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LMX2491: Continous Ramp Function Problem

Elias Steurer
Prodigy 40 points
Part Number: LMX2491
Other Parts Discussed in Thread: LMX2492EVM

Hi,

I am currently facing problems in generating a continous ramp function using LMX2491. The reference oscillator is running at 40MHz and the VCO's feedback frequency is 1.5GHz.. I've done the setup of registers using the TI TICS Pro Software. Unfortunately the generated output function is a ramp burst(see attached picture), repeating at approx. 100ms and ramps configured to 100us. Any idea what I've done wrong here? Thanks for any help in advance!

Output Waveform

Ramp Settings:

User Controls:

Register Settings:

R141    0x008D00
R140    0x008C03
R139    0x008B20
R138    0x008A00
R137    0x008900
R136    0x008800
R135    0x008700
R134    0x008600
R133    0x008503
R132    0x008420
R131    0x008300
R130    0x008200
R129    0x008100
R128    0x008000
R127    0x007F00
R126    0x007E03
R125    0x007D20
R124    0x007C00
R123    0x007B00
R122    0x007A00
R121    0x007900
R120    0x007800
R119    0x007703
R118    0x007620
R117    0x007500
R116    0x007400
R115    0x007300
R114    0x007200
R113    0x007100
R112    0x007003
R111    0x006F20
R110    0x006E00
R109    0x006D00
R108    0x006C00
R107    0x006B00
R106    0x006A00
R105    0x006903
R104    0x006820
R103    0x006700
R102    0x006600
R101    0x006500
R100    0x006400
R99    0x006300
R98    0x006203
R97    0x006120
R96    0x00603F
R95    0x005FFD
R94    0x005E85
R93    0x005D1F
R92    0x005C04
R91    0x005B03
R90    0x005A20
R89    0x005900
R88    0x005802
R87    0x00577A
R86    0x0056E1
R85    0x005500
R84    0x005400
R83    0x005300
R82    0x005210
R81    0x005120
R80    0x005000
R79    0x004F00
R78    0x004EF8
R77    0x004DC0
R76    0x004C00
R75    0x004B00
R74    0x004A00
R73    0x004900
R72    0x004800
R71    0x004700
R70    0x004608
R69    0x004500
R68    0x004400
R67    0x004380
R66    0x004200
R65    0x004100
R64    0x004000
R63    0x003F08
R62    0x003E40
R61    0x003D00
R60    0x003C00
R59    0x003B00
R58    0x003A01
R57    0x003900
R45    0x002D00
R44    0x002C00
R43    0x002B00
R42    0x002A00
R41    0x002900
R40    0x002800
R39    0x002722
R38    0x002620
R37    0x002510
R36    0x002408
R35    0x002361
R34    0x002204
R33    0x002120
R32    0x002000
R31    0x001F2E
R30    0x001E04
R29    0x001D00
R28    0x001C0F
R27    0x001B08
R26    0x001A00
R25    0x001905
R24    0x001800
R23    0x001703
R22    0x0016E8
R21    0x001580
R20    0x001400
R19    0x001300
R18    0x00122C
R17    0x001100
R16    0x0010BB
R15    0x000F00
R14    0x000E00
R13    0x000D00
R12    0x000C00
R11    0x000B00
R10    0x000A00
R9    0x000900
R8    0x000800
R7    0x000700
R6    0x000600
R5    0x000500
R4    0x000400
R3    0x000300
R2    0x000201
R1    0x000100
R0    0x000018

Best regards,

Elias

  • 0
    TI__Expert 5370 points
    Dear Steurer,

    thank you for reaching out. Let me check your config. I will get back to you before Monday the 5th of March.

    Regards, Simon.
  • 0 in reply to Simon Damphousse
    TI__Guru* 96830 points

    Hi Steurer,

    I saw in your configuration there is only one ramp profile, so the output will be a continuous ramp. 

    Here is an example. The loop filter being used is same as LMX2492EVM. 

    Same ramp profile as yours.

    Click the Ramp Enable check box, it will ramp continuously.

    Zoom in view.

  • 0 in reply to Noel Fung
    Prodigy 40 points
    Thank you Noel for helping me regarding this issue. Unfortunately I still have the same problem and the same output waveform as previously described in above post. Any further ideas why the device is behaving like that?
    Thanks for any help in advance!

    Kind regards,
    Elias
  • 0 in reply to Elias Steurer
    TI__Guru* 96830 points
    Hi Elias,

    Our chip does not support "burst mode" operation, I encourage you to check your system to see if there is something toggling.
    BTW, I assume you were measuring the Vtune pin to see the ramp. One thing looks strange to me is that sometime the Vtune voltage is zero, are you turning the chip off periodically?
  • 0 in reply to Noel Fung
    Prodigy 40 points

    Hi Noel,

    thank you very much for your help! I thought I already disabled the toggle-feature inside the firmware. Did a double-check and saw that this was not the case. Unfortunately I still measure a high amount of noise on the Cpout pin. Attached you find a measurement of the cpout pin.(averaging enabled) Thanks for your help in advance!

  • 0 in reply to Elias Steurer
    TI__Guru* 96830 points
    Noel, thank you for grabbing this question. This gentleman has a follow up question. regards, Simon.
  • 0 in reply to Noel Fung
    TI__Guru* 96830 points
    Hi Elias,

    The variation in the "persistence" trace is very big, I don't think this is real. Were you using a probe with long ground wire? Could you use some kind of "spring ground" that is attached very close to the probe tip?
  • 0 in reply to Noel Fung
    Prodigy 40 points

    We are not using a spring ground, but the ground wire of the probe is short. My thoughts are that the external

    loop-filter has to be adjusted. Do you have any suggestions how this filter should look like. We are using LMX2491 together

    with the BGT24 Radar Transceiver. Thanks for your help in advance!

    Elias

  • 0 in reply to Elias Steurer
    TI__Guru* 96830 points

    Hi Elias,

    The loop filter that we used in LMX2492EVM is a good start. You can also download our simulation tool to design your filter. 

    http://www.ti.com/tool/PLLATINUMSIM-SW