I have implemented a CPFSK transmitter succesfully using GNU RADIO.
The blocks are PRBS Source->Scrambler (G3RUH)-> CPFSK Mod-> UHD Sink. The reference HW receiver receives and demodulates the signal quite well.
Then I remove the PRBS Source and place a UDP Source+UART block as the data source.
When the connected client sends continous data, everything is fine (except at the start of transmission where the receiver needs time to lock to the incoming signal).
When I send some bytes from the client and stop sending, the transmission becomes burst-like and the receiver can demodulate pretty much nothing.
I found the culprit which is the Scrambler, when there is no input to the Scrambler, there is no output, so there is no RF output.
I have also developed a FPGA Continous CPFSK transmitter in the past and the scrambler there was producing output every clock cycle as there was 0 in its Input, so that guaranteed continous output.
The real problem is: What should I do to force some Input bits to the Scrambler, even there is no real data?