The 8T49N28X UFT devices have two-stage fractional output dividers. For the Fractional N (FracN) output dividers, the output divide ratio is given by:
Output Divide Ratio = (N.F)x2
N = Integer Part: 4, 5, ...(218-1)
F = Fractional Part: [0, 1, 2, ...(228-1)]/(228)
The output frequency is given by:
For integer operation of these outputs dividers, N = 3 is also supported.
The relevant register settings are given by the following two formulas:
N_Qx= Integer portion of the Output Divide Ratio
NFRAC_Qx = round(F*2^28)
In the following example, a 156.25MHz output and 10ppm offset will be used.
1) Setup 8T49N28X to generate a 156.26MHz output on a fractional output divider, derived from a VCO running at 3750MHz. The output divider is 24.
24=(N.F)*2 → N=12, F=0
2) Use the target ppm offset to calculate the required Noffset value using the following formula.
PPM_offset= (N0ppm-Noffset)/N0ppm *1e6
Noffset= N0ppm -(PPM_Offset/1e6* N0ppm)
Noffset = New N divider for ppm offset
N0ppm = 0ppm offset divider
For the example:
Noffset= 24 -(10/1e6* 24)= 23.99976
Next, decompose the effective output divider into the corresponding register settings:
Noffset =(N.F)x2= 23.99976
N_Qx= 11, F= 0.99988
NFRAC_Qx = round(F*2^28)= 268403244
Update the complete N_Qx and NFRAC_Qx register range using an I2C block write.
When measuring the frequency update, trigger the measurement equipment off the rising edge of the I2C write “STOP” bit.