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LMX2594: Lock time is 2 to 3 times the simulated lock time

Prodigy 120 points

Replies: 11

Views: 154

Part Number: LMX2594

When tuning using the LMX2594 (uses the same schematic design as the LMX2594 eval board) I am getting lock times that vary from 120us to > 200us despite the PLLatinum simulation predicting lock times in the ~70us range.  The Fosc is 100 MHz and the loop bandwidth is 387kHz.  Partial assist is also being used so I would have expected faster than the simulated lock time.  I'm trying to achieve a lock time of < 100us.  Any advice is appreciated.

  • Hi Josh,

    I believe the uncertainty comes from VCO calibration. You may need to use full assist to bypass VCO calibration and therefore have a more manageable lock time.

  • In reply to Noel Fung:

    I am now using full assist and still getting a max of 180us lock time when tuning from 15000 MHz to 9500 MHz.  Are there any settings I should check that could be causing a much larger lock time than expected?  The simulation software shows a lock time with assist of 56 us, which is 1/3 of what I'm actually seeing on the board.

    To use full assist I am setting bit D14 and D11 of register 08 to 1 and bit D10 or register 20 to 1.  Is there anything else I have to do?

    Thanks

  • In reply to Josh Merti:

    Hi Josh,

    What is the settling frequency tolerance you used for lock time measurement? For example, +/-1kHz or +/- 100Hz.

    I will setup a bench test next week to verify.

  • In reply to Noel Fung:

    I'm looking at a +/-100 kHz window.

  • In reply to Josh Merti:

    Hi Josh,

    I measured 13.5µs lock time jumping from 15000MHz to 9500MHz.

    OSCin = fpd = 100MHz, default EVM loop filter, CHDIV = 4.

    My programming sequence.

    1. R16 (VCO_DACISET)

    2. R19 (VCO_CAPCTRL)

    3. R20 (VCO_SEL)

    4. R36 (PLL_N)

    At 15000MHz: VCO_SEL = 7; VCO_CAPCTRL = 32, VCO_DACISET = 297.

    At 9500MHz: VCO_SEL = 2; VCO_CAPCTRL = 47, VCO_DACISET = 292.

  • In reply to Noel Fung:

    Thanks for measuring that, it gives me hope that I can get the lock time I need.  Was fractional mode enabled?  I know the two frequencies that I'm tuning between are integer-N cases but that was just a test case.  My synthesizer is fractional with a fractional denominator of 20.  Would that affect the tune time measurement?

    My programming sequence is R19, R16, R20, R36, R37 (PFD_DLY_SEL), R43 (F), R14 (CP), R0 (FCAL_EN).  CHDIV = 1.

    I'll take a look at my setup and make sure there isn't something else masking the tune time.

    Thanks for your help.

  • In reply to Josh Merti:

    Hi Josh,

    Fractional or integer mode does not matter, the difference is you need to write more registers in fractional mode (fractional NUM). Forgot to tell you, I was using SPI clock of 25MHz, it takes 1µs to write a register. 

    Do you really need to change the CP from time to time? If not, then no need to write this register.

    R0 is not needed as we are not doing auto-calibration anymore.

  • In reply to Noel Fung:

    Hello,

    I got pulled onto something else for a few weeks but I'm back on this.  I was able to get ~ 60us lock time using forced settings.  My state machine takes ~ 14us to run, so I'm at about 46us for the device to lock.  That lock time would be fine but I am also trying to use SYNC and it seems SYNC requires auto calibration and that will again take hundreds of microseconds.  Can I use the SYNC function with the forced settings?

    I also measured the lock time of the 15000 to 9000 MHz jump on the eval board for the LMX2594.  The result was ~ 187us and the strange thing was that it didn't matter if I used partial assist, lock time was unaffected.  To use partial assist in TICS Pro I set the frequency to the value I wanted, then I hit the "Estimate Values" button which wrote to the 3 starting value registers, then I toggled FCALC_EN.  Am I doing that correctly?

  • In reply to Josh Merti:

    Josh,

    Yes, this is how you do partial assist.  TICSPro calculates using the formulas given for this in the datasheet.

    Regards,

    Dean

  • In reply to Dean Banerjee:

    Hi Josh,

    SYNC will not touch the VCO calibration, so yes, SYNC will works in full assist mode.

    187µs is full assist or partial assist? If it is partial assist, it is not surprise to me because partial assist will do calibration.