Generalized Noise Shaping in Delta Sigma Modulators

Abstract

Ever increasing bandwidth demands in modern cellular networks are becoming difficult to meet, as there is no longer room to expand within the current commercial frequency allocation. As a result, an important part of current and future cellular protocols, such as LTE-A and 5G, is Carrier Aggregation---the ability to communicate on multiple channels at once. However, when those channels lie in separate bands, they can be separated by 100’s of MHz. This complicates the analog front-end design, since it is difficult for a single ADC to convert such a wide bandwidth, to say nothing of the inefficiency in converting undesired spectrum along with desired. As a result, multiple power-hungry front-ends are typically required.

To reduce the need for separate analog front-ends, we introduce a new class of ΔΣ modulator—the Multi-Band ΔΣ modulator—that uses custom-designed noise shaping to digitize multiple bands simultaneously without wasting valuable noise shaping resources on undesired portions of the spectrum. The prototype Multi-Band ΔΣ Modulator (MB-ΔΣM) is fabricated in 40nm Complementary Metal Oxide Semiconductor (CMOS) technology and demonstrates two simultaneous bands: one at baseband and one at bandpass. These two bands are separated by 500MHz, have an aggregate bandwidth of 90MHz, with up to 55dB measured SNDR. In addition to reducing the number of ADCs, this new approach promises further system-level power savings by simplifying the RF front-end. The system-level power savings from requiring fewer analog mixers, LNAs, filters, and ADC drivers can be even more than the ADC power reduction.

We further develop two theoretical results that, when taken together, greatly simplify the design of this complex, high-order modulator. The first result introduces an easy-to-use, closed-form solution for Continuous-Time (CT) to Discrete-Time (DT) loop filter conversion which fully accounts for Excess Loop Delay (ELD) and can be used with all standard ELD compensation techniques. Our solution also allows for arbitrary Digital to Analog Converter (DAC) pulse shapes and is architecture agnostic.

The second result shows that any modulator using an ELD compensation path implemented in CT has limited noise shaping capacity. We also show the types of noise shaping that are available to such systems, making it possible for the designer to reliably implement more complex noise shaping.