Abstract: Apparatus and methods for timing offset compensation of frequency synthesizers are provided herein. In certain embodiments, an electronic system includes a frequency synthesizer, such as a fractional-N phase-locked loop (PLL), which generates an output clock signal based on timing of a reference clock signal. Additionally, the electronic system includes an integer PLL configured to compensate for a timing offset, such as a phase offset and/or frequency offset, of the frequency synthesizer based on timing of the output clock signal.

Abstract: Apparatus and methods for timing offset compensation of frequency synthesizers are provided herein. In certain embodiments, an electronic system includes a frequency synthesizer, such as a fractional-N phase-locked loop (PLL), which generates an output clock signal based on timing of a reference clock signal. Additionally, the electronic system includes an integer PLL configured to compensate for a timing offset, such as a phase offset and/or frequency offset, of the frequency synthesizer based on timing of the output clock signal.

Abstract: Provided herein are apparatus and methods for phase-locked loops (PLLs). In certain configurations, a clock system includes a PLL, a control circuit, and a holdover circuit that is electrically coupled to an input of the PLL's loop filter via a holdover switch and a variable resistor. The control circuit generates an input clock signal for the PLL based on a selected reference clock signal. When the control circuit determines that the selected reference clock signal is unreliable, the control circuit disables the PLL's feedback loop and turns on the holdover switch. After the selected reference clock signal is changed or otherwise becomes reliable, the control circuit enables the PLL's feedback loop while keeping the holdover switch turned on, and controls a resistance of the variable resistor over time to provide a soft transition from holdover to reacquiring phase lock.

Abstract: Provided herein are apparatus and methods for system ready in a clock distribution chip or system. In certain configurations, a communication system includes a clock generation circuit having a divider and phase control circuit to provide output clock signals. The communication system further includes a system ready circuit to provide a system ready signal indicative of whether all of the output clock signals are ready.

Abstract: Provided herein are apparatus and methods for fast charge pump holdover on signal interruption. In certain configurations, a clock generator system includes a phase-locked loop (PLL), a fast detect circuit, and a switch electrically coupled to an input of the PLL's loop filter. The fast detect circuit relatively quickly detects when an input signal to the PLL is lost. The fast detect circuit can quickly detect the loss of phase lock and can place the PLL into a holdover such that the frequency of a clock signal generated by the PLL remains within an acceptable range.

Abstract: Provided herein are apparatus and methods for system ready in a clock distribution chip or system. In certain configurations, a communication system includes a clock generation circuit having a divider and phase control circuit to provide output clock signals. The communication system further includes a system ready circuit to provide a system ready signal indicative of whether all of the output clock signals are ready.

Abstract: Provided herein are apparatus and methods for phase-locked loops (PLLs). In certain configurations, a clock system includes a PLL, a control circuit, and a holdover circuit that is electrically coupled to an input of the PLL's loop filter via a holdover switch and a variable resistor. The control circuit generates an input clock signal for the PLL based on a selected reference clock signal. When the control circuit determines that the selected reference clock signal is unreliable, the control circuit disables the PLL's feedback loop and turns on the holdover switch. After the selected reference clock signal is changed or otherwise becomes reliable, the control circuit enables the PLL's feedback loop while keeping the holdover switch turned on, and controls a resistance of the variable resistor over time to provide a soft transition from holdover to reacquiring phase lock.

Abstract: Provided herein are apparatus and methods for fast charge pump holdover on signal interruption. In certain configurations, a clock generator system includes a phase-locked loop (PLL), a fast detect circuit, and a switch electrically coupled to an input of the PLL's loop filter. The fast detect circuit relatively quickly detects when an input signal to the PLL is lost. The fast detect circuit can quickly detect the loss of phase lock and can place the PLL into a holdover such that the frequency of a clock signal generated by the PLL remains within an acceptable range.