An agile ISM band frequency synthesizer with built-in GMSK data modulation

In this paper, a high-resolution fractional-N RF frequency synthesizer is presented which is controlled by a fourth-order digital sigma-delta modulator. The high resolution allows the synthesizer to be digitally modulated directly at RF. A simplified digital filter which makes use of sigma-delta quantized tap coefficients is included which provides built-in GMSK pulse shaping for data transmission. Quantization of the tap coefficients to single-bit values not only simplifies the filter architecture, but the fourth-order digital sigma-delta modulator as well. The synthesizer makes extensive use of custom VLSI, with only a simple off-chip loop filter and VCO required. The synthesizer operates from a single 3-V supply, and has low power consumption. Phase noise levels are less than -90 dBc/Hz at frequency offsets within the loop bandwidth. Spurious components are less than -90 dBc/Hz over a 19.6-MHz tuning range.