Described are apparatus and methods for fractional synchronization using direct digital frequency synthesis (DDFS). A DDFS device includes a memory with N address spaces, a write port circuit configured to sequentially write a digital desired pattern into the N address spaces, a read port circuit configured to readout the digital desired pattern from the N address spaces using continuous sequential automatic addressing from 0 to N−1 at a memory operating frequency clock, where the memory operating frequency clock is based on a sampling frequency clock used for high-speed data processing, and an analog signal processing circuit configured to process a readout digital desired pattern into an analog representation; and output a synthesized frequency clock from the analog representation to a digital core, where the synthesized frequency clock is fractionally synchronized with the sampling frequency clock.

The operation of the phase-locked loop includes a startup phase where a reference signal having a duty cycle of 50% is applied to a phase comparator of the loop. A first divider of an output signal of the voltage-controlled oscillator of the loop is reset at each first type signal edge of the reference signal. The phase comparator receives the reference signal and a feedback signal from the first divider and generates a control pulse at each second type signal edge of the reference signal that causes a control voltage of the oscillator to increase.

An apparatus includes a plurality of monitoring circuits and a reset circuit. The monitoring circuits may each be configured to determine a status of one of a plurality of input signals, transmit one of the input signals to a PLL circuit and generate a loss signal in response to the status. The reset circuit may be configured to receive the loss signal and generate a reset signal in response to the loss signal. One of the input signals may be a primary input used by the PLL circuit. One of the input signals may be a secondary input that has been selected to replace the primary input. The reset signal may be configured to reset a feedback clock divider of the PLL circuit.

An apparatus includes a plurality of monitoring circuits and a reset circuit. The monitoring circuits may each be configured to determine a status of one of a plurality of input signals, transmit one of the input signals to a PLL circuit and generate a loss signal in response to the status. The reset circuit may be configured to receive the loss signal and generate a reset signal in response to the loss signal. One of the input signals may be a primary input used by the PLL circuit. One of the input signals may be a secondary input that has been selected to replace the primary input. The reset signal may be configured to reset a feedback clock divider of the PLL circuit.

A phase lock loop (PLL) includes a phase detector configured to output a signal indicative of a phase difference between a reference signal and a feedback signal, a loop filter configured to filter an output of the phase detector, and a voltage-controlled oscillator (VCO) configured to output an oscillating signal having a frequency corresponding to an output of the loop filter. The PLL further includes a frequency divider configured to output the feedback signal by frequency dividing the oscillating signal output by the VCO, and a reset circuit configured to reset the frequency divider in an initialization mode such that a phase difference between the reference signal and the feedback signal corresponds to a lock angle of the PLL.

Locking time for a phase-locked loop is decreased by selectively controlling a division value of the feedback divider during the first division cycle to reduce the initial phase error. The division value of the feedback divider during the first division cycle is selectively set such that the locking phase relationship between the two phase detector input signals is achieved at the end of the first division cycle.

A clock data recovery circuit configured to receive an input data signal that includes an embedded clock signal includes a clock recovery circuit including a phase detector configured to detect a phase of the embedded clock signal and to generate a recovery clock signal from the input data signal based on the detected phase; and a data recovery circuit configured to generate a recovery data signal from the input data signal by using the recovery clock signal. The phase detector includes a sampling latch circuit configured to output a first sample signal and a second sample signal from the input data signal; and an edge detection circuit configured to generate a phase control signal based on the first sample signal and the second sample signal and output the phase control signal in a period in which the second sample signal is output from the sampling latch circuit.

A clock data recovery circuit configured to receive an input data signal that includes an embedded clock signal includes a clock recovery circuit including a phase detector configured to detect a phase of the embedded clock signal and to generate a recovery clock signal from the input data signal based on the detected phase; and a data recovery circuit configured to generate a recovery data signal from the input data signal by using the recovery clock signal. The phase detector includes a sampling latch circuit configured to output a first sample signal and a second sample signal from the input data signal; and an edge detection circuit configured to generate a phase control signal based on the first sample signal and the second sample signal and output the phase control signal in a period in which the second sample signal is output from the sampling latch circuit.

The operation of the phase-locked loop includes a startup phase where a reference signal having a duty cycle of 50% is applied to a phase comparator of the loop. A first divider of an output signal of the voltage-controlled oscillator of the loop is reset at each first type signal edge of the reference signal. The phase comparator receives the reference signal and a feedback signal from the first divider and generates a control pulse at each second type signal edge of the reference signal that causes a control voltage of the oscillator to increase.

A Phase-locked loop circuit including: a local oscillator, configured to generate a timing signal; a variable-length shift register, controlled by the timing signal; and a feedback control circuit, which receives a pulsed input signal and receives a local signal from the shift register. The feedback control circuit detects whether each pulse of the input signal respects a condition of temporal proximity with a corresponding pulse of the local signal and detects, for each pulse of the input signal that respects the proximity condition, whether the edge falls early, late, or within a predefined portion of the corresponding pulse of the local signal. The feedback control circuit controls the length of the shift register and the frequency of the timing signal, as a function of the detections made.