LMX2820: LMX2820

Full assist still directly writes the values of the VCO_SEL, CAPCODE, and DACISET, in addition to other parameters like the N-dividers. Instant calibration is something else - in this case we build a lookup table of several of these coefficients at startup, then recall coefficients by programming an instant calibration numerator and writing R0. The lookup table of VCO_SEL, CAPCODE, and DACISET results means these values no longer have to be written, and no external per-device lookup table needs to be generated - the LMX2820 can generate the per-part lookup table entirely on its own. Since the instant calibration frequency jump is triggered rather than spread out across a sequence of register writes (frequency-affecting field writes can be double-buffered), in many cases this allows much faster frequency switching (no extra steps to tri-state charge pump, tinker with PFD_DLY_SEL, change output dividers or input path scalars, etc). Additionally, the instant calibration triggered frequency jump also means there's no interval where the VCO tuning voltage significantly deviates from the nominal lock value - contrast with full assist where the period during frequency jumps necessarily unlocks the PLL and can cause the charge pump voltage to get stuck high or low for some time between frequency jumps, increasing settling time for the lock (unless extra steps are taken to tri-state charge pump in between operations).

I recommend reading through these application notes (if you haven't already) in order, keeping the first one in mind as you read through the second one:

• Streamline RF Synthesizer VCO Calibration and Optimize PLL Lock Time - https://www.ti.com/lit/an/snaa336a/snaa336a.pdf
• Dramatically Improve Your Lock Time with VCO Instant Calibration - https://www.ti.com/lit/an/snaa342/snaa342.pdf