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PLLATINUMSIM-SW: Loop filter of LMX2485E

Part Number: PLLATINUMSIM-SW
Other Parts Discussed in Thread: LMX2485E

Hello,

I tried to simulate LMX2485E as input frequency is 10MHz, output frequency is 50MHz, PLL loop filter cut-off is 1kHz.

When calculate Loop Filter button pressed, I feel capacitor value is too big. In below case, C1 is 4.7uF, C2=27uF.

Could you review the simulation?

LMX2485E_50MHz_filter1kHz.zip

Best regards,

Toshihiro Watanabe

  • Hello,

    The loop filter capacitors are largely dependent on the open loop gain and the desired loop bandwidth.

    Reducing the loop gain such as by decreasing the charge pump current will scale the required passive values down for an equivalent bandwidth.

    Regards,

  • Hi Watanabe-san,

    Since your loop bandwidth is small, you don't need to use very high charge pump current or high phase detector frequency, they will result in high capacitor value in the loop filter.

    If 1kHz loop bandwidth is desired, then use 100µA charge pump current is enough. You can further reduce the capacitor value if you reduce the phase detector frequency to 1MHz.