Design of a fractional phase locked-loop frequency synthesiser using a Motorola based microcontroller

A Motorola M58HC11EVB board is used in the design of a frequency synthesiser using a fractional phase-locked loop technique which synthesises frequencies that are integer plus fractional times higher than the reference frequency. The method uses a 68HC11 microprocessor to generate very stable reference frequencies. A software program is designed to give great flexibility in selecting the reference over a desired operating range of frequencies. The ability to operate at low reference frequencies ( approximately=10 Hz) enables the system to step in 1 Hz increments. A frequency synthesiser over the bands of 10-150 Hz, 100-1500 Hz, 1-15 kHz, and 10-150 kHz in steps of +or-1 Hz, +or-10 Hz, +or-100 Hz and +or-1 kHz, respectively, is accomplished. The spectrum purity of the waveform for different frequencies is measured. Sideband frequencies of up to 50 dB below the synthesised frequency are observed.