LTC6948 datasheet(27/38 Pages) AD

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LTC6948

6948fa

For more information www.linear.com/LTC6948

DESIGN AND PROGRAMMING EXAMPLE

This programming example uses the DC1959 with the

LTC6948-2. Assume the following parameters of interest:

fREF = 100MHz at 7dBm into 50Ω

fSTEP = 50kHz

fRF = 1921.650MHz

From the Electrical Characteristics table:

fVCO = 3.080GHz to 4.910GHz

KVCO% = 4.7%Hz/V to 7%Hz/V

Determining Divider Values

Following the loop filter design algorithm, first determine

all the divider values. The maximum fPFDwhileinfractional

mode is less than 100MHz, so R must be greater than 1.

R = 2

Then, using Equations 5 and 7, calculate the following

values:

O = 2

fPFD = 50MHz

Then using Equation 6:

N+F =

2•1921.650MHz

50MHz

= 76.866

Therefore:

N = 76

F = 0.866

Then, from Equation 4,

NUM = 0.866 • 218 = 227017

Also, from Equation 1 and Tables 8 and 9 determine B:

B = 48 and BD[3:0] = h5

A calibration cycle takes 12 to 14 clock cycles of fCAL. This

gives a VCO calibration time of approximately:

tCAL ≅

fCAL

= 14•

fPFD

= 13.4µs

Selecting Filter Type and Loop Bandwidth

The next step in the algorithm is choosing the open loop

bandwidth. Select the minimum bandwidth resulting from

the below constraints.

1) The OSR must be at least 10 (sets absolute maximum

BW).

2) Theintegratedphasenoiseduetothermalnoiseshould

be minimized, neglecting any modulator noise.

3) The loop bandwidth must be narrow enough to ad-

equately filter the modulator’s quantization noise.

FracNWizard reports the loop bandwidths resulting from

each of the above constraints. The quantization noise con-

strained results vary according to the shape of the external

loop filter. FracNWizard reports an optimal bandwidth for

several filter types.

FracNWizard reports the thermal noise optimized loop

bandwidth is 211kHz. Filter 3 (fourth order response) has

a quantization noise constrained BW of 150kHz, making

it a good choice. Select Filter 3 and use the smaller of the

two bandwidths (150kHz) for optimal integrated phase

noise. Use Equation 11 to calculate OSR:

OSR =

50MHz

150kHz

= 333.3

applicaTions inForMaTion

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