[Discuss-gnuradio] Frequency recovery of known signal

Hi ,
I have a

​two ​

known signal with some frequency offset

​between them ,

 without time delay

​ difference​

, i can conjugate-multiply two signals and get a tone proportional to offset value (pls find attached

​picture

 for flow-graph

​ i used​

)

​ ​

.

​

I think i need some sort of PLL and NCO to correct frequency offset and lock to known signal .​

​As i know , it's impossible in gnu radio flow graph to make closed loop systems , but how i can implement that in one block ? are there any example block that implemented some thing like that ?

Thank you .