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LMX2571: FSK Deviation at 30MHz

Andrew Small
Prodigy 20 points
Part Number: LMX2571

Hi,

We are experimenting with the LMX2571's FSK feature to FM modulate a frequency band of 30 to 50MHz. We need a quite big FM deviation of 5KHz, Is this possible?

We are using a 25MHz oscillator and Multiplier set to 4 (FPD =100) also set the prescaler to 4 to try to get the deviation range we need. With the fpd=100 and prescaler 4, the VCO locking is hit and miss across the 30 to 50MHz band.

Any recommendations to do this properly, or is that deviation not really possible with the lower frequencies?

Thanks,

Andrew Small  

  • 0
    TI__Guru** 108130 points

    Hi Andrew,

    At 30MHz output, we can do at most 4.8kHz deviation. Higher output frequencies can support 5kHz deviation.

    Can you use 26MHz reference clock? This will increase fpd slightly to 104MHz, then you can get 5kHz deviation at 30MHz output.

  • 0
    Prodigy 20 points

    Hi Noel,

    Yes we can use a 26MHz Clock. One other issue we are having when using the 25MHz reference and when the fpd is at 100, prescaler 4. Is that we are having a difficult time getting the VCO to lock. Using the EVAL board we can get it to lock sometimes at 30MHz and 50MHz, other times it will not lock at all. We need to cycle the power to the EVAL board then it will lock again at those freqs. If we want to tune to say 31MHz it seems no matter what setting we use the VCO will not lock, It tunes to a random frequency that is close to 31MHz. 

    Thanks,

    Andrew

  • 0 in reply to Andrew Small
    TI__Guru** 108130 points

    Hi Andrew,

    What is the FRAC_ORDER setting?

  • 0 in reply to Noel Fung
    Prodigy 20 points

    Hi Noel,

    We have it set to 2nd order at the moment. Attached are all the settings we are putting into the GUI, as well as spectrum plots. When tuned to 30MHz with the current settings it will lock properly, trying to tune to 32.5MHz or many other frequencies in the 30 to 50MHz band it will not lock. 

    LMX2571 Settings and Plots.zip

    Thanks,

    Andrew

  • 0 in reply to Andrew Small
    TI__Guru** 108130 points

    Hi Andrew,

    to switch the frequency from 30 to 32.5MHz, only N-divider is required to change. did you program it as below?

    1. program PLL_N (from 12 to 13)

    2. Program R0 (with FCAL_EN = 1)

  • 0 in reply to Noel Fung
    Prodigy 20 points

    Hi Noel,

    Yes all we did to tune to 32.5MHz is change N to 13 and FCAL to 1. I sent screenshots of all the settings I have for both programming 30MHz and 32.5MHz in the previous post. 30MHz locks okay, 32.5MHz does not lock, same goes for most frequencies across the 30 to 50MHz range. I can get it to lock reliably at 30MHz and 50MHz anything in between is hit or miss.

    Changing to Prescaler 2 from 4, I can reliably get it to lock across the band so I'm guessing it's something to do with that. Only issue with prescaler 2 is we can not get the FM deviation we need. It's not a huge deal as we can just modulate it with an IQ modulator, It's just easier if we can do it with the VCO.

    Thanks,

    Andrew

  • 0 in reply to Andrew Small
    TI__Guru** 108130 points

    Hi Andrew,

    I also suspect this is due to the prescaler although I don't have proof. 

    Between 30MHz and 50MHz output, do you have lock issue only with the highlighted channels?

    If that is the case, try set R59 (a hidden register) from the default of 0x0084 to 0x0087, see if this will fix the issue.