Q: How to improve adjacent channel selectivity on AX5043?
A: Try activating the VCOI calibration (top left in the RadioLAB Pinconfig panel).
The calibration requires a connection from the FILT pin to pin 25 via 12k Ohm and a 10 nF capacitor from pin 25 to GND. On our DVK module v1.7 these components are NC, but there are pads on the PCB. R1=12 kOhm, C2= 1 nF.
If you want to make a “manual” experiment which does not require the above connection, you could measure the spectrum in TX mode and vary the PLLVCOI register until you find the best phase noise performance. [If you lack a spectrum analyzer you can also enable bit 3 of the PLLLOOP register and measure the voltage at pin 8. The best PLLVCOI setting is the one with where the voltage is minimal.]
In addition to the PLLVCOI setting you can also try the following: Change “Channel spacing” from 50k to 25k. (This causes PLLCPI in RX mode to change from 0x20 to 0x2 and PLLLOP in TX mode to change from 0x0b to 0x09]. This should improve the selectivity close in, but at the price of worsening it for df > 100 kHz.
For more details on the VCOI Current Calibration routine check out the AND9858 application note.