ADF4156 won't lock with spur dithering active

I am using ADF4156 and it is causing noise on another circuit. If I turn on dithering (0x60000000 register 3) the noise goes away. But if I do it as part of the chip configuration it wont lock. But if I wait a short period of time and then turn it on it will lock is this normal? I do not see anything in the data sheet that addresses this and I am worried what it will do across temperature. Yes I write the registers in order 4,3,2,1,0. But for the time delayed write to turn on dithering I only write to register 3.


In all candor, I’m not familiar enough with the '4156 to speak to that question confidently; the folks at ADI would probably be better equipped to speak to the matter.

Product Specialist wants the invoice or customer file, as ADI will ask us this.

Our customer number is 245560
This product has been in production over 10 years so I am sure we have ordered this part through you at some point.

Hi user55,

There are a few things that you could try first.

  1. Test if reducing the charge pump current (R2) will lock.
  2. Try different frequency setting (R0). Maybe the charge pump is not working at the linear region.
  3. There seems to be “Analog lock detect”-signal available for the MUXOUT. Probing that with an oscilloscope
    could perhaps tell what is going on there.

Heke, Asamalab

I abandoned the dithering because it added too much noise to this receiver. Tri-stating the MUXOUT helped quite a bit. Not doubling the Ref Osc moved the spur out of band for the adjacent circuitry which is all I care about right now.

1 Like