• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 21 subscribers
  • Views 382 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMX2572: Direct VCO output OK but divided output spectrum very poor and broad

Stuart Rumley
Prodigy 250 points
Part Number: LMX2572
Other Parts Discussed in Thread: LMX2594, LMX2595

Tool/software:

The synthesizer chip behaves normally with good spectrum and phase noise but when using the divider the spectrum is very poor and broad.   However, when I use a foam tipped swab with cool spay, the divided spectrum is normal.  I've tried reflowing the chip with no improvement. 

I've successfully used the LMX2594 and LMX2595 with similar layout with no problems.

Shown below is the 5000MHz VCO output

Show below is the same synthesizer with the divide by 4 selected.

reg  0 = 0x209c   (default=0x209d)
reg  5 = 0x20c8   (default=0x20c8)
reg 10 = 0x13f8   (default=0x1378)
reg 14 = 0x1840   (default=0x1840)
reg 29 = 0x0000   (default=0x0000)
reg 30 = 0x18a6   (default=0x18a6)
reg 36 = 0x0047   (default=0x0028)
reg 37 = 0x0205   (default=0x0205)
reg 38 = 0x0000   (default=0xffff)
reg 39 = 0x0007   (default=0xffff)
reg 42 = 0x0000   (default=0x0000)
reg 43 = 0x0003   (default=0x0000)
reg 44 = 0x22a2   (default=0x22a2)
reg 45 = 0xc600   (default=0xc600)
reg 46 = 0x07f3   (default=0x07f3)
reg 52 = 0x0421   (default=0x0421)
reg 57 = 0x0020   (default=0x0020)
reg 71 = 0x0080   (default=0x0080)
reg 75 = 0x0840   (default=0x0800)
reg 78 = 0x0065   (default=0x0065)
F(MHz) = 1250.0 = 5000.0 / 4 = 70 * (71 + 3 / 7) / 4  CP=8  PWR=34

  • 0
    TI__Guru** 108550 points

    Hi Stuart,

    I don't have problem locking the device to 1250MHz with your configuration. The only modification I did to your configuration is R10, the correct value should be 0x0A10F8. 

  • +1 in reply to Noel Fung
    TI__Guru** 108550 points

    Hi Stuart,

    Just found out, R46 is not correct, we should not use the reserved option.

    If RFOUTA is VCO output, set RFOUTB to VCO output as well. Similarly, if RFOUTA is divided down output, set RFOUTB also to divided down output.

    When RFOUTA is divided down output and RFOUTB is reserved.

    Change RFOUTB to divided down output.

  • 0 in reply to Noel Fung
    Prodigy 250 points

    Yes, thanks Noel.   I had the unused OUTB_MUX in the "tri-state" mode which really messes up the spectrum.  I don't know how anyone would find that since there is no mention of that OUTB_MUX either in the datasheet or in TICs-Pro.  But yes, thanks, so in general OUA_MUX and OUTB_MUX should be set the same.