WO2002030012A2

WO2002030012A2 - Analog-to-digital spurious noise improvement using a pseudo-random binary sequence generator

Info

Publication number: WO2002030012A2
Application number: PCT/US2001/031078
Authority: WO
Inventors: Leo Montreuil; Lamar West; Joseph Graham Mobley
Original assignee: Scientific-Atlanta, Inc.
Priority date: 2000-10-06
Filing date: 2001-10-04
Publication date: 2002-04-11
Also published as: WO2002030012A3

Abstract

This invention relates to a noise mitigator (400) for use with an analog-to-digital converter (435) for reducing the spurious noise that is generated as a byproduct of the conversion process. The noise mitigator (400) includes a pseudo random binary sequence generator (410) that provides noise signals at a frequency outside of the frequency range that is providing information. The noise signals are combined with the analog signals before the input to the A/D converter (435), where the combintation of the signals reduces the spurious noise signals.

Description

ANALOG-TO-DIGITAL SPURIOUS NOISE IMPROVEMENT USING A PSEUDO-RANDOM BINARY SEQUENCE GENERATOR

INVENTORS: Leo Montreuil

Lamar West

Joseph Graham Mobley

FIELD OF THE INVENTION

This invention relates generally to broadband communications, such as cable television systems, and more specifically to electrical devices that utilize analog to digital converters, such as broadband digital reverse products, within the cable television systems.

BACKGROUND OF THE INVENTION

A communication system 100, such as a two-way cable television system, is depicted in FIG. 1. The communication system 100 includes headend equipment 105 for generating forward signals that are transmitted in the forward, or downstream, direction along a communication medium, such as a fiber optic cable 110. Coupled to the headend 105 are several hubs 115 that serve sites that may be miles away from the headend 105. Included within the hubs 115 is fiber equipment for further transmission of the optical signals to optical nodes 120 that then convert the optical signals to radio frequency (RF) signals. The RF signals are further transmitted along another communication medium, such as coaxial cable 125, and are amplified, as necessary, by one or more distribution amplifiers 130 positioned along the communication medium. Taps 135 included in the cable television system split off portions of the forward signals for provision to subscriber equipment 140, such as set top terminals, computers, and televisions. In a two-way system, the subscriber equipment 140 can also generate reverse electrical signals that are transmitted upstream, amplified by any distribution amplifiers 130, converted to optical signals by the optical node 120, and provided to the headend equipment 105. More recently, however, new cable applications, such as interactive multimedia, Internet access, and telephony, are increasing the demand for additional reverse path capability. Cable operators are redesigning the networks 100 to increase the total reverse bandwidth and further refine the network to become two-way active. Some of the difficulties in the growth of the reverse path are that the conventional methods used to transmit reverse signals from a hub 115 to a headend 105 continue to become more complex and expensive as the numbers of reverse paths grow. Networks 100 are also beginning to increase the physical territory to include areas that may not have been previously serviced. In addition, networks 100 are constructing one "super" headend by consolidating several existing headends 105; therefore, in light of the redesigns and upgrades, the distance between the headend 105 and a hub 115 is extending further than in older networks. Consequently, it is becoming increasingly more difficult for the cable operators to ensure the transmission of a robust and reliable reverse path signal at a manageable cost.

To address the increased demands on the reverse path, a solution is to convert the analog signals to digital signals within the reverse frequency range, such as 5 MHz to 42 MHz. A digital reverse converter is depicted in FIG. 2. A reverse transmitter 205 receives analog electrical signals and converts the signals to digital optical signals. A reverse receiver 210, further upstream in the network 200, then converts the received digitized optical signals back to analog electrical signals. Digitizing the reverse bandwidth as shown allows the operator to increase the reverse path capacity that is demanded by the growing interactive applications. It will be appreciated that the reverse transmitters 205 and receivers 210 can be utilized in a number of broadband communications products and applications, such as digital reverse transmission from an optical node 120 to the headend 105 or from a hub 115 to the headend 105.

A major concern, however, is the impact of noise on the network 100 (FIG. 1), since such noise often becomes noticeable at the headend 105. It is known that digitizing analog signals through use of an analog-to-digital (A/D) converter generates spurious noise as a byproduct. For example, digitizing analog signals in cable television products, such as through use of an A/D converter 215 in the reverse transmitter 205, generates spurious noise as a byproduct. The reverse signals are then transmitted along with the generated noise to a laser 220 for transmission further upstream. When the reverse signals are received at the reverse receiver 210, a photodiode 225 receives the reverse signals and provides the digitized electrical signals, which include the generated spurious noise, to a digital-to-analog (D/A) converter 230, which converts the signals back to analog electrical signals. From there, the signals are further transmitted upstream to the headend 105. At the headend 105, the generated noise that is included with the reverse signals, and which also cumulates with other existing noise within the network 100, can cause errors when processing the signals. For example, if the headend 105 processes the reverse signal incorrectly due to a spur, the interactive response to a cable modem downstream may be incorrect. More importantly, with the advent of telephony, noise may cause a telephone call to be dropped, or at the very least, degrade in quality.

One conventional method for mitigating the effects of the spurious noise is to design a circuit with a noise diode 235 coupled to the A/D converter 215. The noise diode 235 generates broadband frequency signals 305 that are depicted in FIG. 3. The diode 235 generates noise signals 305 that are injected into the received analog signals prior to the digitizing step achieved by the A/D converter 215 (FIG. 2). The addition of the signals from the noise diode 235 mitigates the spurious noise that is generated as a byproduct from the A/D converter 215 by providing dither of the analog input signals and, consequently, causes the spurious noise signals to be spread within the frequency range. Since the diode 235 generates noise signals 305 at frequencies overlapping with the frequencies of the information signals, however, a sophisticated low pass filter (not shown) is required. For example, if the reverse spectrum in which information signals are provided is between 5MHz and 42MHz, the injected noise signals should be outside this range, such as at a frequency of less than 5MHz. If the low pass filter is not used correctly or is designed incorrectly, signals other than the desired frequency of the injected noise signal, for example, greater than 5MHz, may be included with the reverse signals, and this would only add increased noise to the dynamic range.

Consequently, the noise diode 235 will not effectively mitigate the noise generated by the A/D converter 215 unless additional complex, expensive, and space-consuming circuitry is provided. Specifically, the noise diodes 235 are very costly, and the circuitry required for the low pass filter is not only time- intensive to design and test, it also requires a large number of components. Thus, what is needed is an improved way of mitigating the spurious noise that is generated as a byproduct when analog signals are converted to digital signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional broadband communication system, such as a cable television system.

FIG. 2 is a block diagram of a digital reverse system that is used in cable television systems of FIG. 1. FIG. 3 is a graph of an output frequency response of a noise diode.

FIG. 4 is a block diagram of a digital reverse system in accordance with the present invention utilizing a pseudo random binary sequence generator to mitigate generated spurious noise.

FIG. 5 is a graph of output frequency responses of a pseudo random binary sequence generator controlled by varying clock rates in accordance with the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

As mentioned briefly in the Background of the Invention, analog-to-digital converters inherently generate spurious noise as a byproduct. In broadband communication networks, such as cable television systems 100, analog-to-digital converters 215 are used in reverse digital transmitters 205. Conventional methods, such as use of a noise diode 235, to mitigate the spurious noise require a great deal of time designing and testing the supporting circuitry including the low pass filter. In addition, significant costs and space for the components are required. If attempts are not made to mitigate the generated spurious noise, the noise is transmitted along with the reverse digital signals and later converted to reverse analog signals. The converted signals, including the noise signals, are then processed at the headend 105 and, as a result, incorrect responses to the received signals from the headend 105 back to the subscriber equipment 140 may be provided, or processing errors at the headend 105 may occur.

In accordance with the present invention and as depicted in FIG. 4, using a noise mitigator 400 reduces spurious noise generated by an A/D converter. The noise mitigator 400 uses components that, for example, can flexibly be included on a motherboard along with the A/D converter or within an existing integrated circuit package. Advantages of the present invention over conventional methods are a low cost of implementation and a flexibility in design. In addition, the low pass filter used in the noise mitigator 400 of the present invention is relatively easy to design and utilizes very few components, as will be discussed further below. As a result, use of the noise mitigator 400 saves time, money, and space while also minimizing the effects of spurious noise in a system using an analog-to-digital converter. Less noise throughout the network is becoming more important as the number and complexity of reverse signals increase due to the demand on the reverse path resulting from new cable applications. FIG. 4 shows a spurious noise mitigator 400 that is included within an optical transmitter

405. The noise mitigator 400 generates a noise signal via a pseudo random binary sequence generator 410 that operates at a specified clock rate. The frequency of the clock signal, generated by clock 415, is used to limit the response of the generator 410.

Referring next to FIG. 5, the frequency response of the output of the sequence generator 410 is derived by the formula sin(x)/x, which determines the amplitude of the noise signals versus frequency. Since a clock, such as clock 415, regulates the frequency response, the frequency response 510 that is regulated by a faster clock rate, such as a 6MHz clock rate, can be altered to output a frequency response 520 by using a slower clock rate, such as a 2MHz clock rate. As shown in FIG. 5, the frequency response drastically decreases in both amplitude and frequency at a much lower frequency point compared with conventional frequency responses, such as the noise diode output response shown in FIG. 3. As a result, the pseudo random binary sequence generator 410, when clocked at lower clock rates, provides improved performance when used to inject noise signals into reverse signals to mitigate the spurious noise that is generated by an A D converter. This improved performance is due, in part, to the fact that the frequencies of the injected signals overlap with the frequencies of the information signals to only a small extent.

Referring back to FIG. 4, a low pass filter 420 is used to further reduce any noise signals at undesired frequencies, such as noise signals greater than 5MHz that may fall within the frequency range of the reverse information signals. The low pass filter 420, however, is a very simple design that can include as few as three components, for example, inductance in series with a shunt capacitor, due to the output response of the sequence generator 410 as shown in FIG. 5. The limited number of required components reduces the design time, cost of components, and the space required within the noise mitigator 300 without sacrificing performance.

Analog electrical signals are received at an input port 425 of the optical transmitter 405 and are provided to a summer 430 that sums the received signals and the signals received from the noise mitigator 400. The combination of injected signals and the received analog signals is then provided to an analog-to-digital (A/D) converter 435 for digitizing. The effects of the generated spurious noise are drastically decreased due to the injected noise signals. More specifically, the injected noise provides dither of the analog electrical signals and, consequently, causes the spurious noise signals to be spread over a relatively large frequency range. As a result of the spurious noise signals being spread throughout the frequency range, and because the amplitudes of the noise signals are drastically reduced as opposed to the amplitudes present before noise mitigation, signal quality will be improved further upstream at the headend 105, which will be better able to correctly process noise-limited signals. A laser 440 in the reverse transmitter 405 is used to convert the digitized combined signals to an optical signal that is then provided at the output of the transmitter 405 for further transmission.

Further upstream in the network, an optical receiver 450, which may be present in a headend or a hub, receives the optical signals. A photodiode 455 receives the optical signals and converts them back to an electrical signal. The electrical signals are then provided to a digital-to- analog (D/A) converter 460 for converting the digitized signals back to an analog signal. The signals then follow the upstream path in a conventional manner until they reach the headend 105, which processes the reverse signals accordingly. In summary, many interactive applications require the transmission of a large number of two-way signals that are generated either from the headend 105 or the subscriber equipment 140 and received at the opposite end of a communication system. Any noise that is carried along with the signals can affect the quality of the recovered information and, as a result, the response and noise can be carried in both forward and reverse directions. In accordance with the present invention, the noise mitigator 400 limits the generated spurious noise that can adversely affect the recovery of information in a receiver. Additionally, the noise mitigator 400 can be used in any analog-to-digital signal conversion system that is sensitive to the spurious noise that is generated as a byproduct of the signal conversion process. What is claimed is:

Claims

1. A noise mitigator for use with an analog-to-digital (A/D) converter, comprising: a signal generator for providing a signal at an output rate; a filter coupled to the signal generator for filtering the signal to within a desired frequency range to generate a filtered signal; and a summer, comprising: a first input port coupled to the filter for receiving the filtered signal; a second input port for receiving a second signal; a summing circuit for combining the filtered signal and the second signal into a combined signal; and an output port for providing the combined signal to the analog-to-digital converter, wherein the combination of the filtered signal with the second signal reduces spurious noise signals that are generated as a result of the processing by the A/D converter.

2. The noise mitigator of claim 1, further comprising: a clock with a clock rate coupled to the input of the signal generator for controlling the output rate of the signal.

3. The noise mitigator of claim 1 , wherein the signal generator is a pseudo random binary sequence generator.

4. The noise mitigator of claim 1, wherein the filter is a low pass filter.

5. A transmitter, comprising: an analog-to-digital (A/D) converter for receiving analog signals including information, and for converting the analog signals to digital signals; transmission means for transmitting the information; and a noise mitigator for providing noise signals to the A/D converter, wherein the noise mitigator comprises: a signal generator having an input and an output, and for providing the noise signals at an output rate; and a summer for combining the noise signals and the analog signals into a combined signal, comprising: a first input port coupled to the output of the signal generator for receiving the noise signals; a second input port for receiving the analog signals including information; and an output port for providing the combined signal to the A/D converter, wherein the combination of the noise signals and the analog signals reduce spurious noise signals that are generated when converting the analog signals to the digital signals.

6. The transmitter of claim 5, wherein the noise mitigator further comprises: a clock with a clock rate coupled to the input of the signal generator for controlling the output rate of the noise signals.

7. The transmitter of claim 5, wherein the noise mitigator further comprises: a filter coupled to the output of the signal generator for filtering the noise signals to within a desired frequency range, wherein the filter further includes an output for providing filtered noise signals to the first input port of the summer.

8. The transmitter of claim 7, wherein the filter is a low-pass filter.

9. The transmitter of claim 5, wherein the signal generator is a pseudo random binary sequence generator.

10. The transmitter of claim 5, wherein the transmission means is an optical laser.