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 ()