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

2.5 Dealing With Case When Phase Detector Frequency is Changed Without Re-generating the Lookup Table

There can be use cases where PFD needs to be changed. In that case, re-creating LUT again can be avoided with following changes:

Example: Above example is with PFD as 200MHz but let’s assume that PFD is changed to 100MHz. Following sequence shows the steps followed to change PFD and how to take care of PFD change to avoid generating LUT again.

  1. LUT_GEN_SEL=1(R4[15])
  2. PLL_R_PRE(R14[11:0]) as 1
  3. OSC_2X(R11[4]) as 0
  4. FCAL_HPFD_ADJ(R0<10:9]) as 0
  5. R36<14:0], PLL_N as 56
  6. OUTA_MUX(R78[1:0]) as 1
  7. R42(MSB),R43(LSB) as 500(NUM)
  8. R38(MSB),R39(LSB) as 1000(DEN)
  9. R44(MSB),R45(LSB) as 2^31 (2^32*(NUM/DEN) INSTCAL_PLL_NUM)
  10. Write R0 with FCAL_EN as 1
  11. INSTCAL_EN=1(R1[0])
  12. INSTCAL_EN=0(R1[0])
  13. INSTCAL_EN=1(R1[0])
  14. R0[4][FCAL_EN] as 1
  15. R0[4][FCAL_EN] as 0
  16. User needs to see Frequency as 5.65GHz at the output.

INSTANT_CAL toggling procedure is needed whenever PFD changes. If PFD is not changed, after doing sufficient frequency change related settings, just toggling FCAL is enough.