LMX2581: LMK2581 Datasheet

Hello Meng,

It appears your image didn't get attached.  Can you try again?

For the image you capture, can you be sure to share the configuration settings for:

• Fpd = 100 MHz (?)
• PLL NUM = 3(?)
• PLL DEN = 100(?)
• Fractional order = ?

If fractional order = 1st order, then no sub-fractional spur is expected.

Meng Liu said:

I also has other problem about why phase detector frequency increase, the loop bandwidth can increase too?

If you don't change your loop filter values, then increasing the loop bandwidth will decrease your total feedback divider (N).  The loop

An approximation rule of them for loop bandwidth is that loop bandwidth is proportional to sqrt ( Charge pump gain * VCO gain / N ).  So by decreasing N, you will increase the loop bandwidth.  If you increase the charge pump gain by the same factor you increase the N value, you will find you keep the same loop bandwidth.  Otherwise, you will have to re-solve the Rs and Cs of your loop filter to keep the same loop bandwidth with different N value.

73,
Timothy

Hi Timothy

thank you explanation!

for question 1:

Fpd=100MHz

PLL num=3

PLL den=100

SDM=2 or 3

I want to know

if SDM=2 ,the sub-fractional spur can appear 0.5M or multiples 0.5M?

If SDM=3, the sub-fractional spur can appear 0.5M ,because 100 don't divisible by 3.so sub spur don't appear 100/3.

the following is LMX 2581 datasheet, l am confuse about why no sub-fractional spurs for a first order modulator or sub-fractional spurs at multiples of 1.5 MHz for a 2nd or 3rd order modulator would be expected?

8.3.15.4 Fractional Spur - Sub-Fractional Spurs

These spurs appear at a fraction of fPD / PLL_DEN and depend on modulator order. With the first order modulator, there are no sub-fractional spurs. The second order modulator can produce 1/2 sub-fractional spurs if the denominator is even. A third order modulator can produce sub-fractional spurs at 1/2,1/3, or 1/6 of the offset, depending if it is divisible by 2 or 3. For instance, if the phase detector frequency is 100 MHz and the fraction is 3/100, no sub-fractional spurs for a first order modulator or sub-fractional spurs at multiples of 1.5 MHz for a 2nd or 3rd order modulator would be expected.