Each of the LMK03328 PLLs include a VCO that is designed to use high-Q
monolithic inductors to oscillate between 4.8 GHz and 5.4 GHz and have low phase
noise characteristics. Each VCO must be calibrated to verify 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 LMK03328 and the host system can
perform the following steps in a VCO calibration sequence:
- Normal Operation - When the LMK03328 is in normal
(operational) mode, the state of the power-down pin (PDN) is high.
- Entering the reset state - If the user wants 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, with a software reset (R12.7) through the I2C, or by removing and restoring device power. Pulling the PDN pin 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 or reloaded into the device register bank. Invoking software reset
through R12.7 does not re-initialize the registers, but allows the device to retain the
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 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 does the device initiate a VCO calibration. This
verifies that the device power supplies and reference inputs have stabilized
prior to calibrating the VCO.
- VCO Calibration - The LMK03328 calibrates the VCO. During the
calibration routine, the device mutes output channels configured with the
respective auto-mute control enabled so the channels do not generate spurious
clock signals. After a successful calibration routine, the PLL locks the VCO to
the selected reference input.