The present invention relates generally to analog-to-digital (A/D) converters, and more particularly to sigma delta A/D converters.
It is generally known that an analog signal can be converted to a digital signal (or vice versa) when the sampling frequency, f.sub.s, of general A/D (or D/A) converters is selected to be about twice the signal frequency bandwith, f.sub.BW. This relationship between the sampling frequency, f.sub.s, and the signal frequency bandwith, f.sub.BW, is the familiar Nyquist's theorem.
In conventional oversampling A/D converters, the sampling frequency, f.sub.s, is set higher than twice the signal frequency bandwith, f.sub.BW, which would be established by Nyquist's theorem, in order to achieve improved conversion precision by reducing conversion errors. Thus, when a sampled analog input signal is quantized into a digital signal in a conventional A/D conversion stage, a conversion error (or quantization error) occurs which is the difference between the analog input voltage and a digital output (quantized) step voltage. Such quantization error is given as a random value falling between an amplitude range of +V.sub.q /2 and -V.sub.q /2 with respect to a minimum quantized step voltage, V.sub.q.
As a result, the frequency spectrum of the quantization noise produced by such quantization errors is spread in a uniform manner over the interval from 0 Hz to half the sampling rate, or f.sub.s /2. Filtering is then used to eliminate the noise power outside of the desired signal bandwidth.
The well known sigma delta converter uses feedback to shape the quantization noise into a highpass characteristic. As a result, the quantization error is suppressed most at low frequencies, where the loop gain is highest. However, because the total root-mean-square (RMS) quantization error is constant and ultimately limited by the D/A conversion step size, the reduction in the low frequency quantization noise which effects the reduced quantization error is therefore accompanied by an increase in quantization noise at high frequencies. Hence, digital filters are generally used following the sigma delta converter to attenuate this undesired quantization noise at high frequencies, namely those above the cutoff frequency of one or more integrator stages within the sigma delta converter.
Furthermore, it is generally known that if additonal conversion precision is required beyond that available from a single integration, first-order sigma delta converter, a second integration stage can be incorporated to effect a second-order sigma delta converter. Such sigma delta A/D converters are well known for their ability to reduce inband noise power within a lowpass characteristic, and hence such converters operate best upon analog input signals operating at baseband. A baseband signal is defined here as one having a lowpass characteristic. Examples of ways to arrive at a baseband signal include down-converting or demodulating an RF signal with various known detection methods.
One application of particular interest for sigma delta A/D converters is in mobile radios utilized in modern communication systems. In such applications, a baseband signal can be provided when a received signal is down-converted to an intermediate frequency (IF) signal having a center frequency equal to zero Hertz (0 Hz), or when a signal, such as an IF signal, is detected to produce a baseband signal (i.e., falling between 0 Hz and an upper cutoff frequency, f.sub.c, having a bandwith f.sub.BW), with the carrier signal removed. Carrier signal is defined broadly herein as referring to the center frequency of RF signals or IF signals.
However, several disadvantages become apparent when attempting to convert a baseband analog signal to a digital signal utilizing a conventional sigma delta A/D converter having a lowpass characteristic. Namely, there is an inherent ambiguity in distinguishing between signals occuring at 0 Hz and DC offset voltages existing in active stages within the sigma delta A/D converter. Also, the unavoidable crosstalk between the in-phase and quadrature (or I/Q) channels of a zero-IF receiver can mix undesired out of band signals into the desired passband. A further disadvantage is that the noise present in active circuits is always higher at low frequencies due to flicker or 1/f noise. As a result, there are serious limitations placed upon the ultimate signal-to-noise ratio, and hence the dynamic range, that can be obtained in a given mobile radio application. Such limitations have been only partially overcome by various known arrangements which, at the expense of greater complexity, attempt to deal with the ambiguity created by the DC offset component, I/Q crosstalk, and added noise.
Accordingly, there exists a need for an improved, yet simpler, sigma delta A/D converter that provides greater dynamic range while avoiding the ambiguities, undesired signals, and added noise caused by attempting to convert signals occuring at 0 Hz. This permits the signal processing functions occuring thereafter to be performed digitally, including the required mixing, filtering, and demodulating functions. Such need exists for many applications requiring relatively fast, analog-to-digital conversion with low quantization error, including radio receiver applications.