HMC778 Integer Mode Freq Tuning Equation

Hi there,

Equation 5 & 6 are correct - the problem arises in that the term prescaler or perhaps the variable 'fps' isn't defined as clearly as we might like. I typically refer to fixed frequency dividers (like the divide-by- 1 or divide-by-2) as prescalers since historically many PLL"s use 4/5 or 8/9 prescalers. In this case however the datasheet is referring to the N Divider as the prescaler. I'll explain.

The RFOUT Freq is always divided by 2 (inside the VCO) and brought out of the VCO as RFOUT /2  as indicated in the block diagram. The maximum input frequency of the N Divider is 4GHz so if RFOUT / 2 is greater than or equal to 4GHz then the internal divide-by-2 of the PLL, REG0x08[17] must be enabled or set. If RFOUT / 2 is less than 4GHz REG0x08[17] = 0 for divide-by-1 or bypass mode.

Fxtal = 10MHz

R = 40 so PFD (fpd) frequency = 10MHz / 40 = 250kHz

RFOUT = 10001 MHz   = fvco

RFOUT / 2 = 5000.5  which is >= 4.0GHz of N Divider so REG0x08[17] =1 (internal divide-by-2 enabled, k = 2)

Input frequency to N Divider (Fps) = 5000.5 / 2 = 2500.25MHz

N  = 2500.25MHz / 250kHz = 10001 = Nint  (Nfrac = 0)

fps = (10E+6 / 40) (10001) = 2500.25

Hope this helps,

Marty