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LMX2571 spurs behavior

Other Parts Discussed in Thread: LMX2571

Dear all,

I'm testing LMX2571 evaluation board for synthesizing frequencies between 800MHz and 940MHz. On the RF output it is possible to detect a lot of spurs, depending on the set frequency. After making a lot of tests, it is evident that the spurs are generated at a frequency offset from the generated carrier, that is equal to the distance (in frequency) of the internal oscillator from the nearest n-th harmonic of the reference oscillator (osc in). This spur is directly coupled (by a kind of crosstalk) to the output RF line, without being affected by the loop filter.

These spurs are present at a significant level of -60/-70dBc up to about 10MHz of distance from the generated carrier and it is not possible to avoid this effect by loop filter, because the crosstalk is jumping it.

This is not affordable for a base station application. Please can someone help me in finding a possible solution to this problem?

Thank you,

best regards,

Mauro

  • Hi Mauro,

    Thank you for your interest in our products. We received your question. I forwarded your query to the applications engineer that is most familiar with this device.

    Thanks,

    Julian

  • Hi Mauro,

    Would you let us know the following?
    OSCin frequency
    PDF frequency
    Pre-scaler
    VCO frequency
    channel spacing
    exact output frequency
  • Hi,
    here following some of the test conditions:

    TEST A (worst case)
    OSCin freq = 110MHz
    PDF freq = 110MHz
    Pre-scaler = 2
    VCO freq = 4620,49995 MHz
    Channel div = 5
    output frequency = 924,1MHz

    TEST B
    OSCin freq = 110MHz
    PDF freq = 110MHz
    Pre-scaler = 2
    VCO freq = 4614,99995 MHz
    Channel div = 5
    output frequency = 923MHz (spurs at +/-5MHz)

    TEST C
    OSCin freq = 110MHz
    PDF freq = 110MHz
    Pre-scaler = 2
    VCO freq = 4610,4999875 MHz
    Channel div = 5
    output frequency = 922,1MHz (spurs at +/-9,5MHz)

    TEST D (worst case)
    OSCin freq = 110MHz
    PDF freq = 110MHz
    Pre-scaler = 2
    VCO freq = 4400,49995 MHz
    Channel div = 5
    output frequency = 880,1MHz

    TEST E
    OSCin freq = 110MHz
    PDF freq = 110MHz
    Pre-scaler = 2
    VCO freq = 4956,6 MHz
    Channel div = 6
    output frequency = 826,1MHz (spurs at +/-0,55MHz)

    Thank you very much for your support.
    Best regards,
    Mauro
  • Hi Mauro,
    Please make the following change to your configuration:
    1. fractional DEN --> 2^24-1
    2. Dithering --> Disabled
    3. FRAC_ORDER --> 3rd order
    For TEST D, change the Channel div to 6.
  • Dear Noel,

    we changed the setting as you suggested. I used a signal generator by Marconi Instruments to get the frequency of 110MHz for "osc in". (It is a sinusoidal signal, biased at 1.65V).

    The results are the following:

    TEST A

    freq=924.1MHz

    spurs=-47dBc@+/-500kHz

    TEST B

    freq=923MHz

    spurs=-69.5dBc@+/-5.3MHz

    TEST C

    freq=922.1MHz

    spurs=-74dBc@+/-9.5MHz

     TEST D

    freq=880.1MHz (with div=6)

    spurs=-49dBc@+/-100kHz; -60dBc@+/-300kHz; -58dBc@+/-600kHz

    TEST E

    freq=826.1MHz

    spurs=-74dBc@+/-6.5MHz

    I also tried to change the  level and the amplitude of "osc in" and I also tried to use the onboard 20MHz reference oscillator, without significant changes.

    Unfortunately with these results, I cannot use it as synthesizer for TX, nor as L.O. for RX into a DMR base station, because ETSI normatives require a maximum spurious level not greater than -80dBc. 

    Please can you further help me?

    Thank you very much.

    Best regards,

    Mauro

  • Hi Mauro,
    Need more time to find a fix on this.
    In the mean time, would you advise your TX, RX start and stop frequencies? I assume channel spacing is 12.5kHz?
    Is the OSCin frequency fixed to 110MHz? Can I change it if there is a better solution?
  • Hi Noel,
    thank you for your interest. It would be very important for me to get a fix for it.
    My TX frequency is from 806 to 941 MHz; the RX frequency is 70MHz lower, that is from 736 to 871. Channel spacing is 12.5KHz (for DMR application); the OSCin frequency can be modified as needed, there are no restrictions for it, you can change it.
    Best regards,
    Mauro