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

From: w w
Subject: Re: [Discuss-gnuradio] DBPSK and Carrier Suppressed or Not?
Date: Thu, 26 Mar 2009 13:45:36 -0400

As another follow up, I fixed the garbled text problem by putting an AGC block on the RX side.

On Fri, Mar 20, 2009 at 10:57 AM, w w <address@hidden> wrote:
As a follow up....I did some popping and swapping and found that the RFX 900 board was bad.   I put in a RFX 2400 card and the the very pronouced carrier I saw before dissappeared and the signal transmitted out of the RFX brd looked liked the signal coming out of the modulator.   I was able send a text file over the RFX brd and rx it on the other side.   I guess there is nothing that can be done to bring the other board back online...I didn't see any pots on it.
Even though I was able to transmit a text file, parts of it was garbled on the other end.....any suggestions for tighting things up to take out the garble?  This test took place on one usrp with two RFX 2400 brds and using a cable to connect the transmit side of one card to the receive side of the other card.   I will next try between two usrps via coax and then OTA.
Also I had to take out the channel filter because the signal out of it would start up and then would die after about 10 to 20 seconds ( Channel filter = FFT filter -> Decimanation=1, TAPS = lowpass(1,2,1, .5, WIN_HAMMING))  With the RFX 900 I was tranmitting at 800Mhz and with the RFX 2400 I'm tranmitting at 2.5 GHz...Mod - DBPSK....Any thoughts or suggestions?

On Wed, Mar 18, 2009 at 5:23 PM, Bill Stevenson <address@hidden> wrote:

From: Eric Blossom <address@hidden>
To: w w <address@hidden>
Cc: address@hidden
Sent: Wednesday, March 18, 2009 4:30:04 PM
Subject: Re: [Discuss-gnuradio] DBPSK and Carrier Suppressed or Not?

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]

      -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)
                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
      -v, --verbose

    -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
                make sequential packets use different whitening
                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]
Hi, w w

You donot need to add other channel filters because there is a rrc filter as well as a channel filter in DEMOD.
Pleas note that: you should decrease the TX power before connecting them via coax.

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