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Original question:

LMX2820: MUX_OUT_DOUBLER Issue in Fast VCO Mode

LMX2820: Using Full Assist VCO Cal in the Doubler Path

0dBc
Prodigy 190 points
Part Number: LMX2820

A similar question has already been asked before: https://e2e.ti.com/support/clock-timing-group/clock-and-timing/f/clock-timing-forum/1028354/lmx2820-mux_out_doubler-issue-in-fast-vco-mode

No useful answer was given. So I am going to ask one more time:

Using Full assist VCO calibration, we are able to switch between frequencies in a few tens of microseconds for fundamental frequencies and below (f ≤ 11300 MHz). However, it seems that we cannot use the full assist mode in the doubler path i.e., f > 11300 MHz. The tracking filter needs to be calibrated as well, but not much information is given about the tracking filter in the datasheet. It is mentioned that "The calibration for this tunable filter is automatically triggered whenever the VCO calibration is done."
Is it possible to use the full assist VCO cal in the doubler path? If yes how?

Thanks.

  • 0
    TI__Guru** 108130 points

    Hi There,

    Sorry for the confusion, let me clarify.

    If your frequencies are all beyond 11300MHz, you need to use VCO doubler. If you make DBLR_CAL_EN = 1 and execute a VCO calibration, both VCO and the tracking filter will be calibrated. 

    So during instant calibration, you should enable DBLR_CAL_EN, this will make the calibration time a bit longer though. After instant calibration, when you switch the frequency, you don't need VCO and tracking filter calibration anymore, the corresponding VCO parameters as well as tracking filter characteristic will be loaded automatically.

  • 0 in reply to Noel Fung
    Prodigy 190 points

    Hi,

    I didn't say I was confused. There is no confusion. 
    When DBLR_CAL_EN bit is enabled, instant calibration is no longer active. You say that "this will make the calibration time a bit longer". How much longer? a bit longer doesn't have any engineering significance.

  • 0 in reply to 0dBc
    TI__Guru** 108130 points

    Hi there,

    The instant calibration is a one time process, so the time taken for this process should not be matter. 
    After the calibration is complete, when you switch the frequency, there is no VCO and tracking filter calibration time anymore. 

  • 0 in reply to Noel Fung
    Prodigy 190 points

    The time taken for initial one-time calibration doesn't matter. That is right, but I am not talking about this.
    After the calibration is complete, switching between frequencies does require some calibration time due to the tracking filter calibration. In fact what we are measuring right now is substantially larger than the advertised datasheet spec of 5us. So that leaves us two choices: either our measurements are somehow wrong or what you say is not correct.
    Are you %100 sure that "After the calibration is complete, when you switch the frequency, there is no VCO and tracking filter calibration time anymore." ?

  • 0 in reply to 0dBc
    TI__Guru** 108130 points

    Hi There,

    After instant calibration, when you change the frequency:

    1. Write the values for INSTCAL_PLL_NUM, PLL_N, PLL_NUM and PLL_DEN.

    2. Write R0 to trigger calibration (with DBLR_CAL_EN = 0, FCAL_EN = 0).

    In step 2 above, both calibration enable bits are set to zero, there should be no additional time for calibration.