• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 21 subscribers
  • Views 164 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMX2820: Summary of known issues

Christer Andersson
Intellectual 280 points
Part Number: LMX2820

Tool/software:

Hi,

I need to implement a reliable and fast-hopping wideband synthesizer using LMX2820. I just want to confirm that the following two posts cover all known LMX2820 issues, particularly in relation to instant calibration.

This patch is reported to be necessary to support particular transitions from one VCO bank to another:

https://e2e.ti.com/support/clock-timing-group/clock-and-timing/f/clock-timing-forum/1419808/lmx2820-software-patch-that-must-be-inserted-when-using-instant-cal?tisearch=e2e-sitesearch&keymatch=LMX2820%20software%20patch#

Related to this someone has reported that instant calibration can fail when the doubler is enabled:

https://e2e.ti.com/support/clock-timing-group/clock-and-timing/f/clock-timing-forum/1434291/lmx2820-calibration-failures-and-retry-requirements?tisearch=e2e-sitesearch&keymatch=LMX2820%2520software%2520patch#

My basic questions are:

  1. Do the above posts cover and solve all known instant calibration issues?
  2. The second post concers the doubler.
    1. As I understand it, the calibration might fail and need a reattempt, but once calibration is successful there should be no issue to quickly keep changing the frequency (until device reset).
    2. Is the instant calibration only valid for the doubler state during calibration? Is it possible to use instant calibration (enable/disable the doubler) to hop across the ~11 GHz boundary?

Thanks,

Christer

  • 0
    TI__Guru* 98220 points

    Hi Christer,

    Yes, the above links cover all the known issue with Instant Cal. 

    Instant Cal requires a one-time calibration. As long as the calibration (with or without doubler calibration) is succeed, the calibration is completed and no more calibration will be executed when we use Instant Cal to change frequency. 

    If your application requires VCO doubler frequencies, you should enable doubler calibration (INSTCAL_DBLR_EN = 1) in the one-time calibration so that this calibration will capture doubler database as well. 

  • 0 in reply to Noel Fung
    Intellectual 280 points

    Okay, so that means I should also touch R78 (0x4E) to enable/disable the doubler when jumping across ~11GHz, but how is this done in practice?

    The datasheet says the following registers are double buffered: PLL_N, PLL_NUM, PLL_DEN, MULT, PLL_R, PLL_R_PRE, MASH_ORDER, and PFD_DLY.

    So I guess the R78 (0x4E) register is not double buffered? So at what point do I toggle the OUTA_MUX VCO doubler flag in that register? It needs to be done before writing to R0. Does a change to the doubler state only take affect after writing to R0? I want to avoid any output glitches when touching the doubler state and I'd like to be able to output the current frequency until R0 is written to.

    Is the below fine or should the R78 write be placed later in the sequence?

    • R78 (0x4E) - enable/disable the doubler depending on destination frequency
    • PLL_NUM (0x2A, 0x2B)
    • INSTCAL_PLL_NUM (0x2C, 0x2D)
    • PLL_DEN (0x26, 0x27)
    • PLL_N (0x24)
    • R0 (0x0)

    Thanks!

  • +1 in reply to Christer Andersson
    TI__Guru* 98220 points

    Hi Christer,

    OUT_MUX is double buffered by  DBLBUF_OUTMUX_EN = 1. 

    InstaCal requires double buffer for all the necessary register change. The change will only effective after R0 is programmed.

    Your sequence is correct, everything should be put before R0.