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LMX2592EVM: LMX2592EVM rise in phase noise

Victor Levy
Intellectual 2436 points
Part Number: LMX2592EVM

Hello, 

i have a customer that is working with the LMX2592EVM,

As he Is checking phase noise between 10Hz and 1kHz,

He sees a huge rise in phase noise after reaching 300Hz ,

It is not immediate comedown in the phase noise, it keeps rising ,

 

The reference clk he uses is 40MHz,  and output Is 6-7.5GHz.

 

Have you heard of problems like this before with this EVM?

  • 0
    TI__Guru** 107030 points

    Hi Victor,

    Below 1kHz, the phase noise is basically dominated by the reference clock. Where is the 40MHz clock came from?

    BTW, do you have a screen shot of the measurement? 

    Could you also provide the TICS Pro configuration? 

  • 0 in reply to Noel Fung
    TI__Intellectual 2436 points

    Fullscreen 2642.HexRegisterValues.txt Download 
    R64	0x400077
R62	0x3E0000
R61	0x3D0001
R59	0x3B0000
R48	0x3003FC
R47	0x2F08CF
R46	0x2E0FA3
R45	0x2D0000
R44	0x2C0000
R43	0x2B0000
R42	0x2A0000
R41	0x290001
R40	0x280000
R39	0x278204
R38	0x260438
R37	0x254000
R36	0x240011
R35	0x23021F
R34	0x22C3EA
R33	0x212A0A
R32	0x20210A
R31	0x1F0401
R30	0x1E0034
R29	0x1D0084
R28	0x1C2924
R25	0x190000
R24	0x180509
R23	0x178842
R22	0x162300
R20	0x14012C
R19	0x130965
R14	0x0E0FFF
R13	0x0D4000
R12	0x0C7001
R11	0x0B0148
R10	0x0A10D8
R9	0x090302
R8	0x081084
R7	0x0728B2
R4	0x041943
R2	0x020500
R1	0x010808
R0	0x00221C

  • 0 in reply to Victor Levy
    TI__Intellectual 2436 points

    Hey Noel,

    As for the 40Mhz clock, it is coming from the EVM.

  • 0 in reply to Victor Levy
    TI__Guru** 107030 points

    Hi Victor,

    The on-board XO frequency in the EVM is 100MHz.

    This XO is a cheap, low performance XO, its close-in phase noise is known to be bad. 

    If you want better <1kHz offset performance, replace this XO with a high performance XO such as Vectron VC-708.

    BTW, their TICS Pro configuration is not optimized. Although OUTB is not used, Output Mux should be set to "VCO". Fpd is too low, integer mode is still ok but the phase noise will be bad in fractional mode. We do not suggest use Charge pump gain greater than 1.5x. 

  • 0 in reply to Noel Fung
    TI__Intellectual 2436 points

    This is the phase noise of our TCXO measured on our board (including all distribution systems etc).

     


     

     

     

    should we sew this on the EVM and check what happens?

  • 0 in reply to Victor Levy
    TI__Guru** 107030 points

    Hi Victor,

    Your plot is not taken from the TCXO output, it is the PLL/synthesizer output. 

    I just checked the system, we never change the original on-board XO, so its frequency is 100 MHz. 

    Anyway, to improve <1kHz noise, a good quality reference clock is needed. 

    Here is a plot with a pretty good quality reference clock. I used TICS Pro "Default configuration" but changed the Charge Pump Gain box value to "2.5mA".

  • 0 in reply to Noel Fung
    TI__Intellectual 2436 points

    This is the result with out TCXO on the EVB.

    The TCXO phase noise spec is

    100Hz               ≤ -117 dBc/Hz

    1kHz                 ≤ -137 dBc/Hz

    10kHz               ≤ -150 dBc/Hz

     

    If you think there is a better configuration to use I’ll be happy to get instruction.

    We’ve tried the points raised and there was a very minor change.

    The result of the phase noise from LMX using this reference (40MHz) is:

     

  • 0 in reply to Victor Levy
    TI__Guru** 107030 points

    Hi Victor,

    I want to emphasize again that the <1kHz noise is determined by the reference clock, we can do nothing, in the PLL, to improve it. 

    However, I noticed that their overall measured phase noise at 5.4GHz is worse than my test data. Are they still using 5MHz phase detector frequency? This is not a good idea. Can they try with 100MHz phase detector frequency?