10.4.11 VCO Calibration
The PLL of the LMK03318 includes an LC VCO that is designed using high-Q monolithic inductor to oscillate between 4.8 GHz and 5.4 GHz and has low phase-noise characteristics. The VCO must be calibrated to ensure that the clock outputs deliver optimal phase noise performance. Fundamentally, a VCO calibration establishes an optimal operating point within the tuning range of the VCO. While transparent to the user, the LMK03318 and the host system perform the following steps comprising a VCO calibration sequence:
- Normal Operation - When the LMK03318 is in normal (operational) mode, the state of the power-down pin (PDN) is high.
- Entering the reset state - If the user wishes to initialize the selected pin mode default settings (from ROM, EEPROM, or register default) and initiate a VCO calibration sequence, then the host system must place the device in reset through the PDN pin, or through software reset (R12.7) through I2C, or by removing and restoring device power. Pulling the PDN pin low low or setting R12.7 = 0 places the device in the reset state.
- Exiting the reset state – The device calibrates the VCO either by exiting the device reset state or through the device reset command initiated through the host interface. Exiting the reset state occurs automatically after power is applied and/or the system restores the state of the PDN or R12.7 from the low to high state. Exiting the reset state using the PDN pin causes the selected pin mode defaults to be loaded/reloaded into the device register bank. Invoking software reset through R12.7 does not reinitialize the registers; rather, the device retains settings related to the current clock frequency plan. Using this method allows for a VCO calibration for a frequency plan other than the default state (that is,. the device calibrates the VCO based on the settings current register settings). The nominal state of this bit is high. Writing this bit to a low state and then returning it to the high state invokes a device reset without restoring the pin mode.
- Device stabilization – After exiting the reset state as described in Step 3, the device monitors internal voltages and starts a reset timer. Only after internal voltages are at the correct level and the reset time has expired will the device initiate a VCO calibration. This ensures that the device power supplies and reference inputs have stabilized prior to calibrating the VCO.
- VCO Calibration - The LMK03318 calibrates the VCO. During the calibration routine, the device mutes output channels configured with their respective auto-mute control enabled, so that they generate no spurious clock signals. After a successful calibration routine, the PLL will lock the VCO to the selected reference input.