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LMX25412690EVAL/NOPB 900 MHz sinewave generation issue

Yuwei Jin
Prodigy 30 points
Other Parts Discussed in Thread: LMX25412690EVAL, LMX2541

Currently, I am trying to use the LMX25412690EVAL/NOPB to generate a 900 MHz sinewave synchronized by our local oscillator. the parameters can been seen below. the reference clock is generated by our local oscillator.

To check the quality of this signal. I mixed this signal with our 900.000001 MHz local socillator (sinewave, synchronized with the same 10MHz clock) to monitor the 1 Hz beating frequency. Here is the reslut:

I can see the 1Hz beating clearly but superimposing with a lot of spurs and the reason is not clear. Could anybody give some solutions for this? The figure below shows the FFT of this signal.

 

 

 

 

 

  • 0
    TI__Mastermind 27581 points

    Yuwei,

    The LMX2541 has excellent spur performance and does not generate spurs in integer mode.   However, if you have the fraction modulator on and dithering enabled it can.  The solution to this is disable ditheriring in our device.

    I am suspicious of whatever is generating the 900.000001 MHz signal from 10 MHz.  In general, when you produce one frequency from another, expect spurs at the greatest common divisor of the two frequenies.  So GCD(900.000001,10) = 1 Hz.  In other words, I suspect that this 900.000001 MHz signal could have very high spurs at 1 Hz offset.

     

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    Prodigy 30 points

    Dean,

    Thank you for your replay.

    I am using a R&S®SMB100A RF and Microwave Signal Generator to generate the 900.000001MHz.

    Here is the link.

    http://www.rohde-schwarz.com/en/product/smb100a-productstartpage_63493-9379.html

    From the back panel of this device, there is an 10 MHz ref output. I am using this 10 MHz output as the reference of the LMX2541 board.

    Waiting for your further comments.

    Regards,

    Yuwei

  • 0 in reply to Dean Banerjee
    Prodigy 30 points

    Dean,

    Thank you for your replay.

    I am using a R&S®SMB100A RF and Microwave Signal Generator to generate the 900.000001MHz.

    Here is the link.

    http://www.rohde-schwarz.com/en/product/smb100a-productstartpage_63493-9379.html

    From the back panel of this device, there is an 10 MHz ref output. I am using this 10 MHz output as the reference of the LMX2541 board.

    Waiting for your further comments.

    Regards,

    Yuwei