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LMX2594: spurious

SM
Expert 1780 points

Part Number: LMX2594

Hello,

    We have made a custom board with the LMX2594 with the schematic being almost exactly the same as the EVM (the 100MHz reference being VC-708-EDE-FNXN-100M000000)

3.3V to the oscillator and the LMX2594 are being generated on-board using the LDO  AP7361C-FGE-7. 

There are two issues that we noticed:

1) The frequency in not 100% locked. It keeps shifting by a few hundred Hz. 

2) As we change the supply to the LDO (from 4V to 5.5V) the close in spurious keep changing frequency and levels (do not know if they are the PFD spurs). 

We are using the TI GUI to program our board (and are using the default settings)

Following is a single capture 

Looking forward to your inputs in fixing the problem. Thank you for your help in advance,

Regards,

  • 0
    TI__Guru** 121130 points

    Hi SM,

    The span of your spectrum analyzer is 10MHz, I cannot tell from the video that the signal is drifting in a few hundred Hz. Anyway, I am sure that the synthesizer is already locked because if it is unlock, the drifting will be in MHz. If you see xxxHz drifting, that must be due to the reference clock. 

    With default EVM configuration, the fpd is 200MHz. So the fpd spurs will also be 200MHz. In addition, the default configuration is an integer channel, there will be no fractional spurs. 

    From the second picture, the spurs frequency is something around 200kHz. Could you use a scope to spot where in your circuitry has 200kHz noise?

  • 0 in reply to Noel Fung
    Expert 1780 points

    Hello Noel,

             You were right about the clock drift. We have a XO VC-708-EDE-FNXN-100M000000 (100MHz XO). As the temperature of the board changes the frequency also changed. This can probably be fixed by using an appropriate XO (so a TCXO or OCXO) or the reference mounted on a different board. 

    However I am unable to understand the spurious, it is a few tens of KHz away and changes with the supply voltage to the LDO (which feeds the LMX). Also, the spurious are not static but keep fluctuating (as can be seen in the video). 

    Looking forward to your inputs and thanks again,

  • 0 in reply to SM
    TI__Guru** 121130 points

    Hi SM,

    Can you try with an external power supply?