Q: Internal or External VCO inductor with AX5043.
A: Frequencies from above 400MHz can easily be generated by the totally internal VCO of the chip.
Frequencies below 400MHz require an external VCO inductor. With one given inductor value the tuning range is about 10% (+/- 5%) around the center frequency. The main reason to do this is when phase noise requirements of an application are not met with the internal VCO. To Enable fully external VCO, in addition to FLT[1:0], the correct bits in PLLVCODIV need to be set (p.46 AX5043 programmer’s manual, p. 18 AX5043 Datasheet). VCO control voltage is output on pin FILT.