Issue with ADF4159 + HMC1161 PLL Setup: Vtune Stuck at +15V, No Lock, and Op-Amp Acting as Comparator

I am using the ADF4159 PLL with an external HMC1161 VCO and an active loop filter designed using ADISimPLL. The loop filter parameters are:

Loop Bandwidth: 735 kHz

Phase Margin: 48.2°

Components: R1=220Ω, C1=33pF, R2=1.2kΩ, C2=560pF, R3=1kΩ, C3=15pF, C4=68pF

Symptoms:

Vtune is stuck at +15V (op-amp positive rail).

CPOUT voltage is 0.821V when connected, and OPBIAS is set to 1.421V.

When CPOUT is disconnected:

If inverting input voltage 1.42V, Vtune = 0V.

LD pin remains low, indicating no lock.

Troubleshooting Done:

Verified ADF4159 register settings:
ICP=2.5 mA , PFD frequency = 100 MHz, N-divider set for HMC1161’s 9.1–10.2 GHz range.

Checked op-amp functionality: Works as a comparator but not as an integrator.

Confirmed loop filter connections: No open traces or cold joints.

Tested VCO independently: Works fine with manual tuning.

Questions:
Why is the op-amp acting as a comparator instead of an integrator?

Could the ADF4159 charge pump be misconfigured? Are there specific register settings I should double-check?

Are there any common pitfalls in the ADF4159 + HMC1161 + active loop filter setup that I might be missing?

How can I ensure the loop filter is stable with the given phase margin (48.2°)? Should I add a damping resistor or adjust component values?