What is the format of the tuning word?
AN31 “Inductor Design for the Si41xx Synthesizer Family” describes how to read an 18 bit value from the serial interface which contains the tuning code. However, the tuning word (coarse varactor) range is only 0 to 0x7FF (11 bits). The nominal value will vary from board to board depending on the manufacturing tolerance of both the RF Synthesizer and the external tuning inductor.
The coding is as follows:
Maximum capacitance is all bits on. The MSB represents slightly less than half the total capacitance (perhaps 5% less). The next bit represents less than half the remaining capacitance, and so on, excepting the least two bits which are 2:1.