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LMX2581E: Lock frequency accuracy

Kazuki Itoh
Genius 9795 points
Part Number: LMX2581E
Other Parts Discussed in Thread: LMX2581

Hello team,

I'd like to know LMX2581 lock frequency accuracy.

In my configuration, VC-TCXO 30.72MHz is used.

The phase noise doesn't look good below 10Hz. So I'm looking for proper setting to address this issue.

Also, the lock frequency shift by 40Hz is observed with the spectrum analyzer.

So, could you please help us to find proper LMX2581 setting to resolve the shift and phase noise issue?

Regards,

Itoh

  • 0
    TI__Guru* 90880 points

    Hi Itoh-san,

    if the PLL is unlock, it will not just shifted by 40Hz. This shift must be came from the reference clock. For example, if the output frequency of LMX2581 is 3072MHz and if the reference clock is shifted by 1Hz (30.72MHz + 1Hz), then the output of LMX will shift by 100Hz [(3072 / 30.72) * 1Hz].

    The phase noise at 10Hz offset is dominated by the reference clock, nothing we can do on the LMX to improve the phase noise.

    VCTCXO output is clipped sine wave, the slew rate is not pretty good. If the customer can use CMOS output VCTCXO, the overall LMX output phase noise should be better.