SNAA449 June   2025 LMX2820

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Various Options in Instant Calibration
    1. 2.1 When Delay for Instant Calibration Needs to be Increased From 2.5μs to 5μs
    2. 2.2 Generating the Lookup Table for Instant Calibration With the VCO Doubler
    3. 2.3 Reading Back the Instant Calibration Lookup Table Through SPI
    4. 2.4 Writing the Lookup Table Through SPI
    5. 2.5 Dealing With Case When Phase Detector Frequency is Changed Without Re-generating the Lookup Table
  6. 3Summary
  7. 4References

1 Introduction

For changing from one frequency to another in LMX2820, there are multiple ways to do that namely No assist, Full assist, Partial assist, and Instant calibration. There are some limitations with each approach. No assist takes more settling time. Full assist requires user to know before what to write for VCO and doubler settings. Partial assist helps in faster settling but not as fast as full assist. To get better of all the three approaches, there is a new approach in LMX2820 is called Instant Calibration. This document includes the instant calibration feature.

The instant calibration feature of the LMX2820 is very popular and allows the user to switch frequencies at blazing fast speeds. This feature works by storing VCO calibration values to a lookup table. There are use cases and special restrictions that are not stated in the data sheet that are discussed in this application note.

  1. When delay for instant calibration needs to be increased from 2.5μs to 5μs.
  2. Special handling when generating the lookup table when the VCO doubler is used
  3. Read back the look up table through SP interface
  4. Write the look up table through SP interface
  5. Dealing with the case where the phase detector frequency is changed without forcing the need for the lookup table to be re-generated.