Re: [Discuss-gnuradio] DBPSK and Carrier Suppressed or Not?

[Eric Blossom]

On Wed, Mar 18, 2009 at 02:55:41PM -0400, w w wrote:
> I'm putting together a DBPSK Tx and Rx and running into problems when the
> modulated signal hits the USRP.   When I receive the signal on the USRP
> there's  a very pronouced carrier....I've confirmed with a spectrum analyser
> that the signal on the FFT is the one that I'm recieving.   It seems to me
> that the sidebands should be close to the highest amplitute and the carrier
> should be suppressed.....is this correct?  Shouldn't the output of the DPSK
> modulator FFT plot look very similar to the FFT plot on the RX side of the
> usrp?   I'm modeling the block connections after the benchmark samples in
> the digital example section.   I put a channel filter inline with the usrp
> and the dpsk demod block just like the example.....do I need to put more
> filtering in place to suppress the carrier?  If so, do you have any
> examples?
> 
>  If I take the output of the modulator and go directly to the demod thru the
> channel filter, data flows just fine(bypassing the usrp).
> 
> I'm only using one usrp and taking the output of the TX via coax and going
> into the RX side...that shouldn't make a difference should it?
> 
> Any thoughts?
> 
> Cheers,
> Jody

Before writing your own, did you take a look at the implementation of
dbpsk that's already in the tree?

See the -m dbpsk option below


    $ ./benchmark_tx.py --help
    Usage: benchmark_tx.py [options]

    Options:
      -h, --help            show this help message and exit
      -m MODULATION, --modulation=MODULATION
                            Select modulation from: cpm, d8psk, qam8, dbpsk,
                            dqpsk, gmsk [default=gmsk]
      -s SIZE, --size=SIZE  set packet size [default=1500]
      -M MEGABYTES, --megabytes=MEGABYTES
                            set megabytes to transmit [default=1.0]
      --discontinuous       enable discontinous transmission (bursts of 5 
packets)
      --from-file=FROM_FILE
                            use file for packet contents
      -f FREQ, --freq=FREQ  set Tx and/or Rx frequency to FREQ [default=none]
      -r BITRATE, --bitrate=BITRATE
                            specify bitrate.  samples-per-symbol and 
interp/decim
                            will be derived.
      -w WHICH, --which=WHICH
                            select USRP board [default=0]
      -T TX_SUBDEV_SPEC, --tx-subdev-spec=TX_SUBDEV_SPEC
                            select USRP Tx side A or B
      --tx-amplitude=AMPL   set transmitter digital amplitude: 0 <= AMPL < 32768
                            [default=12000]
      -v, --verbose

      Expert:
        -S SAMPLES_PER_SYMBOL, --samples-per-symbol=SAMPLES_PER_SYMBOL
                            set samples/symbol [default=none]
        --tx-freq=FREQ      set transmit frequency to FREQ [default=none]
        -i INTERP, --interp=INTERP
                            set fpga interpolation rate to INTERP [default=none]
        --log               Log all parts of flow graph to file (CAUTION: lots 
of
                            data)
        --use-whitener-offset
                            make sequential packets use different whitening
        --excess-bw=EXCESS_BW
                            set RRC excess bandwith factor [default=0.35] (PSK)
        --no-gray-code      disable gray coding on modulated bits (PSK)
        --bt=BT             set bandwidth-time product [default=0.35] (GMSK)
        -B FUSB_BLOCK_SIZE, --fusb-block-size=FUSB_BLOCK_SIZE
                            specify fast usb block size [default=0]
        -N FUSB_NBLOCKS, --fusb-nblocks=FUSB_NBLOCKS
                            specify number of fast usb blocks [default=0]