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LMX2595EVM: The reference signal passing through the LMX2595 generates spurious signals at integer multiples of the reference frequency

Mark
Prodigy 57 points
Part Number: LMX2595EVM
Other Parts Discussed in Thread: LMX2595, , LMK61PD0A2

Hello,  I observed a phenomenon that puzzled me.

When my LMX2595's reference signal is set to 125MHz, no matter what output frequency is, there is always a 3.5GHz spurious output (nearly -70dBm), which seems to be higher order harmonics of the reference frequency. I didn't find any specifications about LMX2595 reference frequency spurious or PFD spurious. Is it normal performance of LMX2595?

Also, when I tested LMX2595EVM, using 125MHz reference provided by SMW200A and Reference Pro( LMK61PD0A2), I observed the same phenomenon. It is my TICS Pro setting and the 3.5GHz spur is nearly -75dBm.

When I change reference frequency to 130MHz, the 3.5GHz spurious disappear. It's preliminarily determined that this spurious signal is caused by OSCin or PFD. Can this spurious signal be eliminated or reduced?

  • 0
    TI__Guru* 87210 points

    Hi Mark,

    After the PLL is locked, try program R2[10]=0, this will stop the internal state machine clock which is equal to fosc in your configuration.

    Make sure to set this bit back to =1 before a VCO calibration is executed. 

  • 0 in reply to Noel Fung
    Prodigy 57 points

    Hi Noel,

    Thanks for your instant reply. Program R2[10]=0 does work. It has improved the spurious of N×125MHz, especially spurious at 125MHz and 375MHz, which are reduced from -70dBc to -90dBc. However, it has no significant influence on spurious at 3.5GHz. 

    Is there any other possible ways to reduce spurious at 3.5GHz?

  • +1 in reply to Mark
    TI__Guru* 87210 points

    Hi Mark,

    Try set OUT_ISET = 3, this will reduce the overall output power by approx. 1dB, but the 3.5GHz spurs will reduce approx. 5dB. These are the data taken from my EVM.

  • 0 in reply to Noel Fung
    Prodigy 57 points

    Hi Noel,

    It seems not working on my EVM and little influence on 3.5GHz spur. However, spurs at 125MHz and 375MHz have been reduced a lot. I think what you suggest is still a good way to improve OSCin/PFD spurs at low frequency. It's good enough for me to know that.

    Thank you so much!