Hello everyone!

Hope that you are staying healthy and at a safe place, despite the ongoing lockdown.

I implement  Digital Costas Loop for phase and frequency shifts correction of BPSK  in space communication.

I have used the standard realisation of the loop

The signal is BPSK with variable phase and frequency shifts under low SNR. I have noticed that I can get a delay of the signal ( samples) between a filter and the Costas loop.

Could someone explain this phenomenon? What can I use to eliminate it?

Wishing you all the best!

Fank L

The LPF FIR filters have delays proportional to the length to the peak of the impulse response.  e.g., if it is a symmetric impulse response, the delay is N/2.   As long as that delay is much less than the time constant of the loop response, which it typically is, it doesn't cause any problems.  It is not usually necessary to do anything about the filter delay.