250Hz noise from LFRX


#1

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 ()