Question asked by HankZ on Jul 15, 2011
Latest reply on Jul 18, 2011 by PeterW

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,