaptive analog filter that provides different degrees of high frequency boosts to the received signal, while retaining a relatively constant phase shift for each boost setting. The response of the equalizer is controlled by a contro

Affiliated with

Also associated with

Rating

Integrated analog adaptive equalizer does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Integrated analog adaptive equalizer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Integrated analog adaptive equalizer will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3034093