At present, i want to use your company's phase-locked loop chip ADF4356 to generate a fixed frequency of 137.37MHz, reference frequency is 50MHz.

When the parameter MOD2 is calculated from the datasheet of the ADF4356: MOD2=fPDF/GCD(fPDF,fCHSP), (datasheet page 12)

The question is: What we need to output is a fixed frequency. How is the value of fCHSP determined? How to determine the value of MOD2 and FRAC2?

Hi,

fCHSP can be assumed as '1' initially; which will give MOD2=fPFD. FRAC2 and MOD2 can be divided down in the same ratio afterwards.

This can be formalized as follows:

N = VCO_Freq / PFD_Freq;

INT = (uint)N;

FRAC1 = (uint)((N - INT) * MOD1);

residue = MOD1 * (N - (INT + FRAC1 / MOD1));

MOD2 = (PFDFreq * 1000000); // Just convert to Hz where "PFDFreq" is in MHz.

if (MOD2 > MOD2max) MOD2 = MOD2max;

FRAC2 = residue * MOD2;

FRAC2 = Math.Round(FRAC2, 0, MidpointRounding.AwayFromZero);

// optional steps:

uint D = (uint)GCD(MOD2, FRAC2); //greatest common divisor

MOD2 = MOD2 / D;

FRAC2 = FRAC2 / D;

Regards,

Kazim