BPSK receiver issue

GNU Radio
1

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

2

On Mon, Nov 4, 2013 at 10:25 AM, Vanush V. [email protected]
wrote:

→ AGC
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

The PSK demod has the FLL, Costas loop, and RRC filter in it. That’s
what each of those bandwidth settings are for (frequency, phase, and
timing). But also, going in at 20 sps is really high. You should put a
channel filter in front that both removes any extra noise and
downsamples (by a factor of 5 or 10 to get to 4 or 2 sps).

Tom