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LMX2820: Moving spurs when using large PLL fraction term

yingju chen
Prodigy 10 points
Part Number: LMX2820

Hello,

We are using LMX2820, but observed spurious signals when using a large PLL fraction term (2097152001) as the denominator (see Video).

Video 1. 5.74GHz output frequency, with 40MHz as reference input and PLL_DEN=2097152001.

Although this issue can be alleviated by using lower PLL fraction term (65536000), we are curious about the reason and wondering if there is another solution to this issue, so that we can generate signals with higher precision in frequency.

Here is the configurations we used.

LMX2820_5.74G_40Mref_higher.tcs

Thank you!

  • +1
    TI__Guru** 108130 points

    Hi Yingju,

    Running spurs is a normal behavior of a delta sigma PLL when the fractional denominator is a very large number, especially when the fraction cannot be simplified. That is, with denominator = 65536000, the fraction can be simplified to 3/4, there will be no running spurs. However, when you set denominator to 65536001, there will be running spurs.

    You can try use Third order MASH, this should improve running spurs issue but phase noise may get hurt. There is no other straight forward solution to tackle this problem, you may consider to use different denominator value when running spurs happen. 

  • 0 in reply to Noel Fung
    Prodigy 10 points

    THanks for the help!

    I'll try the third order MASH first.