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Delta-Sigma FDC Based Fractional-N PLLs with Multi-Rate Quantizing Dynamic Element Matching

  • Author(s): Venerus, Christian
  • et al.
Abstract

Fractional-N phase-locked loop (PLL) frequency synthesizers are ubiquitous in modern communication systems, where they are used to synthesize a signal of high spectral purity from a reference signal of much lower frequency. In order to meet the requirements of wireless communication standard, strict limitation are placed on the spectral content of the synthesized signal. In recent years, PLL based on time-to-digital converters (TDC-PLLs) have been proposed that aim at moving the complexity of the design from the analog section to the digital section of the synthesizer : the advantages are a reduction in area, cost and power consumption over competing architectures based on delta-sigma modulation and charge pumps ([Delta][Sigma]-PLLs). Although TDC-PLLs with good performance have been demonstrated, TDC quantization noise has so far kept their phase noise and spurious tone performance below that of the best comparable [Delta][Sigma]-PLLs. An alternative approach is to use a delta-sigma frequency-to-digital converter ([Delta][Sigma]FDC) in place of a TDC to retain the benefits of TDC-PLLs and [Delta][Sigma]-PLLs. Chapter 1 describes a practical [Delta][Sigma] FDC based PLL in which the quantization noise is equivalent to that of a [Delta][Sigma]-PLL. It presents a linearized model of the PLL, design criteria to avoid spurious tones in the [Delta][Sigma] FDC quantization noise, and a design methodology for choosing the loop parameters in terms of standard PLL target specifications. Chapter 2 presents a multi-rate quantizing dynamic element matching (DEM) encoder for digital to analog converters (DACs) that allows a significant reduction in the encoder power consumption with respect to a conventional encoder for oversampling DEM DACs, at the expense of a minimal signal- to-noise ratio reduction. In Chapter 3, the implementation details of a [Delta][Sigma] FDC based fractional-N phase- locked loop prototype are shown. The PLL was built to showcase the capability of the architecture analyzed in Chapter 1 to comply with the most stringent wireless communication standards. The prototype extends the architecture described in Chapter 1 by including an FDC quantization noise cancelling algorithm, and an hardware efficient implementation of a multi-rate quantizing DEM encoder for digital to frequency conversion

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