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Original question:

LMX2820: LMX2820 instant calibration

LMX2820: EVM

Gurusatwik Bhatta N
Prodigy 20 points
Part Number: LMX2820

Tool/software:

To perform Instant calibration in LMX2820EVM is it necessary to check VCO_CAPCTRL_FORCE, VCO_SEL_FORCE and VCO_DACISET_FORCE?

I performed the following steps
Initialization register setting
1. Set DBLBUF_PLL_EN, DBLBUF_CHDIV_EN, DBLBUF_OUTBUF_EN, DBLBUF_OUTMUX_EN = 1
2. Set DBLR_CAL_EN = 0; INSTCAL_SKIP_ACAL = 0
3. If VCO doubler is required, set INSTCAL_DBLR_EN = 1, otherwise set this bit to 0
4. Set INSTCAL_DLY = T x fosc (in MHz) / 2^CAL_CLK_DIV, where T = 2.5 x CBIASVCO / 0.47µF. CBIASVCO is the bypass capacitor at pin 3
5. Configure other registers to lock to 5.65GHz without any calibration assist
6. Set INSTCAL_PLL_NUM = 2^32 x (PLL_NUM / PLL_DEN)
Programming
7. Vcc power up LMX2820
8. Program all the registers, LMX2820 should lock to 5.65GHz
InstCal calibration
9. Program INSTCAL_EN = 1
9.5 run Index routine
10. Program R0 (with FCAL = 1), calibration will begin
11. Wait 100ms
12. Program R0 (with FCAL = 0) to complete the calibration
13. If LD pin did not turn HIGH, program RESET = 1 to reset LMX2820. After LMX2820 is reset, repeat Step 8 to 12
Changing VCO frequency
14. Program INSTCAL_PLL_NUM, PLL_N, PLL_NUM and PLL_DEN (if their value have change)
15. Program R0 (with FCAL = 0) to change VCO frequency

Yet my lock time is still 43.26us. Isn't it supposed to be 2.5us as mentioned in the datasheet?

  • 0
    TI__Guru** 107870 points

    Hi There,

    Total lock time = programming time + InstCal processing time + analog PLL lock time.

    We can ignore programming time because double buffering is enabled in InstCal, frequency will not change until we program R0. 

    After R0 is programmed, the device will pick up the correct VCO_CAPCTRL, VCO_SEL, VCO_DACISET and then wait until INSTCAL_DLY is elapsed. This is the 2.5µs we advertised in the datasheet.

    At this moment, the frequency is closed to the programmed frequency. The device will let the PLL to eventually lock to the target frequency. This is referring to the analog PLL lock time and is dependent on the loop filter bandwidth.

    Please check below post for an example lock time, you should probably get 7µs lock time.

    https://e2e.ti.com/support/clock-timing-group/clock-and-timing/f/clock-timing-forum/1507807/lmx2820-lmx2820-lock-time

    VCO_CAPCTRL_FORCE, VCO_SEL_FORCE and VCO_DACISET_FORCE are not used in InstCal.