A Radio Frequency Bandpass ΣΔ A / D

1 Direct conversion of a radio signal into digital representation is highly desired. Such a system would allow all radio signal processing to be done completely in the digital domain. This is beneficial for many reasons. First, it allows for an RF system to take full advantage of the availability of inexpensive digital computation brought by modern digital CMOS processes. Second, computation in the digital domain is scalable to accommodate large signal dynamic range with adequate resolution. Last and most importantly, digital signal processing of radio signal is highly program-mable. With the increasing demand for multi-standard capable wireless devices, this approach provides a good platform to support any of the existing wireless standards as well as to adapt to the needs of future wireless standards. So far, such an approach is still not a viable option. The main problem with this approach is that the performance achievable in today's A/D technology is no where near the type of performance needed to directly convert a radio signal. Because of this reason, a radio receiver typically precedes an A/D. The radio receiver effectively preconditions the radio signal for analog-to-digital conversion. This is done by performing frequency downconversion as well as filtering to reduce the radio signal's dynamic range. A bandpass ΣΔ A/D offers an attractive alternative in achieving direct digital conversion of radio signals. A conventional A/D architecture requires both a large dynamic range and a large band-Chapter 1 Introduction 2 width of operation in order to convert a radio signal with sufficient fidelity. On the other hand, a bandpass ΣΔ A/D is able to selectively enhance conversion resolution at a specific frequency band, by moving the quantization noise energy away from that band. Effectively, a bandpass ΣΔ A/D is able to capture an RF signal faithfully with using a low resolution conversion hardware. The flip side of this approach is the need for a loop filter in order to appropriately suppress quan-tization noise at the desired frequency band. Implementing a bandpass ΣΔ A/D that can operate at radio frequencies still presents a difficult challenge. First, the loop filter has to be able to operate at a high frequency; which pushes the limits of filter design with the available integrated circuits technologies. Second, in order to accommodate for operation in multiple bands, some flexibility has to be built into the filter so that it can be tuned to different frequency bands. In …