I have a question about Phase noise results using the ADIsimPLL software program. I included a 6 page Word document to help with the question showing results from the ADIsimPLL. I am trying to find out why when I change my PLL from generating 92.16MHz to 98.304MHz, my phase noise results (in the ADIsimPLL tool) get worse by about 7dB, when my calculation using a literature reference (not Analog Devices) says it should be about 0.6dB (19.9 – 19.3).

My literature reference says that in a fractional multiplication PLL, you subtract to the incoming reference oscillator phase noise 20log(numerator) and add 20log(denominator).

Reference input = 10MHz

1.0 98.304MHz Design

Calculated N/R = 6144/625.

Phase Noise (PN) = +20log(625) – 20log(6144) = 55.9dB – 75.8dB = -19.9dB

In the ADISimPLL Results table I see

Phase Noise Table

Freq Total VCO Ref Chip Filter

100 -103.2 -- -- -104.0 -111.4

1.00k -92.74 -- -- -101.1 -93.42

10.0k -118.8 -- -- -128.5 -119.2

100k -158.6 -- -- -168.4 -159.1

1.00M -198.6 -- -- -208.4 -199.1

2.0 92.16MHz Design

Calculated N/R = 1152/125.

Phase Noise (PN) = +20log(125) – 20log(1152) = 41.9dB – 61.2dB = -19.3dB

In the ADISimPLL Results table I see

Phase Noise Table

Freq Total VCO Ref Chip Filter

100 -111.1 -- -- -111.1 -139.6

1.00k -110.8 -- -- -111.4 -119.7

10.0k -107.0 -- -- -111.1 -109.1

100k -143.5 -- -- -148.0 -145.5

1.00M -183.5 -- -- -188.0 -185.4

Any help is appreciated,

The in-band phase noise of a PLL is usually dominated by noise sources associated with the digital logic and the charge pump, and is given by

PN floor in-band = FOM + 20logN + 10logFpd

This is given in the helpfile along with references.

In your PLL, you are changing both N and Fpd whilst keeping the output frequency about constant, the net effect is that the phase noise varies as -10logFpd. Your two cases have Fpd changing from 16kHz to 80kHz, a factor of 5 that results in a 7dB variation in phase noise.

The fact that the phase noise increases as you reduce Fpd is one of the main reasons for Fractional-N PLLs.