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LMX2594: LMX2594 randomly outputs spurs after reprogram

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Replies: 3

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Part Number: LMX2594

Hello TI Experts

I'm struggling with the LMX2594 which at some frequencies sometimes randomly outputs a spur.I have two LMX2594 as LO's in a double superhet receiver. First I thought this was leakage and cross products. However, the problem appears also if I only run the LMX2594 alone (no other components running apart from external refclock).

The following examplifies the issue: I have the LMX2594 set to 7.097GHz with a reference clock of 143.36MHz. The registers I use for programming are attached.

I dump the registers to the device and sometimes the result is with spur at 400kHz offset:

Then after re-programming with the same registerfile the result is without spurs:

Attached also phase noise measurement results: phasenoise_lmx2594_nok.pdfphasenoise_lmx2594_ok.pdf

In both cases the device reports a "Locked" state.

Any help with this issue is very much appreciated!

Registerfile: (decimal)

registers_LMX2594.txt

  • Jonas Reber1
    Registerfile: (decimal)

    for your convenience I have converted it to HEX for import in TICS pro. And yes: I do set R0 again after 10ms with FCAL set to true. registers_LMX2594_hex.txt

  • In reply to Jonas Reber1:

    Jonas,

    I was able to download your files to TICSPro and have the following comments:.

    1.I note that 204.8 MHz/512 = 400 kHz, so this looks like a fractional spur.

    What may be happening is that the initial modulator state is being changed.  Have you tried toggling the MASH_RESET_N bit (R44[5]) to see if this makes the spur go away?

    2.  I see that you have ACAL_CMP_DLY set to 10 and TICSPro is warning that this might be too small (recommends >=14)-- possibly leading to less consistent VCO calibrationFor diagnostic purposes, you might want to read back rb_VCO_SEL, rb_VCO_DACISET, and rb_VCO_CAPCTRL and compare the good and bad spur case to see if there is an obvious pattern. This frequency does not seem to be at the boundary of two cores, but if you see a significant difference in any of these settings that is consistent between good and bad spur settings, it points to something related to the VCO calibration.

    Regards,

    Dean

    1. 

  • In reply to Dean Banerjee:

    Hi Dean

    Thanks a lot for your input and the hints. For both comments I implemented a "solution" that I would like to share with you and the community. For 1) a adjusted the driver to set ACAL_CMP_DLY according to the datasheet's minimum formula - for the specific case it is 14. The phasenoise of the output changes slightly towards the better but spur performance is not improved significantly. Plot: phasenoise_lmx2594_acal_cmp_delay.pdf

    for 2) I now implemented a large fraction - similar to what you do in TICS pro. So instead of the 157/512 I get 658505728/2147483649 for example. This solves the issue of the spurs (at a higher phase noise level though...). Plot: phasenoise_lmx2594_largefraction.pdf

    If you have one more trick up your sleeves to combine the best of both worlds (no spurs & low phase noise) I'd be happy to be a spectator :)

    Best,

    Jonas