I have BPSK flowgraphs that work fine in simulation as well as between two USRP boards at 1200 bps and sample rate of 48000 baud. In the transmitter flowgraph, the transmitter reads HDLC bits from a file, NRZI-encode them and sends them via the USRP. The vice-versa is repeated by the receiver flowgraph which is connected to another USRP. The task at hand is to modify the receiver to work with the ICOM 910H transceiver instead of the USRP. Below is the new setup. Transmission frequency on both the USRP and the ICOM transceiver is 436Mhz.
The modified transmitter is shown below. Since the ICOM radio has been set to USB, I have translated the signal by multiplying it with a complex sine wave with frequency 1500Hz. The receiver is modified as shown below. The signal is now received from the audio jack at 48000 baud. The FIR xlating filter shifts the signal down by 1500 and low-pass filter it (firdes.low_pass(1, samp_rate, 1300, 500) Unfortunately, this time, the receiver won't decode any HDLC packets. Actually, it seems like the Costas loop is not locking as shown by the constellation diagram below. Any ideas, suggestions or links to resolve this issue will be highly appreciated.