ADF4159 Charge Pump Polarity Issue

Hi, The datasheet does not explain this but for a positive tuning VCO (so increasing vtune = increasing frequency) the CP polarity would need to be programmed positive only for a passive or non-inverting active loop filter. The eval board has uses an inverting active loop filter so the PFD polarity is flipped. So you would need to use a negative PFD polarity with the eval board.

Have you designed/populated your own loop filter? Can you send the ADIsimPLL file if so. What VCO are you using?

This is my loop filter, the polarity is positive, and, for single-frequency output, the polarity of the charge pump should not affect the output.

Hi, The PFD polarity still needs set correctly even for fixed frequency applications.

To confirm:

If using a noninverting loop filter and a VCO with a positive tuning slope, set the PD polarity to positive.
If using an inverting loop filter and a VCO with a negative tuning slope, set the PD polarity to positive.
If using a noninverting loop filter and a VCO with a negative tuning slope, set the PD polarity to negative.
If using an inverting loop filter and a VCO with a positive tuning slope, set the PD polarity to negative.

Your loop filter design looks acceptable with 2.5mA charge pump current but above software screenshot it looks like you have programmed for 5mA charge mump current. With the components you have selected this gives a phase margin of 35 degrees. Phase margin should be at least 45 degrees for stability, so it would be better to program CP current = 2.5mA.

What op amp are you using in the active loop filter? 

Regards,

Alex

This active loop filter is non-inverting and a VCO with a positive tuning, the phase margin is 45 degrees, and the charge pump current settings areare tried at 2.5, 3.75, 5 mA .Both AD820 and ths4031 op amps have been used, and the effect is the same。
On the left side of the attached image is the loop filter details

sorry，I tested the op amp alone，it is inverting。Now I want to know how my near end noise can't go down, no matter how to adjust the charge pump current and leakage current

Hi, what reference are you using?

I should also add, the AD820 may not be a good choice here since its bandwidth is likely not sufficient for such a wide LBW filter. Generally the rule is GBP (gain bandwidth product of the op amp) should be at least 10*LBW. OP184 or ADA4084-1 might be a better choice

I should also add, the AD820 may not be a good choice here since its bandwidth is likely not sufficient for such a wide LBW filter. Generally the rule is GBP (gain bandwidth product of the op amp) should be at least 10*LBW. OP184 or ADA4084-1 might be a better choice

Thanks for your suggestions, the reference input signal provided by the signal source used in the previous picture

Hi，I did a comparison test according to your suggestion and the results are as follows 。

This picture is the result of testing with AD820

This picture is the result of testing with ADA4084-1，which is better than the previous result