Electronically controllable slope equalizer

Details Electronically controllable slope equalizer Electronically controllable slope equalizer

2002-02-28

2003-06-17

Nguyen, Patricia T. (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including frequency-responsive means in the signal...

C330S284000, C330S306000, C330S144000, C330S151000

Reexamination Certificate

active

06580327

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to telecommunication line conditioning devices. More particularly, the invention relates to an electronically and remotely controllable slope equalizer for wireline signal transmission, such as for twisted pair local loops.
BACKGROUND OF THE INVENTION
1. The Local Loop
In telephony systems, telephone subscriber equipment, such as telephones and modems, are connected to the telecommunication network by way of twisted pairs of copper wires. The wire connection from the telephone network to the customer premise is commonly referred to as the local loop. The local loops from numerous locations terminate at a “central office” (“CO”) of the local telecommunication provider or at remote telecommunication pedestals. Channel banks, within the CO, such as exemplary D
4
channel banks, convert analog signals from a plurality of loops into a digital form.
Various impairments on the local loop can interfere with communication. One source of impairment, the frequency response characteristic of the local loop, among other things, is a function of the length of the wiring connecting the subscriber to the channel card. That is, as the cable length increases, the varying amounts of attenuation at various frequencies can distort the local loop signals. Typical local loop wiring can range from a few hundred feet, or up to five or six miles, depending on the proximity of the customer premises to the CO.
Typically the attenuation caused by the cable increases with frequency, resulting in significantly more attenuation at higher frequencies. This causes the frequency response to exhibit a “slope” (as typically graphically illustrated as gain versus frequency, measured as the dB difference between two reference frequencies, typically 1004 Hz and 2804 Hz). Some local loops include coils placed along the wires that improve the frequency response characteristic by flattening out this slope. These lines are referred to as “loaded” cables. Even loaded cables, however, can exhibit frequency characteristics with unacceptable amounts of slope. To correct for the slope in the frequency response, slope equalizers are often used.
2. Slope Equalizers
Slope equalizers are used to compensate for the additional attenuation at higher frequencies. By providing varying amounts of gain (or attenuation) with a slope opposite to the frequency response of the cable or transmission line, the slope equalizer may compensate for the frequency response of the local loop.
Variations in the loop characteristics, caused by varying cable lengths and qualities, the presence of loading coils, other line conditions, and so on, make it desirable to have adjustable slope settings for the slope equalizers, to accommodate the characteristics of any given individual loop. Optimal slope settings can be determined from directly measuring the line characteristics. Alternatively, these settings can be prescribed based on features such as whether the cable is loaded or unloaded, the cable gauge, and the cable length.
Local loop slope equalizer settings have been standardized.
FIG. 1
is a block diagram illustrating a prior art, manually adjustable slope equalizer
100
. The prior art manually adjustable slope equalizer
100
utilizes an operational amplifier (“op amp”)
150
, having a switch and resistor arrangement
145
fed back into one input, to form a variable-gain portion, with the second input having an input signal V
IN
, followed by a high-pass filter (HPF)
160
and low-pass filter (LPF)
170
. The switch and resistor arrangement
145
, more particularly, consists of segments of parallel switch and resistor arrangements, with each parallel segment coupled to the others in series (and referred to herein as a series arrangement). The equalization slope is manually set using four designated and standardized switches, referred to in the art as SW
1
(
105
), SW
2
(
110
), SW
4
(
115
), and SW
8
(
120
). The slope setting, k, is determined by the sum of the index numbers on the open switches SWi. As an example, a slope setting of six is achieved when SW
1
and SW
8
are shorted or closed, and SW
2
and SW
4
are open, resulting in a short across the R
1
(
125
) (787&OHgr;) and R
8
(
140
) (6.19 K&OHgr;) resistors and open switches across the R
2
(
130
) (1.58 K&OHgr;) and R
4
(
135
) (3.16 K&OHgr;) resistors.
The variable slope portion of the equalizer is typically followed by a high-pass filter
160
and a low-pass filter
170
. In the example shown in
FIG. 1
, the 0.326 uF and 1.96 K&OHgr; resistor form a high-pass filter
160
with a 3 dB high-pass corner frequency at 249 Hz. This provides a modest amount of additional slope that tends to flatten out the low frequency response of the channel by bringing it down slightly. The low-pass filter
170
of the manual, prior art equalizer of
FIG. 1
is typically an RC filter. An additional operational amplifier is preferably added in the low-pass filter for isolation.
The switches SW
1
, SW
2
, SW
4
, and SW
8
are typically manually configurable switches, such as DIP switches. Manual switches, however, have the disadvantage of requiring a technician to set the slope values by hand. This increases costs of configuring the slope equalizers, considering the many thousands of line cards in a typical central office. Relays or transistors may be used for the switches shown in
FIG. 1
, thereby providing the added feature of electronic control (which also might be accomplished remotely), but having inherent disadvantages. Mechanical relays, for example, have a relatively large size, high cost, and are susceptible to failure due to the mechanical nature of the devices.
Transistors are smaller, cheaper, and more reliable than relays, but also have significant disadvantages, particularly with regard to introduced distortion and errors due to their non-negligible on-resistance. Inexpensive transistors such as Field Effect Transistors (such as FETs of MOSFETs) have a relatively high on-resistance; as a consequence, when they are in the conductive mode (i.e., the switch is “on”), the resistance across the transistor is non-zero and thereby contributes to the overall feedback resistance. This significant on-resistance introduces error, affecting the desired equalization slope, and resulting in inaccurate settings.
With reference to
FIG. 1
, as an example, consider the case of slope=1, where all switches except SW
1
(
105
) are closed. The on-resistance of the remaining three switches appears in series with the R
1
(
125
), the 787&OHgr; resistor. When the four switches
105
,
110
,
115
and
120
are implemented as a typical low-cost FET transistor package, such as an 74HC4316 with four FET switches, each switch has a rated, worst case on-resistance of about 170&OHgr;. Three of these resistors in series with 787&OHgr; causes an effective feedback resistor of 787+3(170)=1297&OHgr;, effectively resulting in the equalizer having an approximate slope=2, rather than the desired slope of 1, introducing a significant error in the frequency response.
In addition, even if the error in the effective resistor value were reduced (by, for example, modifying the scale of the component values throughout the circuit), the variation in the FET's on-resistance as a function of signal voltage would still cause unacceptable amounts of non-linear distortion. While alternative transistor switches for use as SW
1
-SW
8
may be used that will mitigate to some extent the problems discussed above, however, they are considerably more expensive and may be commercially impractical.
As a consequence, a need remains for an improved slope equalizer structure that may be configured electronically and remotely, rather than manually, that has variable slope settings, that has significantly lower distortion and is more accurate than analog switches would be in conventional slope equalizers, and that may be implemented in a small space, in a commercially reasonable manner and at low cost. In addition, a need remains for a slope equalizer struct

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