LTC6948 datasheet(26/38 Pages) AD

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LTC6948

6948fa

For more information www.linear.com/LTC6948

applicaTions inForMaTion

Alternatively,tocalculatethenumeratorstepsizeNUMSTEP

needed to produce a given frequency step fSTEP(FRAC), use

Equation 9:

NUMSTEP =

fSTEP(FRAC) •R•O•218

fREF

(9)

The output frequency resolution fSTEP(INT) produced by a

unit change in N while in integer mode is given by Equa-

tion 10:

fSTEP(INT) =

fREF

R•O

(10)

LOOP FILTER DESIGN

AstablePLLsystemrequirescareindesigningtheexternal

loopfilter.TheLinearTechnologyFracNWizardapplication,

available from www.linear.com, aids in design and simula-

tion of the complete system.

The loop design should use the following algorithm:

1) DeterminetheoutputfrequencyfRFandfrequencystep

size fSTEP based on application requirements. Using

Equations 3, 5, 7, and 8, change fREF, N, R, and O until

the application frequency constraints are met. Use the

minimum R value that still satisfies the constraints.

Then calculate B using Equation 1 and Tables 8 and

2) Select the open loop bandwidth BW constrained by

fPFD and oversampling ratio OSR. The OSR is the ratio

of fPFD to BW (see Equation 11):

OSR =

fPFD

BW =

fPFD

OSR

(11)

where BW and fPFD are in Hz.

A stable loop, both in integer and fractional mode,

requires that the OSR is greater than or equal to 10.

Further, in fractional mode, OSR must be high enough

to allow the loop filter to reduce modulator quantiza-

tion noise to an acceptable level.

Choosing a higher order loop filter when using the Δ∑

modulator allows for a smaller OSR, and thus a larger

loop bandwidth. Linear Technology’s FracNWizard

helps choose the appropriate OSR and BW values.

3) Select loop filter component RZ and charge pump cur-

rent ICP based on BW and the VCO gain factor, KVCO.

BW (in Hz) is approximated by the following equation:

BW ≅

ICP •RZ •KVCO

2 • π •N

RZ =

2 • π •BW •N

ICP •KVCO

(12)

where KVCO is in Hz/V, ICP is in Amps, and RZ is in

Ohms.KVCOisobtainedfromtheVCOTuningSensitivity

in the Electrical Characteristics. Use ICP = 5.6mA to

lower in-band noise unless component values force

a lower setting.

4) Select loop filter components CI and CP based on BW

and RZ. A reliable second-order loop filter design can

be achieved by using the following equations for the

loop capacitors (in Farads).

CI =

3.5

2 • π •BW •RZ

(13)

CP =

7 • π •BW •RZ

(14)

Use FracNWizard to aid in the design of higher order

loop filters.

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