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Original question:

LMX2595: frequency synthesizer

LMX2595: Fast calibaration

Pradeep Kumar68
Intellectual 420 points
Part Number: LMX2595

Hi ti,

We are using LMX2595 to generate 500-18000 MHz output in steps of 1MHz. We are using 6mA charge pump current with 50KHz loop bandwidth. This is choosen to reduce near by spurious.

With this we are getting less than 200us lock time for all frequency band expect for 11900-12100MHz frequency band.

So for 11900-12100MHz we are thinking of doing fast calibration.

But in page 26 of LMX2595 datasheet there is note stating

"In the range of 11900 MHz to 12100 MHz, VCO assistance cannot be used, and the settings must be: VCO_SEL = 4, VCO_DACISET_STRT = 300, and VCO_CAPCTRL_STRT = 1. Outside this range, in the partial assist for the VCO calibration, the VCO calibration runs. This means that if the settings are incorrect, the VCO still locks with the correct settings. The only consequence is that the calibration time might be a little longer. The closer the calibration settings are to the true final settings, the faster the VCO calibration will be."

What does it mean, we canot do full assist calibration for this band?

  • 0
    TI__Guru** 107190 points

    Hi Pradeep,

    That statement is for partial assist and does not apply to full assist.

  • 0 in reply to Noel Fung
    Intellectual 420 points

    HI Ti,

    Thanks for the reply.

    We have implemented the fast calibration and we are are getting fast lock for most of the cases. 

    We are using this following registers changes for 11.9GHz to 12.1GHz. We are not writing R0 here. R20, R19 and R16 are derived by reading R110,R111 and R112 from PLL.

    VCO_DACISET_FORCE=1  0x086800 R8 0x086800
    VCO_CAPCTRL_FORCE=1
    VCO_SEL_FORCE=1,VCO core LUT R20 0x14EC48
    Cap code LUT R19 0x1327B7
    PLL_N TBC R36 0x24003C
    PLL_NUM[15:0] TBC R43 0x2B1D4C
    PFD_DLY_SEL 0x250404 R37 0x250404
    VCO2X_EN 0x1B0002 R27 0x1B0002
    CHDIV 0x4B0800 R75 0x4B0800
    OUTB_MUX,Channel divider,VCO,SysRef 0x2E07FD R46 0x2E07FD
    OUTA_MUX,Channel divider,VCO,VCO2X 0x2DC8DF R45 0x2DC8DF
    VCO amplitude  LUT R16 0x1000EA

    For other bands between 500-180000MHz we are using below registers along with R0 at the last

    R75 0x4B0800
    R46 0x2E07FD
    R45 0x2DC8DF
    R43 0x2B1D4C
    R37 0x250404
    R36 0x24003C
    R27 0x1B0002

    Fast calibration is working fine for particular frequency changes as mentioned below, but from some frequency to 11900 to 12100 MHz frequency band, some time we are getting fast lock and sometime lock time is more. Refer the table below for different lock time for frequency changes.

    Start freq(MHz) End Freq(MHz) Lock time
    7600 12050 or 11950 less than 50us
    8800 12050 or 11950 441us
    10000 12050 or 11950 240us
    12500 12050 or 11950 less than 50us
    12950 12050 or 11950 less than 50us
    14000 12050 or 11950 less than 50us
    11000 12050 or 11950 less than 50us
    135000 12050 or 11950 less than 50us

    Is there any register we are not writing properly here?

    What is the cause for more lock time for some frequency changes?

    0x1B0002
  • 0 in reply to Pradeep Kumar68
    TI__Guru** 107190 points

    Hi Pradeep,

    I got similar result. The end freq in below table is 12100 MHz.

    Start freq(MHz)                     Lock time (within +/-1 kHz)             Lock time (within +/-5 kHz)

    7700                                      38µs                                                30µs

    8900                                      300µs                                              125µs

    10100                                    200µs                                              50µs

    12500                                    34µs                                                26µs

    12900                                    34µs                                                27µs

    14100                                    39µs                                                33µs

    11100                                     104µs                                              33µs

    13500                                     38µs                                                31µs

    Looks to me that the long lock time is due to loop filter characteristic, I think the lock time could be improved if we use film capacitors in the loop filter. 

  • 0 in reply to Noel Fung
    Intellectual 420 points

    HI Ti,

    Thanks for the reply.

    I am using 50KHz loop filter as shown in the below figure

    and I am using below components for my loop filters,

    CAP CER 6200PF 50V C0G/NP0 0603     KEMET         C0603C622J5GAC7867

    CAP CER 0.82UF 10V X5R 0603            KEMET         C0603C824K8PACTU

    CAP CER 0.022UF 50V X7R 0603          KEMET      C0603C223K5RACTU

    RES SMD 13.3 OHM 1% 1/16W 0402         Yageo          RC0402FR-0713R3L

    RES SMD 3.09 OHM 1% 1/16W 0402         Vishay Dale CRCW04023R09FKED

    Is it only because of the loop filter values or is there any thing to with Registers or chip?

    If we use film capacitor will this issue will get resolved for sure? if so we will change the values immediately.

  • 0 in reply to Pradeep Kumar68
    TI__Guru** 107190 points

    Hi Pradeep,

    825nF is big, charging/discharging time will be long. Suggest use a smaller charge pump current in order to reduce the loop filter capacitors value.