LMX2571 in Clock Designer - missing LPF's

Bob Haagensen

Other Parts Discussed in Thread: LMX2571

Just downloaded the new Clock Designer which includes LMX2571. It seems that in the Loop Filter Select dropdown, your only options are 0 Hz, Inf Hz, 4th Order and Brickwall. It would be nice to have 2nd and 3rd Order as well. In the LMX2571 Ext VCO option, these options are present but not for the Internal VCO config. I suppose I can use the Ext VCO option with a VCO similar to the internal VCO and I should get similar results. Anyone see any problems with this workaround? Thanks

Bob

over 10 years ago

0 Dean Banerjee over 10 years ago

TI__Mastermind 28021 points

Bob,

On the LMX2571 operating with the internal VCO there are two poles located on front of the VCO that you are forced to use. You have the option of reducing the resistor values to push these poles farther out to make their impact much less, but you can not bypass them. If you reduce the filter resistors for R3 and R4 to the minimum value, this will be closer to a 2nd order filter.

These 2 integrated poles can restrict your maximum loop bandwidth if you design for something very wide and if you ahve this issue, use a higher charge pump gain and phase detector frequency.

Regards,
en

0 Bob Haagensen over 10 years ago in reply to Dean Banerjee

Intellectual 370 points

Ok, that makes sense - thank you. I'm going for a very narrow LPF - about 50 Hz with a 100 MHz VCXO - so that'll work out well.
While we're on the subject of LMX2571, can I ask what to do with the FSK pins - pins 4, 5, 6 and 7 - when we're not using FSK in any form?
Thanks again for your help
Bob

0 Noel Fung over 10 years ago in reply to Bob Haagensen

TI__Guru** 110530 points

Hi Bob,

You can simply left them floating.

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LMX2571 in Clock Designer - missing LPF's