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

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?

  • 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. 

  • 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?

  • 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.

  • 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!