PLL no lock

Hello everyone,

I made a simple PLL simulink model (link bellow). When the reference frequency is 2010Hz and the VCO is nominally at 2000Hz, the PLL locks successfully. However when the reference is 2020Hz (or more) the PLL does not lock but oscillates. I tried varying several of the model parameters but still no clue why this is so? Any idea or explanation would be appreciated.

http://www.sendspace.com/file/dddy8s

Greetings, PO

[Poky Poky]

No ideas? OK I went a little bit further and figured out that actually the loop filter should be a first order. Then everything with the lock range is as expected. Does anyone has an idea why the loop filter needs to be first order? I am new to this stuff, so simple explanation would be greatly appreciated.

Greeting,PO