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LMX2595: LMX2595 Chirp Mode Frequencies

Richard Elmquist
Mastermind 45250 points
Part Number: LMX2595


I have a customer who is asking the following questions:

I can send a presentation describing the activation of the component - LMX2595. There is a problem running the component in chirp mode for 125MHz. The maximum possible was up to 95MHz.

LMX2595 - possible to create a chirp up to 95MHz-1.pptx

Can you help the customer with this issue? Do we have any application notes that might help with this issue?

Hanks for your help with this!

Richard Elmquist

  • 0
    TI__Mastermind 28021 points

    Richard,

    Note that there is no guarantee about how far the VCO can go without the need to calibrate.

    We do show that the device can typically go for 125 MHz at 12 GHz, but this should not be interpreted as the device can do this much at any VCO frequency or that this applies over process and temperature.

    We give no guarantees in the datasheet how far the VCO can go "calibration free", but I measured on devices and am sharing the data for this.  

    Below is how far I measured one device can go up in frequency "calibration free" in MHz

    Fvco Ta=-40 Ta=25 Ta=85
    7500 76 43 20
    10000 109 58 29
    12000 191 124 64
    15000 231 115 55

    And here is how far it could go down "calibration free" in MHz

    Fvco Ta=-40 Ta=25 Ta=85
    7500 36 70 86
    10000 53 103 123
    12000 64 131 175
    15000 100 216

    260

    So if you see, 125 MHz is about the limit at 12 GHz and at 9.5 GHz, we see it would be less.

    Now if you add margin for part to part variation and temperature variation, this number would be less.

    Regards,
    Dean

  • 0 in reply to Dean Banerjee
    TI__Mastermind 45250 points
    Dean,
    Thanks for all your help!
    I will respond back if the customer has any further questions.
    Have a great day!
    Richard Elmquist
  • 0 in reply to Richard Elmquist
    TI__Mastermind 45250 points

    Dean,

     

    The customer has a couple of additional questions:

    What did you mean by referring to "calibration free" ? 

    Is “FCAL_EN” checked ? 

    If so, you specify that there is a chirp length limit 

    How do you explain that in each VCO (one out of 7), can cover up to 1 GHz? Or is there no connection? 

    Can you specify which registers are required to control chirp creation?

    Thanks for your help with this!

    Richard Elmquist

  • 0 in reply to Richard Elmquist
    TI__Guru** 109470 points
    Hi Rich,
    During ramp, it is possible not to do vco calibration between the start and stop frequencies. This is called calibration free ramping.
    However, as Dean pointed out, this range (start-stop freq) is not guaranteed and is vary between parts and vco core.
    For example, in the datasheet, the typ. plot is showing a 120MHz calibration free ramp range when the VCO output is 12GHz. If the VCO output is 7.5GHz, for sure that the calibration free range will be less than 120MHz.
    In your ppt, your ramp range is 62.5MHz, which I believe is possible as the VCO output is 9.5GHz. The problem you have is the fpd is too high. Max. fpd in ramp mode is 125MHz. So please try disable the doubler to make fpd = 100MHz.
    It is recommended to keep FCAL_EN = 1 all the times unless you have a special use case. In frequency ramp mode, keep FCAL_EN = 1.
    Our VCO cores can support 7.5GHz to 15GHz, and we have 7 vco cores. So, approximately, each VCO core can support around 1GHz tuning range.
  • 0 in reply to Noel Fung
    TI__Mastermind 45250 points
    Noel,
    I think that this should solve the customer's issue.
    If the customer does have further questions I will post them.
    Have a great day!
    Richard Elmquist
  • 0 in reply to Richard Elmquist
    TI__Mastermind 45250 points

    Noel,

    Can you look at the attached document and send me a response?

    LMX2595 Chirp Mode Frequencies.docx

    Thanks for your help with this!

    Richard Elmquist

  • 0 in reply to Richard Elmquist
    TI__Mastermind 45250 points
    Noel,
    Have you had a chance to look at the customer's further questions?
    I have compiled them in a word doc attached to the previous email.
    Please look at them and let me know if you have any questions for the customer.
    Have a great weekend!
    Richard Elmquist
  • 0 in reply to Richard Elmquist
    TI__Mastermind 28021 points

    Richard,

    In response to the document questions:

    I referred to your answer saying that Fpd should be changed to 100MHz, but the chirp could not be received in length of 125MHz, and also, the results were less good than running on Fpd of 200MHz.

    -> The device does work with 100 MHz, so I am not sure exactly what this means.  Now if you ramp through an integer boundary, you will get a glitch on the ramp.  200 MHz Fpd might make this glitch go away, but we have had some issues with the ramp reliably starting for these higher phase detector frequencies and this is why we say Fpd<=125 MHz.

     

    At this point, a result of 95MHz was obtained for chirp length when Fpd is run at 200MHz.

     ->  See response below.

    What is the role of the “Threshold” field in the “VCO Calibration window” that belongs to the “TICS Pro” application?

    "Calibration Free Range" refers to how far the VCO can ramp without the need to recalibrate.  "Threshold" specifies how long you let the VCO go before recalibrating.  Note that:

    Threshold <= Calibration Free Range

    We have a discussion of calibration free range in this thread.  In this case, I see a mention of 95 MHz, which sounds like for this frequency, CalibrationFreeRange=95 MHz.  You set the threshold to 130 MHz, but because this is bigger than 95 MHz, the VCO will go the 95 MHz and re-calibrate and ignore the fact that you want it to go 130 MHz.  The idea is to set the threshold to something smaller, like 50 MHz so then at least you can control when these calibration breaks are.  You can also set the calibration time.  Now if you say 0, then it will just wait for however long it takes the VCO to calibrate.  However, if you set it longer than this, then at least the calibration breaks in the ramp will be a consistent, known time.

    Regards,
    Dean