[Discuss-gnuradio] BPSK receiver issue

[Vanush Vaswani]

Hi all,

I am using GR 3.6.5.1 to receive BPSK modulated data from a third
party RF chip / microcontroller combination and am having a strange
experience. I am implementing a very simple protocol by constantly
transmitting 0x7E constantly and then ASCII data triggered by a serial
connection to the microcontroller, all at 9.6kbps

With the  following configuration
FCDPP source sampling at 192KHz.
-> AGC
 -> Freq Xlate (bring near baseband)
 -> Costas loop (remove carrier)
-> RRC (48 taps, symbol rate = 9600)
-> PSK Demod (sps = 20 as 192/20 = 9.6kbps, everything else default)

I can see my 0x7E bitstream "square wave" in a scope sink.
When I pack the bits and send it to a file sink, I can see my 0x7E's
and also ascii data that I've typed in to the microcontroller serial
port. However - this is rare. Most of the time, I see the bytes of
rotated flag - 0x3F, 0xFC, 0x3F etc. I thought this was ok, I can
simply rotate the whole file by x bits to remake all the flags as
0x7E. But when I do this, my ascii text input is completely garbled,
with only a few letters showing up.

Can anyone give me a hint as to what is going on? I don't think my
transmitter is the issue, as when I get my flag bytes in the output
file, I can see my data.

vanush