Time based data separator zone change sequence

Abstract

The present invention provides a method and an apparatus for controlling initial transients in a frequency synthesizer by controlling the start-up sequence of the device. The start-up sequence comprises several steps. The voltage controlled oscillator(s) (VCO) is reset so that the VCO(s) are in a known state during start-up. The charge pump and phase detector of phase-locked loop (PLL) are disabled. New data values are loaded into counter(s)/register(s) that control the frequency of the VCO(s). Also, a data value is provided to a digital-to-analog converter (DAC) to set the data rate for the PLL. A fixed amount of time is provided as a delay for the DAC to settle (i.e., 1.6 .mu.s). Divide-by-M and divide-by-N counters are then enabled. Also, the phase detector of the phase-locked loop (PLL) is enabled. The VCO is then restarted. By utilizing a start-up sequence, the center frequency of the VCO is already settled when changing frequencies, the divide-by-M and divide-by-N counters are matched, the VCO starts in phase with the reference frequency of the reference signal, and the voltage of the loop filter is prevented from railing. By using dual buffered registers for each counter, loading of the divide-by-M and divide-by-N counters is accomplished without shutting down the VCO. A timer provides a 1.6 .mu.s delay to allow the DAC to settle. Digital logic is used to synchronize signals. Delay compensation circuitry is used to implement delay cancellation for zero phase restart.