The LMX2571-EP includes a fully integrated VCO. The VCO generates a frequency which varies with the tuning voltage from the loop filter. Output of the VCO is fed to a prescaler before going to the N-divider. The prescaler value is selectable between 2 and 4. In general, prescaler equals 2 will result in better phase noise especially when the PLL is operated in fractional-N mode. If the prescaler equals 4, however, the device will consume less current. The VCO frequency is related to the other frequencies and Prescaler as follows:

To reduce the VCO tuning gain, thus improving the VCO phase noise performance, the VCO frequency range is divided into several different frequency bands. This creates the need for frequency calibration to determine the correct frequency band given a desired output frequency. The VCO is also calibrated for amplitude to optimize phase noise. These calibration routines are activated any time that the R0 register is programmed with the FCAL_EN bit equals one. It is important that a valid OSCin signal must present before VCO calibration begins.

This device will support a full sweep of the valid temperature range of 125°C (–40°C to 85°C) without having to recalibrate the VCO. This is important for continuous operation of the synthesizer under the most extreme temperature variation.