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LMX2820: VCO Frequency Ranges

Turker Dolapci
Prodigy 30 points
Part Number: LMX2820

Tool/software:

Hi,

It looks like VCO frequency ranges are different than the ones on the datasheet. For example, in VCO 7 after 11150 MHz, LD fault probability is so high. It is better to use output doubler to have frequencies higher than this frequency. Is it common or does the LMX2820 I have has a problem?

Does the frequency ranges of the VCO's change sample by sample? I only have an eval board, don't have a chance to test multiple LMX2820's.

Do you have any updates or recommendations about the VCO frequency ranges?

Regards,

Turker

  • 0
    TI__Guru** 106770 points

    Hi Turker,

    I have never seen this happen and I also did not get similar report from other customers. 

    Were you using the EVM Default Mode configuration in TICS Pro to get started with?

    Make sure to click the Calibrate VCO button after changing the VCO frequency. 

  • 0 in reply to Noel Fung
    Prodigy 30 points

    Hello,

    Thanks for your quick reply.

    We are using an evaluation board with our own test script in the frequency interval between 5650 MHz to 11300 MHZ (no doubler or divider is used, mux output is VCO output). We have both tried instant calibration and no assist modes. We are always using double buffering. 

    Test setup includes the VCO bands in the datasheet. Frequency is hopped within the bands and between the bands in the randomly generated frequencies by obeying the frequence resolution (ref=100 MHz, OSC_X is active, fractional divider is 200, resolution is 1 MHz). Test script changes the frequency and controls the spectrum analyzer to measure the settling time. At the end we have a 3D bar plot: Initial Frequency vs Final Frequency vs Settling Time.

    When the final frequency is higher than approximately 1150 MHz, lock detect is rarely successful.

    Today, I borrowed another evaluation board from a colleague. This time, when we use no assist settings, LD is successful up to 11300 MHz. However, when we use Instant Calibration settings, frequencies higher than 11120 MHz is mostly problematic, LD fails.

    As you suggested, I tested with TICSPro to see what happens. I tested the inital EVM.

    Sequence:

    Turn on the power supply to energize EVM

    Turn on TICSPro and Click EVM Default Mode

    Change frequency to 11140 MHz and press enter

    Ctrl+L to write all registers

    LD failed, spectrum analyzer shows a signal at 11365 MHz, with sidelobes. Nearest sidelobe is 30 dB less than the peak, which is around 0 dBm.

    Register values exported from TICSPro is given below:


    R122 0x7A0000
    R121 0x790000
    R120 0x780000
    R119 0x770000
    R118 0x760000
    R117 0x750000
    R116 0x740000
    R115 0x730000
    R114 0x720000
    R113 0x710000
    R112 0x70FFFF
    R111 0x6F0000
    R110 0x6E001F
    R109 0x6D0000
    R108 0x6C0000
    R107 0x6B0000
    R106 0x6A0000
    R105 0x69000A
    R104 0x680014
    R103 0x670014
    R102 0x660028
    R101 0x6503E8
    R100 0x640533
    R99 0x6319B9
    R98 0x621C80
    R97 0x610000
    R96 0x6017F8
    R95 0x5F0000
    R94 0x5E0000
    R93 0x5D1000
    R92 0x5C0000
    R91 0x5B0000
    R90 0x5A0000
    R89 0x590000
    R88 0x5803FF
    R87 0x57FF00
    R86 0x560040
    R85 0x550000
    R84 0x540040
    R83 0x530F00
    R82 0x520000
    R81 0x510000
    R80 0x5001C0
    R79 0x4F011E
    R78 0x4E0001
    R77 0x4D0608
    R76 0x4C0000
    R75 0x4B0000
    R74 0x4A0000
    R73 0x490000
    R72 0x480000
    R71 0x470000
    R70 0x46000E
    R69 0x450011
    R68 0x440020
    R67 0x431000
    R66 0x42003F
    R65 0x410000
    R64 0x400080
    R63 0x3FC350
    R62 0x3E0000
    R61 0x3D03E8
    R60 0x3C01F4
    R59 0x3B1388
    R58 0x3A0000
    R57 0x390001
    R56 0x380001
    R55 0x370002
    R54 0x360000
    R53 0x350000
    R52 0x340000
    R51 0x33203F
    R50 0x320080
    R49 0x310000
    R48 0x304180
    R47 0x2F0300
    R46 0x2E0300
    R45 0x2D3333
    R44 0x2CB333
    R43 0x2B02BC
    R42 0x2A0000
    R41 0x290000
    R40 0x280000
    R39 0x2703E8
    R38 0x260000
    R37 0x250500
    R36 0x240037
    R35 0x233100
    R34 0x220010
    R33 0x210000
    R32 0x201001
    R31 0x1F0401
    R30 0x1EB18C
    R29 0x1D318C
    R28 0x1C0639
    R27 0x1B8001
    R26 0x1A0DB0
    R25 0x190624
    R24 0x180E34
    R23 0x171102
    R22 0x16E2BF
    R21 0x151C64
    R20 0x14272C
    R19 0x132120
    R18 0x120000
    R17 0x1115C0
    R16 0x10171C
    R15 0x0F2001
    R14 0x0E3001
    R13 0x0D0038
    R12 0x0C0408
    R11 0x0B0612
    R10 0x0A0000
    R9 0x090005
    R8 0x08C802
    R7 0x070000
    R6 0x060A43
    R5 0x050032
    R4 0x044204
    R3 0x030041
    R2 0x0281F4
    R1 0x0157A0
    R0 0x006470

    Regards

  • 0 in reply to Turker Dolapci
    TI__Guru** 106770 points

    Hi Turker,

    I don't have issue to lock the eval board with your configuration. I also don't have issue with InstCal locking the device to 11.3GHz. 

    What kind of clock source did you use? I am using a 100MHz, 0dBm (also tried -10dBm and 10dBm) signal from a signal generator. 

  • 0 in reply to Noel Fung
    Prodigy 30 points

    Hi Noel,

    100 MHz is fed by a VCXO with approximate slew rate of 1V/ns. Vpp is around 3V. I verfied again. The problem is not in the oscillator input. Then, I decided to change the LMX2820 in the eval board. Now, I have two eval boards. Both of them locks successfully up to 11300 MHz with evaluation software and in my software. But, it is not possible to use it in the full VCO range as I said in my previous posts, when we tried InstantCal. For now, I will use without assist, no InstantCal.

    If you have any idea about the Instant Cal problem at the end of VCO7 , I will wait to hear from you. In forum posts, I’ve come across posts indicating that using VCO doubler could lead to an increase in LD errors. Does the error in my setup and these posts related to each other?

    If InstantCal feature works in my setup, I might consider using it in other projects that require fast frequency tuning. Your assistance is very helpful up to now. 
    Thank you for your help so far and for your future assistance.

    Regards

  • 0 in reply to Turker Dolapci
    TI__Guru** 106770 points

    Hi Turker,

    Here is the procedure to complete an InstCal calibration without using the VCO doubler.

    TICS Pro setup:

    1. Select EVM Default Mode

    2. click this button to refresh the GUI

    3. Change VCO frequency to 5650MHz

    4. uncheck these bits

    5. Enable double buffer bits

    Programming:

    6. Vcc power up the EVM

    7. Provide a 100MHz signal to OSCin

    8. Program all the registers (Ctrl+L or USB Communications --> Write All Registers)

    9. The EVM should lock to 5650MHz and the LD LED should lit

    InstCal calibration:

    10. Check INSTCAL_EN

    11. Click Calibrate VCO button once (LD LED will turn OFF)

    12. Uncheck FCAL_EN (LD LED will lit, calibration is complete)

    To change frequency:

    13. Change VCO frequency

    14. Click Calibrate VCO button once (it should lock to the new frequency)