Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2001-03-23
2003-12-16
Le, Dinh T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S074000
Reexamination Certificate
active
06664841
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to methods and apparatus for conditioning an analog input signal, and specifically to limiting or “clipping” an analog signal that is input to an analog-to-digital converter, the input signal being variant in voltage with time.
2. Description of Related Art
Analog circuits are commonly used to condition signals supplied to or received from other devices or functional units within an integrated device. For example, an analog circuit is often used as a “front end” signal conditioner for an analog-to-digital converter (ADC) implemented in the form of an application-specific integrated circuit (ASIC), the ADC converting the conditioned analog signal to a binary digital format for use by digital circuitry either within or external to the ASIC. In the case of an ADC, it is often necessary to condition the analog input signal in order to obtain improved or even acceptable performance from the ADC, as described in greater detail below. While the following exemplary discussion is cast in terms of an ADC/ASIC, those skilled in electronic art shall appreciate that analog signal conditioning may be utilized in a broad range of electronic applications.
Analog-to-digital converters are well known in the electronic arts. ADC devices take an analog input signal of varying voltage and convert it to a binary digital representation of the input signal for subsequent processing by digital circuitry such as a digital signal processor (DSP). ADC devices can generally be divided into different functional categories, including “over-sampling” ADC devices. Over-sampling converters, as the name implies, sample the analog signal at a frequency that is typically much higher than the Nyquist frequency. The delta-sigma converter (also referred to as a “sigma-delta” converter) is one type of over-sampling converter that is commonly used in applications where the high sampling rate provides intrinsic benefits. Such applications include digital audio and video decoding. As is well known, Delta modulation refers generally to the process whereby the digital output signal represents the change, or “delta”, of the analog input signal. Delta-sigma converters integrate the analog input signal before performing delta modulation. Hence, the integral of the analog input signal is encoded in delta sigma converters. In contrast, only the delta or change in the input signal is encoded in the simple delta modulator. A digital sample rate reduction filter (commonly known as a decimation filter) is also commonly used to provide an output sampling rate that differs from the Nyquist frequency of the signal. The combination of the over-sampling process and the decimation process produces greater resolution than a typical Nyquist converter.
Third order and higher order delta-sigma converters, in contrast to their lower-order counterparts, provide enhanced performance due to their ability to more effectively remove in-band noise from the signal. Hence, a third-order or higher order delta-sigma converter will provide a higher quality digital audio or video signal (i.e., higher Signal-to-Noise Ratios (SNR)) than second-order or lower order counterparts.
Despite their enhanced performance and utility in certain applications, all third order (and higher order) delta sigma converters are inherently unstable. This instability arises from, inter alia, the noise transfer function (NTF) associated with the converter. Typically, this instability is manifested in very harsh and largely unpredictable signal degradation when the relevant threshold condition (ie., input signal voltage) is exceeded. Throughout the remainder of this specification the term “exceed” or “exceeds” is used to describe the condition when the input signal voltage level is either greater than a high threshold voltage or less than a low threshold voltage.
Unlike other types of circuits that may exhibit more “graceful” degradation (e.g., a progressively increasing noise component or distortion present in the output signal) as the threshold voltage is exceeded, third and higher order delta-sigma converters tend to degrade catastrophically. Even small increases in the input voltage above a threshold induce large oscillations within the circuit. This results in an output signal that is almost entirely dominated by noise, and that bears little or no resemblance to the input signal. This type of behavior is especially troubling in applications in which it is desirable to have improved control over the degradation of the output signal, such as in digital audio applications.
Consider, for example, the use of a third-order or higher order delta-sigma ADC in a digital wireless telephone wherein there are no limitations placed on the input signal that is applied to the ADC. When a caller's audio input produces input voltages that are less than the specified threshold value, the noise component within the output signal of the ADC is minimized, and the useful signal is maximized. However, when the input signal exceeds a level that induces oscillation of the converter, there is a rapid and often complete degradation of the signal. In such cases, a very abrupt cessation of voice may become manifest and perceived by the listener. This cessation may be followed by an unintelligible string of voice information until the signal level falls very near or below the threshold value of the ADC. Clearly such circuit behavior is unacceptable and must be avoided.
While third-order and higher-order converters can be made conditionally stable by appropriately restricting the input signal voltage or via system level design, such design and operational restrictions place a significant burden on the system designer. This is highly undesirable from the perspective of labor and man-hours required to implement the restrictions, thereby potentially increasing required die area, external component costs and time-to-market of devices using delta-sigma converters. In many applications, such design restrictions are exceedingly difficult to implement, such as in the case of a tuner circuit whose output (ie., the input to the ADC) may vary hundreds of millivolts. Furthermore, prior art approaches for restricting voltages that are provided as input to the ADC can have significant deleterious effects on the quality and useful range of the input signal.
Some techniques for restricting or conditioning voltages of an analog signal that is input to another device, such as a higher-order ADC, have been proposed in the prior art. These techniques typically require that the input signal voltage be progressively restricted as it approaches a threshold value of interest. For example, one approach utilizes discrete components, such as diodes, to “clip” an input voltage as it approaches a pre-determined threshold voltage. The degree of signal clipping is substantially dependent upon the proximity of the input signal voltage to the pre-determined threshold. At a voltage that is substantially distant from the threshold voltage, there will be very little if any clipping of the input signal. However, as the input signal voltage approaches the threshold voltage, more clipping is applied until the input signal is completely clipped so as to maintain its voltage at or below the threshold level. When completely clipped in this fashion, no amount of increase in the input signal voltage will drive the output voltage to a level that is higher than the threshold voltage.
While effective at clipping the signal so as to avoid exceeding the threshold, the foregoing technique suffers from the significant disadvantage of distorting the input signal when it operates within the voltage range of interest. The degree of signal distortion varies depending on the proximity of the voltage to the threshold. The diodes used by the previous clipping techniques create increased signal distortion as the voltage thresholds are approached. At some point the distortion becomes sufficiently significant such that the resultant signal is no longer useful. At this poi
Cetin Yusuf
Hamilton Graham S.
Yayla Ibrahim G.
Conexant Systems Inc.
Le Dinh T.
Procopio Cory Hargreaves & Savitch LLP
LandOfFree
Method and apparatus for conditioning an analog signal does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for conditioning an analog signal, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for conditioning an analog signal will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3179440