Re: [Discuss-gnuradio] BPSK receiver issue

[Tom Rondeau]

On Mon, Nov 4, 2013 at 10:25 AM, Vanush Vaswani <address@hidden> wrote:
> 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

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