STUW81300 datasheet(25/63 Pages) STMICROELECTRONICS

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DS11314 Rev 7

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STuW81300

Circuit description

The resulting VCO frequency is:

where:

FVCO is the output frequency of VCO

Fref is the input reference frequency

R is the division ratio of reference chain

N is the overall division ratio of the PLL

The N divider accepts input signal frequencies up to a maximum of 6 GHz. When the setup

requires a VCO frequency greater than 6 GHz, the VCO signal provided to the N divider

must be divided by 2 by setting PLL_SEL=’1’ in the ST1 Register . In this case the VCO

frequency is fixed by:

The implementation with programmable modulus allows the user to easily select the desired

fraction and the exact synthesized frequency with no approximation.

The MOD value can be set to very high values, thus the frequency resolution of the

synthesizer can reach very fine steps (down to a few hertz).

A ‘low spur mode’ could be configured by maximizing both FRAC and MOD values, keeping

the same desired FRAC/MOD ratio, and setting the DITHERING bit to ‘1’. The drawback is a

small frequency error, equal to FPFD/(2*MOD) on the synthesized frequency. This error is in

the range of a few hertz (usually tolerated by most applications).

7.4

Fractional spurs and compensation mechanism

The fractional PLL operation generates unwanted fractional spurs around the synthesized

frequency.

The integer boundary spurs occur when the carrier frequency is close to an integer multiple

of the PFD frequency. If the frequency difference between the carrier and the N*FPFD falls

within the PLL loop bandwidth, the integer boundary spur is unfiltered and represents the

worst-case situation giving the highest spur level.

The channel spurs are generated by the delta-sigma modulator operations and depend on

its settings (they are mainly related to the MOD value). The channel spurs appear at a

frequency offset from the carrier, equal to FPFD/MOD and its harmonics, and they are not

integer boundary. If the MOD value is extremely high (close to the maximum value of 221-1)

the channel spur offset is of the order of a tenth of a hertz and it appears as ‘granular noise’

shaped by the PLL around the carrier.

FVCO

Fref

---------- N

⋅

Fref

----------

NINT

FRAC

MOD

----------------- DITHERING

2MOD

⋅

-----------------------------------

⎝⎠

⎛⎞

⋅

FVCO

Fref

---------- 2NINT

FRAC

MOD

----------------- DITHERING

2MOD

⋅

-----------------------------------

⎝⎠

⎛⎞

⋅⋅

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