[Discuss-gnuradio] 250Hz noise from LFRX

[Eric R Menendez]

Hello All--

I am attempting to build a CB radio (26-27 MHz) receiver using the USRP
with the LFRX daughterboard. Currently, a "rubber-ducky" CB antenna is
connected directly to the LFRX and the DDC on the USRP converts the
received signal to baseband.

At the receiver output, I am seeing a roughly 250Hz sinusoid. I am not
sure where this is coming from; if I attach a scope_sink directly to
usrp.source_c(), I still see the unwanted signal. The signal remains no
matter what CB channel I listen to.

I also tried using the BASIC Rx with a 30MHz lowpass filter, with the
same results.

Any ideas as to what I can do to eliminate this noise? My code is below.

Thanks for any help!

Sincerely,
Eric Menendez

#!/usr/bin/env python

from gnuradio import gr, gru, eng_notation, optfir
from gnuradio import audio
from gnuradio import usrp
from gnuradio import blks
from gnuradio.eng_option import eng_option
from gnuradio.wxgui import slider, powermate
from gnuradio.wxgui import stdgui, fftsink, form, scopesink
from optparse import OptionParser
import usrp_dbid
import sys
import math
import wx

def pick_subdevice(u):
    """
    The user didn't specify a subdevice on the command line.
    Try for one of these, in order: TV_RX, BASIC_RX, whatever is on side
A.

    @return a subdev_spec
    """
    return usrp.pick_subdev(u, (usrp_dbid.LF_RX,
                                usrp_dbid.BASIC_RX))


class cb_rx_graph (stdgui.gui_flow_graph):
    def __init__(self,frame,panel,vbox,argv):
        stdgui.gui_flow_graph.__init__ (self,frame,panel,vbox,argv)

        parser=OptionParser(option_class=eng_option)
        parser.add_option("-R", "--rx-subdev-spec", type="subdev",
default=None,
                          help="select USRP Rx side A or B (default=A)")
        parser.add_option("-c", "--channel", type="int", default=1,
                          help="set channel to CHANNEL",
metavar="CHANNEL")
        parser.add_option("-g", "--gain", type="eng_float", default=40,
                          help="set gain in dB (default is midpoint)")
        parser.add_option("-V", "--volume", type="eng_float",
default=None,
                          help="set volume (default is midpoint)")
        parser.add_option("-S", "--squelch", type="eng_float",
default=30.0,
                          help="set squelch level in dB (default is
30)")
        parser.add_option("-O", "--audio-output", type="string",
default="",
                          help="pcm device name.  E.g., hw:0,0 or
surround51 or /dev/dsp")

        (options, args) = parser.parse_args()
        if len(args) != 0:
            parser.print_help()
            sys.exit(1)
        
        self.frame = frame
        self.panel = panel
        
        self.vol = 0
        self.freq = 0

        # build graph
        
        self.u = usrp.source_c()                    # usrp is data
source

        adc_rate = self.u.adc_rate()                # 64 MS/s
        usrp_decim = 200
        self.u.set_decim_rate(usrp_decim)
        usrp_rate = adc_rate / usrp_decim           # 320 kS/s
        chanfilt_decim = 1
        demod_rate = usrp_rate / chanfilt_decim
        audio_decimation = 10
        audio_rate = demod_rate / audio_decimation  # 32 kHz

        if options.rx_subdev_spec is None:
            options.rx_subdev_spec = pick_subdevice(self.u)

        self.u.set_mux(usrp.determine_rx_mux_value(self.u,
options.rx_subdev_spec))
        self.subdev = usrp.selected_subdev(self.u,
options.rx_subdev_spec)
        print "Using RX d'board %s" % (self.subdev.side_and_name(),)

        chan_filt_coeffs = optfir.low_pass (1,           # gain
                                            usrp_rate,   # sampling rate
                                            4e3,         # passband
cutoff
                                            5e3,         # stopband
cutoff
                                            0.1,         # passband
ripple
                                            60)          # stopband
attenuation
        #print len(chan_filt_coeffs)
        self.chan_filt = gr.fir_filter_ccf (chanfilt_decim,
chan_filt_coeffs)
        
        self.squelch = gr.simple_squelch_cc(0, 0.01)
        
        audio_coeffs = gr.firdes.low_pass (1.0,         # gain
                                           demod_rate,  # sampling rate
                                           4e3,         # cutoff freq
                                           1e3,         # width of
transition band
                                           gr.firdes.WIN_HANN)
                                        
        self.guts = gr.fir_filter_ccf (audio_decimation, audio_coeffs)
        
        self.converter = gr.complex_to_float()
        
        self.DCnotch = gr.iir_filter_ffd((1, -1), (1, -0.98))
        
        self.agc = gr.agc_ff(1e-4, 1, 1)
        
        self.volume_control = gr.multiply_const_ff(self.vol)

        # sound card as final sink
        audio_sink = audio.sink (int (audio_rate),
                                 options.audio_output,
                                 False)  # ok_to_block
        
        # now wire it all together
        self.connect (self.u, self.chan_filt, self.squelch, self.guts,
                      self.converter, self.DCnotch, self.agc,
                      self.volume_control, audio_sink)

        self._build_gui(vbox, usrp_rate, demod_rate, audio_rate)

        if options.gain is None:
            # if no gain was specified, use the mid-point in dB
            g = self.subdev.gain_range()
            options.gain = float(g[0]+g[1])/2

        if options.volume is None:
            g = self.volume_range()
            options.volume = float(g[0]+g[1])/2
            
        # set initial values

        self.set_gain(options.gain)
        self.set_vol(options.volume)
        self.set_squelch(options.squelch)
        if not(self.set_channel(options.channel)):
            self._set_status_msg("Failed to set initial frequency")


    def _set_status_msg(self, msg, which=0):
        self.frame.GetStatusBar().SetStatusText(msg, which)


    def _build_gui(self, vbox, usrp_rate, demod_rate, audio_rate):

        def _form_set_channel(kv):
            return self.set_channel(kv['channel'])
        
        def _form_set_squelch(kv):
            self.set_squelch(kv['squelch'])
            return True

        self.show_chan_filt_fft = False
        if self.show_chan_filt_fft:
            self.src_fft = fftsink.fft_sink_c (self, self.panel,
title="Post Chan Filt",
                                               fft_size=512,
sample_rate=usrp_rate)
            self.connect (self.chan_filt, self.src_fft)
            vbox.Add (self.src_fft.win, 4, wx.EXPAND)

        self.show_demod_fft = False
        if self.show_demod_fft:
            post_filt_fft = fftsink.fft_sink_f (self, self.panel,
title="Post Demod", 
                                                fft_size=1024,
sample_rate=usrp_rate,
                                                y_per_div=10,
ref_level=0)
            self.connect (self.converter, post_filt_fft)
            vbox.Add (post_filt_fft.win, 4, wx.EXPAND)

        self.show_agc_scope = True
        if self.show_agc_scope:
            converter = gr.complex_to_float()
            test_scope = scopesink.scope_sink_f (self, self.panel,
title="Post AGC",
                                                 sample_rate=usrp_rate,
v_scale=1)
            self.connect(self.u, converter, test_scope)
            vbox.Add (test_scope.win, 4, wx.EXPAND)

       
        # control area form at bottom
        self.myform = myform = form.form()

        hbox = wx.BoxSizer(wx.HORIZONTAL)
        hbox.Add((5,0), 0)
        myform['channel'] = form.float_field(
            parent=self.panel, sizer=hbox, label="Channel", weight=1,
            callback=myform.check_input_and_call(_form_set_channel,
self._set_status_msg))

        hbox.Add((5,0), 0)
        myform['channel_slider'] = \
            form.quantized_slider_field(parent=self.panel, sizer=hbox,
weight=3,
                                        range=(1, 40, 1),
                                        callback=self.set_channel)
        hbox.Add((5,0), 0)
        vbox.Add(hbox, 0, wx.EXPAND)

        hbox = wx.BoxSizer(wx.HORIZONTAL)
        hbox.Add((5,0), 0)

        myform['volume'] = \
            form.quantized_slider_field(parent=self.panel, sizer=hbox,
label="Volume",
                                        weight=3,
range=self.volume_range(),
                                        callback=self.set_vol)
        hbox.Add((5,0), 1)

        myform['gain'] = \
            form.quantized_slider_field(parent=self.panel, sizer=hbox,
label="Gain",
                                        weight=3,
range=self.subdev.gain_range(),
                                        callback=self.set_gain)
        hbox.Add((5,0), 0)
        vbox.Add(hbox, 0, wx.EXPAND)
        
        hbox = wx.BoxSizer(wx.HORIZONTAL)
        hbox.Add((5,0), 0)
        myform['squelch'] = form.float_field(
            parent=self.panel, sizer=hbox, label="Squelch", weight=1,
            callback=myform.check_input_and_call(_form_set_squelch))

        hbox.Add((5,0), 0)
        myform['squelch_slider'] = \
            form.quantized_slider_field(parent=self.panel, sizer=hbox,
                                        weight=3,
range=self.squelch.squelch_range(),
                                        callback=self.set_squelch)
        hbox.Add((5,0), 0)
        vbox.Add(hbox, 0, wx.EXPAND)


    def set_squelch (self, sql):
        g = self.squelch.squelch_range()
        sql = max(g[0], min(g[1], sql))
        self.squelch.set_threshold(sql)
        self.myform['squelch'].set_value(sql)
        self.myform['squelch_slider'].set_value(sql)

    def set_vol (self, vol):
        g = self.volume_range()
        self.vol = max(g[0], min(g[1], vol))
        self.volume_control.set_k(10**(self.vol/10))
        self.myform['volume'].set_value(self.vol)
        self.update_status_bar ()
                                      
    
    def set_channel(self, channel):
        # CB Channels
        channels = [26.965e6,
                    26.975e6,
                    26.985e6,
                    27.005e6,
                    27.015e6,
                    27.025e6,
                    27.035e6,
                    27.055e6,
                    27.065e6,
                    27.075e6,
                    27.085e6,
                    27.105e6,
                    27.115e6,
                    27.125e6,
                    27.135e6,
                    27.155e6,
                    27.165e6,
                    27.175e6,
                    27.185e6,
                    27.205e6,
                    27.215e6,
                    27.225e6,
                    27.255e6,
                    27.235e6,
                    27.245e6,
                    27.265e6,
                    27.275e6,
                    27.285e6,
                    27.295e6,
                    27.305e6,
                    27.315e6,
                    27.325e6,
                    27.335e6,
                    27.345e6,
                    27.355e6,
                    27.365e6,
                    27.375e6,
                    27.385e6,
                    27.395e6,
                    27.405e6 ]
        self.myform['channel'].set_value(channel)          # update
displayed value
        self.myform['channel_slider'].set_value(channel)   # update
displayed value

        return self.set_freq(channels[int(channel - 1)])
    
    def set_freq(self, target_freq):
        """
        Set the center frequency we're interested in.

        @param target_freq: frequency in Hz
        @rypte: bool

        Tuning is a two step process.  First we ask the front-end to
        tune as close to the desired frequency as it can.  Then we use
        the result of that operation and our target_frequency to
        determine the value for the digital down converter.
        """
        r = usrp.tune(self.u, 0, self.subdev, target_freq)
        
        if r:
            self.freq = target_freq
            self.update_status_bar()
            self._set_status_msg("OK", 0)
            print "r.baseband_freq =",
eng_notation.num_to_str(r.baseband_freq)
            print "r.dxc_freq      =",
eng_notation.num_to_str(r.dxc_freq)
            print "r.residual_freq =",
eng_notation.num_to_str(r.residual_freq)
            print "r.inverted      =", r.inverted
            return True

        self._set_status_msg("Failed", 0)
        return False

    def set_gain(self, gain):
        self.myform['gain'].set_value(gain)     # update displayed value
        self.subdev.set_gain(gain)

    def update_status_bar (self):
        msg = "Volume:%r" % self.vol
        self._set_status_msg(msg, 1)
        if self.show_chan_filt_fft:
            self.src_fft.set_baseband_freq(self.freq)

    def volume_range(self):
        return (-20.0, 0.0, 0.5)
        

if __name__ == '__main__':
    app = stdgui.stdapp (cb_rx_graph, "CB Rx")
    app.MainLoop ()