8.3.3 VCO Calibration
The LC VCO is designed using high-Q monolithic inductors and has low phase noise characteristics. The VCO of the CDCM6208 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 CDCM6208 and the host system perform the following steps comprising a VCO calibration sequence:
- Normal Operation – When the CDCM6208 is in normal (operational) mode, the state of both the power down pin (PDN) and reset pin (RESETN) is high.
- Entering the reset state – If the user wishes to restore all device defaults and initiate a VCO calibration sequence, then the host system must place the device in reset via the PDN pin, through the RESETN pin, or by removing and restoring device power. Pulling either of these pins low places the device in the reset state. Holding either pin low holds the device in reset.
- Exiting the reset state – The device calibrates the VCO either by exiting the device reset state or through the device reset command initiated via the host interface. Exiting the reset state occurs automatically after power is applied and/or the system restores the state of the PDN or RESETN pins from the low to high state. Exiting the reset state using this method causes the device defaults to be loaded/reloaded into the device register bank. Invoking a device reset via the register bit does not restore device defaults; 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 contained within the register bank at the time that the register bit is accessed). The nominal state of this bit is low. Writing this bit to a high state and then returning it to the low state invokes a device reset without restoring device defaults.
- 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 phase locked loops have stabilized prior to calibrating the VCO.
- VCO Calibration – The CDCM6208 calibrates the VCO. During the calibration routine, the device holds all outputs in reset so that the CDCM6208 generates no spurious clock signals.