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LMX2594: VCO selection when 12GHz output

Part Number: LMX2594

Hello team,

I have a question about LMX2594.

My customer has two EVMs and set 12GHz output setting along with datasheet recommendation; VCO_SEL=4, VCO_DACISET_STRT=300 and VCO_CAPCTRL_STRT=1.

However, two EVMs show different VCO selection behavior like below table. One EVM behaves that VCO5 is always selected. but another EVM shows that VCO is selected alternately between VCO4 and VCO5 ever time FCAL_EN is sent.

EVM#1 1st 2nd 3rd 4th 5th 6th
rb_VCO_SEL 5 4 5 4 5 4
rb_VCO_CAPCTRL 183 1 183 1 183 1
rb_VCO_DACISET 195 235 195 235 195 235

EVM#2 1st 2nd 3rd 4th 5th 6th
rb_VCO_SEL 5 5 5 5 5 5
rb_VCO_CAPCTRL 182 182 182 182 182 182
rb_VCO_DACISET 227 228 228 229 229 229

Settings are; Oscin=125MHz, Fpd=62.5MHz, N divider=192, Fvco=12GHz VCO output, integer mode, PFD_DLY_SEL=1, FCAL_LPFD_ADJ: Fpd>=10, FCAL_HPFD_ADJ: Fpd<=100, CAL_CLK_DIV: Div2.

Although I think VCO5 has to be selected at 12GHz output, Is there any missing requirement?

Best regards,

  • Hi Taketo-san,

    As pointed out in page 20 of the datasheet, VCO_SEL must be set to 4 for frequency between 11.9 and 12.1GHz. For the cases where rb_VCO_SEL = 4, can you confirm with customers that the device is locked?

    The reason that VCO_SEL must be set to 4 around 12GHz is that there is not enough overlapping between 4 and 5, and most devices can only lock at 5. For more details, please read this post: e2e.ti.com/.../2867078

    Regards,
    Hao
  • By the way, we recommend setting VCO_SEL = 7, VCO_DACISET_STRT = 300 and VCO_CAPCTRL_STRT = 183 for all other frequencies except 11.9~12.1GHz. For frequencies around 12GHz, change VCO_SEL to 4 and everything else can be the same. User can also change VCO_CAPCTRL_STRT to 1 as noted in the datasheet but that is not important. The critical thing is to switch VCO_SEL, also VCO_DACISET_STRT needs to be >= 250

    You will find the descriptions under "no assist" mode in the rev C of datasheet. The rev C should be published on web this week (probably today).

    Regards,
    Hao
  • Hello Hao-san,

    For the cases where rb_VCO_SEL=4, R110[10:9] shows 2, so device should be locked. Output is proper 12GHz as expected.
    Do you know why VCO5 and VCO4 is selected alternately every time FCAL_EN is sent? Is there any missing requirement?

    Best regards,
  • Hi Taketo-san,

    The VCO frequency is at the edge of core 4 and 5. You can see that the capcode is either 1(0 is highest frequency) for core 4 or 183 (lowest frequency) for core 5. So it is not surprising that it switches between the two because there is very little overlap in between. Sometimes it gets locked at 4 but next time it may miss core 4 and jump to 5. Most likely, in the range of 11.9~12.1 GHz, the device cannot lock in core 4 and can only lock in core 5.

    Regards,
    Hao