LMX2485E: LMX2485E phase detector spurs

TeeHowe Peh1

Part Number: LMX2485E

Hi there,

I designed a PLL to work with my VCO and so far most things are quite okay. One thing that bugs me are the phase detector spurs, especially when using higher cp current. Are there any known techniques that you can share for me to try to minimize the spurs? I have tried supplying both VDD_RFAs separately but didn't notice any improvements.

I've tested these spurs with different cp gains and I'm aware that the loop filter BW drops with lower cp gain possibly making .

over 5 years ago

0 TeeHowe Peh1 over 5 years ago

Prodigy 40 points

For the record, I just copied the power supply scheme from the LMX2485E evaluation board, with some additional filtering.

0 Noel Fung over 5 years ago in reply to TeeHowe Peh1

TI__Guru** 120050 points

Hi TeeHowe,

Your loop bandwidth is 3.6kHz you don't need high charge pump current. Another problem with small bandwidth but high current is you have a big C2 capacitor.

Redesign the loop filter with a smaller charge pump current can reduce the fpd spurs and reduce the size of C2.

Here is an example.

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LMX2485E: LMX2485E phase detector spurs