LTC6946 3rd order filter design

Hi Experts:

We are implement LTC6946 and try to improve the system phase noise performance by design loop filter.

May I know if we could apply 3rd order filter to meet total noise under -100dBc? It seems we could only apply 2rd order filter in FracNWizard.

Could you please advise the 3rd order filter values ( Rz, Ci, Cp, R1, C2) and total noise under -100dBc if possible? Thanks in advance!

Our setting as below

Fref: 50MHz

Fstep: 0.5MHz

Frf: 4267MHz

Loop BW: 350kHz

R Div:3

N Div: 256

  • Hello.

    to meet total noise under -100dBc

    What frequency offsets is this noise referred? You can also convert this number in "jitter - rms", which is more typical parameter for integrated noise. You selected 3rd order filter in its "reverse variant". Is this wanted?

  • +1
    •  Analog Employees 
    on Jul 27, 2021 8:25 PM


    The LTC6946 is an integer part.  To achieve a step size (pfd frequency) of 0.5MHz with 50M reference requires RDIV=100 and NDIV=9254, which puts the in band phase noise in the -85dBc/Hz range.

    I would recommend using the LTC6948-4 Fractional part for this to allow for a faster PFD. Here is the plot and device settings for that configuration.  Notice the LTC6948 is below -100dBc/Hz.  You will need a low phase noise reference to meet the lower frequency offset numbers.  Reference Selection is discussed in the datasheet..

    In regards to the reverse loop filter, you do want to use that with the LTC694x parts.  The LTC694x has a high Kv, the reverse loop filter rolls off the noise from the R1 resistor in the 3rd and 4th order filters in FracNWizard.  Otherwise R1 resistor noise will be multiplied by the Kv and affect VCO PN performance slightly.