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LMX2572: frequency offset

Amy Luo
Expert 6895 points
Part Number: LMX2572

Hi team,

The LMX2572 is used. When the configuration is 1.5GHZ, the frequency offset is very small, only a few 10KHZ, but when the configuration is 1.6GHZ, the measured frequency is 1.594GHZ, with a deviation of more than 6MHZ,

The reference clock used is 38.4MHZ. Please help analyze the reason for such a large frequency offset? The configuration is as follows

Best Regards,

Amy Luo

  • 0
    TI__Guru** 107550 points

    Hi Amy,

    If the programming is done with TICS Pro, after TICS Pro configuration, we should press Ctrl and L keys to load all the register.

    If the programming is done with customer's software, make sure to program all the registers in descending order and the last register to program is R0. 

    From datasheet:

    7.5.1 Recommended Initial Power-On Programming
    7.5.1.1 Programming Sequence
    When the device is first powered up, it must be initialized, and the ordering of this programming is important. The
    sequence is listed below. After this sequence is completed, the device should be running and locked to the
    proper frequency.
    1. Apply power to the device and ensure all the supply pins are at the proper levels.
    2. If CE is low, pull it high.
    3. Wait 500 μs for the internal LDOs to become stable.
    4. Ensure that a valid reference clock is applied to the OSCin pins.
    5. Program register R0 with RESET = 1. This will ensure all the registers are reset to their default values. This
    bit is self-clearing.
    6. Program in sequence registers R125, R124, R123, ..., R1 and then R0.

    7.5.2 Recommended Sequence for Changing Frequencies
    The recommended sequence for changing frequencies in different scenarios is as follows:
    1. If the N divider is changing, program the relevant registers and then program R0 with FCAL_EN = 1.
    2. In FSK and Ramp mode, the fractional numerator is changing; program the relevant registers only.