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LMX2820EVM: chip to multiply 10MHz to 100Mhz and good way to reduce spurs

Adam Daluga
Expert 6205 points
Part Number: LMX2820EVM
Other Parts Discussed in Thread: LMX2820, LMK04832, LMK02000

Hello,

In the system I am working with I am getting a 10MHz reference.  On the eval board for the LMX, it wants 100MHz.  Do you have a good chip that I can use to multiply up the 10MHz to 100MHz that I  can use to drive the reference on the LMX?

Also, we ran the unit with a 20MHz reference and we are seeing spurs 10MHz away from the carrier.  Do you know a good way to reduce the spurs?

Thanks,

Adam

  • 0
    TI__Guru* 99150 points

    Hi Adam,

    LMX2820 can work with 10MHz, 20MHz or 100MHz, the problem is the reference clock frequency will affect the max. possible fpd and which in turn will affect PLL phase noise.

    If you cannot change the clock frequency, at least you need to find a square wave and ultra low phase noise clock. Square wave is highly recommended as the slew rate of the clock will affect PLL noise. 

    To multiply a 10MHz clock to 100MHz without adding noise, we can use a PLL + VCXO and set the loop filter to a few Hz. LMK02000 is one of the options. if your system requires other clock source too, you may consider LMK04832, which contains two PLL/PLL+VCO. The first PLL, in conjunction with an external 100MHz VCXO, can translate 10MHz to 100MHz. This clock can be used to drive LMX2820. The second PLL+VCO is used to generate other clock frequencies up to 3.2GHz.

    With 10MHz input, the max. possible fpd is 20MHz. What is your output frequency? if you are operate the synthesizer in fractional channel, it is possible that you may have spurs output as well.