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LMX2592 switching time

Hello,

Figure 15 of the LMX2592 datasheet suggests a very fast frequency change time of 20 usec.

Measuring the calibration time at the MUXout pin (with calibration status detect) gives a 3msec time @ 125MHz state machine clock.

Minimum time is 1.8 ms @ 200MHz state machine clock (Fast_Cal enabled).

So, measuring the complete frequency change time from the end of the transfer of a new frequency (rising edge of CSB) to the lock detect is around 2 msec (@maximum state machine clock, 200MHz).

That is far away from 20 usec of Figure 15 - Datasheet LMX2592.

Could you please give any advice what can be optimized here.

Best regads

2 Replies

  • Elmar,

    Sorry for the long wait, I just got notified on this one.

    For the LMX2592 the dominant calibration time is really the amplitude calibration portion. Try below first to see a first step difference and get a feel:

    • ACAL_EN (R0[4]) = 0

      • this turns off amplitude calibration

    • VCO_IDAC_OVR (R8[13]) = 1

      • This enables manually selecting the VCO amplitude

    • VCO_IDAC (R19[11:3]) = 350

      • This is the VCO amplitude value, 350 is around where you get a good full swing for good phase noise.

        It’s good to go through this step first to show how fast calibration is if the device doesn’t need to spend time on figuring out the amplitude. After we verify he can see the calibration times on the order of 20us or so, then we can explore the settings that are meant to shorten amplitude calibration as much as possible.

  • In reply to Brian Wang:

    Hallo Brain,

    thanks you very much for your helpful response.

    The fantastic result is, that not using the amplitude calibration speeds up the switching time tremendeously into the range of a few 10 usecs.

    Could you please explain a little more detailed, what the internal amplitude calibration is good for?

    Especially: is there a price to be paid for not using the automatic amplitude calibration - for instance: a higher risk of getting an unlock status?

    Best regrads

    Elmar