New approaches in noise shaping Fractional-N synthesis

• Main Author: Walkington, Robert
• Format: Dissertation
• Language: English
• Published: University of London 2001

Modern mobile communication systems require high stablility high resolution frequency sources able to change frequency at high speed under digital control. The high speed channel changing requirement can result from security requirements and the need for frequency diversity to reduce the apparent effects of multipath fading. For stability the signal source is invariably implemented as a phase locked loop (PLL) or indirect frequency synthesiser. This allows an unstable voltage controlled oscillator to be phase locked to a highly stable crystal oscillator by means of a control loop. The requirements of high resolution and high speed channel change are conflicting and frequently lead to multiplexing two synthesisers to satisfy demanding system specifications. This results in a high component count and demanding isolation requirements between the synthesisers, leading to costly screening. An alternative technique to increase the performance of a single loop is a Fractional-N synthesiser switched by a modulator. The main limitations of Fractional-N loops stem from the need to implement the modulator in digital hardware. Digital multipliers and adders limit the reference frequency of the modulator which limits the acheivable noise spreading performance of the synthesiser system. The need to operate at high reference frequencies also leads to a requirement to truncate the loop filter coefficients to allow implementation as bit shifts which further degrades the noise response. This thesis concerns the study of an alternative system where a modulator sequence is generated offline for each channel and stored in memory, making very high reference frequency operation possible with highly optimised noise shaping. The increased exploitation of the [sigma][delta] noise shaping would allow a single chip solution in many applications currently using dual synthesisers.