Stijn Reekmans

Mismatch Insensitive Double-Sampling Quadrature Bandpass

In a double-sampling quadrature bandpass sigma-delta modulator, path mismatch between the double-sampling branches and between the I/Q paths occurs. In this paper, an analytical study is presented which shows that this causes quantization noise and input signals to fold from the image band into the signal band and that this also results in a self-image component. To reduce the folding from the image band, a novel resonator is presented. This resonator has a bilinear input circuit so that noise and signals exhibits first-order shaping before folding in the band of interest. Next, three different modulator architectures based on the novel resonator are introduced. Finally, the remaining problem of self-image is tackled with a simple, yet efficient offline calibration strategy. Various design examples are shown and simulated to illustrate and prove the effectiveness of the proposed architectures and methods.

\Sigma\Delta

The receiver architecture proposed in this brief seizes the subsampling properties of continuous-time sigma-delta (SigmaDelta) modulators based on distributed resonators to construct a quadrature receiver. The proposed architecture is based on a low-pass SigmaDelta modulator that subsamples an intermediate frequency signal around the sampling frequency and does not require quadrature mixers. Instead, the quadrature mixing is replaced by suitably choosing the sampling instants inside the loop. Two practical circuit implementations are proposed. The first one uses separate circuitry for the I and Q paths. The second architecture introduces an innovative way to produce the I and Q outputs that is immune to path mismatch due to the sharing of all the analog circuitry for both paths. The proposed modulator may be feasible for the typical IF frequencies used in cellular base stations.

Modulation

Quadrature SigmaDelta ADCs require a feedback path for both the I and the Q part of the complex feedback signal. If two separated multibit feedback DACs are used, mismatch among the unit DAC elements leads to additional mismatch noise in the output spectrum as well as an unbalance between the I and Q DAC. This paper proposes a new quadrature bandpass mismatch shaping technique. In our approach the I and Q DACs are merged into one complex DAC, which leads to near-perfect I/Q balance. To select the unit DAC elements of the complex, multibit DAC, the well-known tree structured element selection logic is generalized toward a complex structure and necessary conditions for its correct operation are derived. Finally, a very efficient first-order quadrature shaper implementation is proposed and simulations show the effectiveness of the quadrature bandpass mismatch shaping technique

A Subsampling Quadrature

This paper gives an insight on how to derive the minimum required specifications both for the anti-aliasing filter (AAF) and the analog-to-digital converter (ADC) in the case of an UMTS receiver. Based on the receiver specifications, we come up with a design plan for both discrete time ΣΔ ADC as well as for a continuous time ΣΔ ADC.

\Sigma \Delta

Quadrature bandpass (QBP) SigmaDelta ADCs require a feedback path for both the I and the Q part of the complex feedback signal. A complex DAC could give this feedback with near-perfect I/Q balance. Still, the mismatch between the unit elements of the complex DAC introduces mismatch noise that should be shaped out of the signal band with dynamic element-matching (DEM) techniques. To select the unit DAC elements of the complex multibit DAC, the well-known data directed swapper is generalized towards a complex structure and the necessary constraints for its correct functioning are derived. Additionally, a hardware efficient structure is presented: the reduced butterfly shuffler. Here, some of the QBP swapper cells are replaced by bandpass (BP) swapper cells. Also, great attention is paid to the interconnection pattern of the data directed swapper to prevent instability.

Modulator Based on Distributed Resonators for Use in Radio Receiver

This paper presents experimental results of a quadrature bandpass sigma-delta (SigmaDelta) modulator based on distributed resonators. The modulator employs transmission lines and transconductors as main components and does not require any switches. In addition, the proposed complex modulator does not require a quadrature mixer in the receiver. As main feature, the modulator architecture introduces an innovative way to produce the I and Q outputs that is immune to path mismatch due to the sharing of all the analog circuitry for both paths. The one-bit second-order modulator chip achieves a dynamic range of 60 dB within a 1 MHz signal bandwidth at 25 MHz and at a clock frequency of 50 MHz. Furthermore, it provides an image rejection of 47 dB. The 0.35 mum BiCMOS chip consumes 28 mW at 3.3 V supply voltage.

Quadrature mismatch shaping with a complex, tree structured DAC

Quadrature SigmaDelta ADCs require a feedback path for both the I and the Q part of the complex feedback signal. A complex DAC could give this feedback with near-perfect I/Q balance. Still, the mismatch between the unit elements of the complex DAC introduces mismatch noise that should be shaped out of the signal band with dynamic element-matching (DEM) techniques. In literature, only quadrature DEM techniques for 3-state (I, Q, 0) unit elements are considered. However, in fully differential circuit, also 5-state unit elements are available. In this case, each unit element can be selected plusmn1, plusmnQ or 0. When using these 5-state unit elements, the amount of hardware and power consumption can be reduced significantly. In this paper it is shown how the tree structure, the data directed swapper and the vector quantizer structure can be adapted for the use in fully differential circuits with such 5-state unit elements.