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LMX2541 External reference frequency application

Nidhi Singh
Intellectual 390 points
Other Parts Discussed in Thread: LMX2541, CODELOADER

Hi,

I want to drive LMX2541 with an external OCXO of 10MHz sine wave output as i need to modulate this 10MHz at 3KHz using EFC of OCXO. I need RF output of 200MHz from LMX2541. I already have an OCXO which I am able to modulate at 3KHz. Please guide me how can I use it to drive LMX2541 and what all I need to consider for this application.

Regards,

Nidhi Singh

  • 0
    TI__Expert 4085 points
    Hi Nidhi,

    Thank you for your interest in our products. We received your question and will be answering it as soon as possible. We appreciate your patience.

    Best regards,
    Julian
  • 0
    TI__Mastermind 28001 points

    Nidhi,

    If you just modulate your reference, then this should work.  The main concern I would have is that the LMX2541 VCO is divided into 128 different bands and just ensure that you don't modulate too wide.  The device chooses the correct band whenever the R0 register is programmed.

    If you modulate your 10 MHz at 3 kHz, then at 3 GHz, this modulation would be  3kHz * 3000/10 = 900 kHz.  

    The bands are typically many MHz wide, so it should be able to handle this, but I can't guarnatee that.  Also be aware that if you have the temperature drift without recalibrating the VCO (Programming R0), then the tuning voltage drifts closer to the rail and there is less margin.

    Regards,
    Dean

  • 0 in reply to Dean Banerjee
    Intellectual 390 points

    Dear Dean,

    I am facing an issue with LMX2541. When I am setting the board for 40MHz through Codeloader, it is giving me 48MHz. Same is happening at all the frequencies. It is giving me an additional 7-8MHz frequency. What could be the reason of this?

    Regards,

    Nidhi Singh

  • 0 in reply to Nidhi Singh
    TI__Mastermind 28001 points

    Nidhi,

    We haven't seen this issue.

    Is it possible that somehow the output divider is not being programmed to the correct value? 

    Another possibility is that he VCO frequency is being programmed out of range?

    When you get  the wrong frequency, is this a clean lock with the lock detect signal indicating lock, or is it unlocked?

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    Intellectual 390 points
    Hi Dean,

    When I am using the internal reference oscillator to generate frequency it is giving the correct frequency output but when I am using external reference oscillator to generate same frequency it is giving an additional 7-8MHz jump.

    Case 1: Internal reference oscillator
    Ref input = 100MHz
    R counter = 4
    N counter = 80.01
    Prescalar = 4
    Charge pump gain = 32x
    VCO_Div = 50
    RF_out = 40MHz
    Lock detect indicating lock

    Below are the register values
    R7 0x00000017
    R13 0x0000008D
    R12 0x0000001C
    R9 0x28001409
    R8 0x0111CE58
    R6 0x001F3326
    R5 0xA0000005
    R4 0x88072644
    R3 0x00387F03
    R2 0x04000642
    R1 0xA0000041
    R0 0x00000500

    Case 2: External reference oscillator (Single-ended input)
    Ref input = 20MHz
    R counter = 1
    N counter = 100.01
    Prescalar = 4
    Charge pump gain = 32x
    VCO_Div = 50
    RF_out = 40MHz (on GUI)
    = 47MHz (on Oscilloscope)
    Lock detect not indicating lock

    Below are the register values
    R7 0x00000017
    R13 0x0000008D
    R12 0x0000001C
    R9 0x28001409
    R8 0x0111CE58
    R6 0x001F3326
    R5 0xA0000005
    R4 0x88072644
    R3 0x00387F83
    R2 0x040000A2
    R1 0xA0000011
    R0 0x00010640

    Regards,
    Nidhi Singh