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LMX2594: LMX2594 randomly outputs spurs after reprogram

Jonas Reber1
Prodigy 30 points
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 
LO1 Registers LMX2594
=======================
7340032
7274496
7208960
7143424
7077888
7012352
6946823
6881280
6815744
6750208
6684672
6619136
6553600
6488064
6422528
6359040
6291456
6225920
6160384
6094848
6029312
5963776
5898240
5832704
5767168
5701632
5636096
5570560
5505024
5439488
5373952
5308416
5242880
5177344
5112075
5046272
4980748
4917248
4849664
4784191
4718596
4653185
4637520
4521984
4457448
4390912
4325876
4259840
4199304
4128768
4064034
3997864
3933160
3866625
3833857
3735584
3670016
3604480
3538944
3473408
3409952
3342464
3276800
3228032
3146496
3080960
3016700
2998514
2896419
2818205
2752512
2686976
2621440
2556416
2490368
2425860
2359365
2301473
2228224
2170401
2098067
2032620
1978764
1913228
1836168
1769474
1707440
1639972
1574682
1507452
1441793
1377281
1357896
1255351
1179748
1114414
1048704
984655
925296
868352
806919
720920
660184
595460
532480
458930
444418
327880
264771
198210
132352
67592
9628

  • 0
    Prodigy 30 points

    Jonas Reber1 said:
    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 
    0x700000
0x6F0000
0x6E0000
0x6D0000
0x6C0000
0x6B0000
0x6A0007
0x690000
0x680000
0x670000
0x660000
0x650000
0x640000
0x630000
0x620000
0x610800
0x600000
0x5F0000
0x5E0000
0x5D0000
0x5C0000
0x5B0000
0x5A0000
0x590000
0x580000
0x570000
0x560000
0x550000
0x540000
0x530000
0x520000
0x510000
0x500000
0x4F0000
0x4E010B
0x4D0000
0x4C000C
0x4B0800
0x4A0000
0x49003F
0x480004
0x470081
0x46C350
0x450000
0x4403E8
0x430000
0x4201F4
0x410000
0x401388
0x3F0000
0x3E0322
0x3D00A8
0x3C03E8
0x3B0001
0x3A8001
0x390020
0x380000
0x370000
0x360000
0x350000
0x340820
0x330080
0x320000
0x314180
0x300300
0x2F0300
0x2E07FC
0x2DC0F2
0x2C3223
0x2B009D
0x2A0000
0x290000
0x280000
0x270200
0x260000
0x250404
0x240045
0x231E21
0x220000
0x211E21
0x200393
0x1F03EC
0x1E318C
0x1D318C
0x1C0488
0x1B0002
0x1A0DB0
0x190624
0x18071A
0x17007C
0x160001
0x150401
0x14B848
0x1327B7
0x120064
0x11012E
0x100080
0x0F064F
0x0E1E70
0x0D4000
0x0C5007
0x0B0018
0x0A12D8
0x091604
0x082000
0x0700B2
0x06C802
0x0500C8
0x040A43
0x030642
0x020500
0x010808
0x00259C

  • 0 in reply to Jonas Reber1
    TI__Mastermind 27581 points

    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. 

  • 0 in reply to Dean Banerjee
    Prodigy 30 points

    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