Pulse or digital communications – Equalizers
Reexamination Certificate
2000-05-08
2003-11-04
Vo, Don N. (Department: 2631)
Pulse or digital communications
Equalizers
C375S316000
Reexamination Certificate
active
06643324
ABSTRACT:
This invention relates to a high frequency digital signal receiver for an integrated circuit (IC), and more particularly to a new and improved equalizer receiver having size and power requirements which allow it to be integrated into the IC, preferably as a part of a pad cell circuit of the IC. The new and improved equalizer receiver and its method combine the beneficial effects of gain enhancement and frequency equalization to reliably detect relatively high frequency digital signals while overcoming the adverse affects from initial symbol interference (ISI) of signals communicated over a cable, without requiring a separate equalizer and/or amplifier as added components to the IC.
BACKGROUND OF THE INVENTION
Modern digital communications systems require the transmission and reception of digital signals at a relatively high rate. The digital signals are high-level and low-level voltages, each of which is a “bit,” and the rate of communication of the digital signals is measured in terms of bits per second (bps). The present bps rate of communication internally between a processor and its bus-connected components within modern personal computers is typically in the range of 100-600 Mbps. Such high internal communication frequencies are possible because of relatively short communication paths, tight control over the characteristics of the signals and the communication paths, and the use of coordinated system components which are designed to interact with one another in a specific manner at high communication rates.
Relatively high digital signal communication rates become more problematic, however, when dealing with peripheral equipment connected by cables to the internal processor and bus-connected components. Examples of peripheral equipment connected by cables include disk drives, memories with mechanical components associated with their-media storage, and other computers which are connected by cabling or by a local area network. For cable connections, certain standards have been enacted to enhance the uniformity of signal communication, thereby facilitating the interconnection of various components. One well-known standard associated with cabling is a standard known as small computer system interface (SCSI). The SCSI standard requires that signals delivered have certain voltage levels, exhibit certain waveform characteristics with respect to time and otherwise comply with a variety of other requirements. For local area network connections, such as ethernet, certain protocols and standards have also been enacted which specify the characteristics of the signals and thereby enhance the ability for diverse and unspecified computer equipment to interact with each other over the local area network.
In both cable and local area network connections, difficulties in achieving high frequency communication rates arise which are not present in communication between the processor and its bus-connected elements. The cable itself attenuates the signals, thereby diminishing the signal strength, particularly if the length of the cable is significant. It is not unusual for a SCSI cable to extend a number of feet or meters, and it is typical of that local area network cables extend many tens of feet or meters. Enough attenuation of a digital signal will result in it being mistaken for a digital signal having the opposite value (high or low voltage).
Because the cable itself is a complex impedance having both inductance and capacitance, the signal transmission characteristics of the cable are frequency-dependent. The cable itself filters and blocks the high frequency components of the digital signals but usually passes low frequency signals without difficulty. As a consequence, the relatively rapid transition of a digital signal between the high and low levels is changed by the cable to a more gradual transition. A sharp transition is achieved by passing high frequency components of the signal, but those high frequency components are blocked by the high frequency filtering characteristics of the cable itself. A gradual transition of the signal is undesirable because it affords the possibility that the voltage level of the digital signal will be mistaken for the opposite value as a result of the gradual transition. To avoid the problem of high frequency attenuation, the communication rate must be reduced, which is counter to the evolution of higher communication rates in modern computer systems.
A further significant problem with the high communication rates over a relatively lengthy cable is initial symbol interference (ISI). In simple terms, ISI is in adverse influence on a presently occurring digital signal as a result of the previous digital signals which have been communicated over the cable. The residual affects from previous signals adversely influence the present signal. The previous signals have charged or otherwise influenced the capacitance of the cable, so that a transition of the present digital signal must overcome the previous charge on the cable. For example, if a series of low-voltage level digital signals had been communicated over the cable, the cable is relatively discharged. The occurrence of a high-level signal will necessitate charging the cable. The time taken to charge the cable will diminish the value of the present digital signal until the cable becomes charged to the high voltage level of the present signal. The necessity to charge the cable may result in mistaking the present digital signal for its opposite value. In very high frequency communication systems, more than one of the previous digital signals have residual ISI influences, although the residual affect diminishes with each subsequent digital signal.
Local area network communications systems have solved many of the attenuation, frequency filtering and ISI problems by employing pre-compensation or pre-emphasis driver circuits and equalization receiver circuits connected at both ends of the cables which form the communication media of such local area networks. A pre-compensation or pre-emphasis driver circuit boosts the signal level or amplitude of each transitional digital signal applied on the cable. For example, after a series of low-voltage level digital signals applied to the cable, the first high-voltage level digital signal will be boosted in magnitude by some appropriate margin, for example 30 percent. The boosted magnitude of the transition signal tends to overpower the effects of ISI and attenuation. An equalization receiver circuit connects complex impedance elements to the cable to cause a tuned or peaked frequency response at the desired high frequency communication rate. Equalization overcomes the high frequency filtering characteristics of the cable by providing a peaked or enhanced response at the frequency of the communication rate.
Local area networks also successfully use differential signaling to overcome adverse influences. Differential signaling involves a pair of conductors whose individual signals move in relatively opposite directions with respect to one another. The difference in signal level determines the digital signal value. Noise induced into the cable has a minimal effect, because the noise equally influences the signal levels on both conductors, thereby canceling or rejecting those adverse influences.
The use of pre-compensation, pre-emphasis, equalization and differential signaling circuits in local area networks is acceptable, because it is relatively easy to accommodate these additional separate components to each end of the single connecting cable. However, difficulties arise in the context of a SCSI cable or other internal multiconductor computer cable were it is necessary to connect the SCSI or other cable to the ICs contained within the computer itself. In these situations, to build a pre-compensation or pre-emphasis driver circuit as a part of an IC would consume a large surface of the chip upon which the IC is fabricated. Since the frequency response characteristics of a pre-compensation or pre-emphasis driver circuit which uses operational amplifiers and operational t
LSI Logic Corporation
Ortiz & Lopez PLLC
Tran Khai
Vo Don N.
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