Costas Loop

A Costas loop carrier recovery module, great for synchronizing to BPSK, QPSK, and 8PSK. The Costas loop locks to the center frequency of a signal and downconverts it to baseband.

• When order=2: used for BPSK where the real part of the output signal is the baseband BPSK signal and the imaginary part is the error signal.
• When order=4: can be used for QPSK where both I and Q (real and imaginary) are output.
• When order=8: used for 8PSK.

The Costas loop can have two output streams:

1. stream 1 (required) is the baseband I and Q;
2. stream 2 (optional) is the normalized frequency of the loop

There is a single optional message input for a noise floor estimate used to calculate the SNR of a sample.

The block uses 'phase_est' tag generated by the Correlation Estimator. This helps to correct the phase offset which is crucial for synchronization process.

More details can be found online:

J. Feigin, "Practical Costas loop design: Designing a simple and inexpensive BPSK Costas loop carrier recovery circuit," RF signal processing, pp. 20-36, 2002.

Parameters

(R): Run-time adjustable

Loop Bandwidth (R)

Internal 2nd order loop bandwidth (~ 2pi/100)

Order

The loop order, either 2, 4, or 8, see above.

Use SNR

Use or ignore SNR estimates (from noise message port) in measurements; also uses tanh instead of slicing.

Example Flowgraph

See QPSK_Mod_and_Demod#Phase_and_Frequency_Correction

Source Files

C++ files

costas_loop_cc_impl.cc

Header files

costas_loop_cc_impl.h

Public header files

costas_loop_cc.h

Block definition

digital_costas_loop_cc.block.yml