LMX2571EVM: Problem with some dividers

Mateusz Drewniak

Part Number: LMX2571EVM
Other Parts Discussed in Thread: CODELOADER, USB2ANY, LMX2571

Good Morning,

I have some problem with set freqencu on LMX2571, we use evo board, usb2any and applications Code Loader 4 to write and read value on register. When we calculate value of some dividers, PLL doesn't lock. I pasted screenshot from CodeLoader and spectrum analyzer.

Example 1 OK

Example 2, different N and PLL N Prescaler -> bad

According datasheet, I don't cross any limits or We don't know that are .

Perhaps I do not include all limits in to my algorytm.

Regards

Mat

over 6 years ago

0 Noel Fung over 6 years ago

TI__Guru* 88030 points

Hi Mat,
What is the delta sigma modulator in example 2? N=11.8 is too small for a 3rd order modulator. You may try again with 2nd order.

0 Mateusz Drewniak over 6 years ago in reply to Noel Fung

Prodigy 80 points

Thanks Noel, You are right.
Have You some articles or publications with sigma delta and PLL theory ? I wanna understand why N is too small for a 3rd order modulator .
Regards
Mat

0 Noel Fung over 6 years ago in reply to Mateusz Drewniak

TI__Guru* 88030 points

Hi Mat,

In general, you can read Dean's book (www.ti.com/.../snaa106), it has a section explaining why there is a min. N value restriction. Every part has different characteristic, so the min. N vs modulator order is also different.

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LMX2571EVM: Problem with some dividers