8.4.2.1 DCXO Mode

In applications that require the LMK61E08 as part of a PLL that is implemented in another device like an FPGA, it can be used as a digitally-controlled oscillator (DCXO) where the frequency control word can be passed along through I²C to the LMK61E08 on a regular basis, which in turn updates the numerator of its fractional feedback divider by the required amount. In such a scenario, the entire portion of numerator for the fractional feedback divider must be written on every attempt MSB first and LSB last to ensure that the output frequency does not jump during the update, as described in Feedback Divider (N). In every update cycle, a total of 46 bits needs to be updated leading to a maximum update rate of 8.7 kHz with a maximum I²C rate of 1 Mbps. The minimum step size of 0.55 ppb (parts per billion) is achieved for the maximum VCO frequency of 5.6 GHz and when reference input doubler is disabled and reference divider is set to 4. The minimum step size of 4.96 ppb (parts per billion) is achieved for the maximum VCO frequency of 4.8 GHz and when reference input doubler is enabled and reference divider is bypassed.