Simple FM transmitter using gnuradio

Simple FM transmitter using gnuradio.

Tools needed:

• gnuradio 3.2
• grc (gnuradio companion)
• mpg123
• USRP

Mono FM transmission is very simple as all necessary blocks already exist in gnuradio. In this example we will show how to make a FM transmission from an mp3 stream for an Internet radio.

GRC Schema

Input file is a unix FIFO file that means that we need to feed it externally from the shell using these commands

Creation of the FIFO file (once):

mkfifo stream_32k.fifo

Decoding of an mp3 Internet radio station stream, conversion and output of raw linear samples to the FIFO:

mpg123 -r32000 -m -s  http://maxxima.mine.nu:8000 >stream_32k.fifo

Option "-m" is to convert stream to mono, "-r32000" perform sample rate conversion to 32kHz.

Python code

Code generated by GRC

#!/usr/bin/env python
##################################################
# Gnuradio Python Flow Graph
# Title: Simple FM Transmitter FIFO input
# Author: Mathias Coinchon
# Description: Simple FM Transmitter using FIFO
# Generated: Sat Jan  2 18:56:38 2010
##################################################

from gnuradio import blks2
from gnuradio import gr
from gnuradio.eng_option import eng_option
from gnuradio.wxgui import forms
from grc_gnuradio import usrp as grc_usrp
from grc_gnuradio import wxgui as grc_wxgui
from optparse import OptionParser
import wx

class FM_tx_FIFO(grc_wxgui.top_block_gui):

	def __init__(self):
		grc_wxgui.top_block_gui.__init__(self, title="Simple FM Transmitter FIFO input")

################################################## # Variables ################################################## self.samp_rate = samp_rate = 32000 self.FM_freq = FM_freq = 107900000

################################################## # Controls ################################################## _FM_freq_sizer = wx.BoxSizer(wx.VERTICAL) self._FM_freq_text_box = forms.text_box( parent=self.GetWin(), sizer=_FM_freq_sizer, value=self.FM_freq, callback=self.set_FM_freq, label="FM Frequency", converter=forms.float_converter(), proportion=0, ) self._FM_freq_slider = forms.slider( parent=self.GetWin(), sizer=_FM_freq_sizer, value=self.FM_freq, callback=self.set_FM_freq, minimum=87500000, maximum=108000000, num_steps=205, style=wx.SL_HORIZONTAL, cast=float, proportion=1, ) self.Add(_FM_freq_sizer)

################################################## # Blocks ################################################## self.blks2_wfm_tx_0 = blks2.wfm_tx( audio_rate=samp_rate, quad_rate=640000, tau=75e-6, max_dev=75e3, ) self.gr_file_source_0 = gr.file_source(gr.sizeof_short*1, "/home/mc/FM_demo/stream_32k.fifo", True) self.gr_multiply_const_vxx_0 = gr.multiply_const_vcc((32768, )) self.gr_multiply_const_vxx_1 = gr.multiply_const_vff((0.00003, )) self.gr_short_to_float_1 = gr.short_to_float() self.usrp_simple_sink_x_0 = grc_usrp.simple_sink_c(which=0, side="B") self.usrp_simple_sink_x_0.set_interp_rate(200) self.usrp_simple_sink_x_0.set_frequency(FM_freq, verbose=True) self.usrp_simple_sink_x_0.set_gain(0) self.usrp_simple_sink_x_0.set_enable(True)

################################################## # Connections ################################################## self.connect((self.gr_multiply_const_vxx_0, 0), (self.usrp_simple_sink_x_0, 0)) self.connect((self.gr_short_to_float_1, 0), (self.gr_multiply_const_vxx_1, 0)) self.connect((self.gr_file_source_0, 0), (self.gr_short_to_float_1, 0)) self.connect((self.gr_multiply_const_vxx_1, 0), (self.blks2_wfm_tx_0, 0)) self.connect((self.blks2_wfm_tx_0, 0), (self.gr_multiply_const_vxx_0, 0))

def set_samp_rate(self, samp_rate): self.samp_rate = samp_rate

def set_FM_freq(self, FM_freq): self.FM_freq = FM_freq self._FM_freq_slider.set_value(self.FM_freq) self._FM_freq_text_box.set_value(self.FM_freq) self.usrp_simple_sink_x_0.set_frequency(self.FM_freq)

if __name__ == '__main__':

parser = OptionParser(option_class=eng_option, usage="%prog: [options]") (options, args) = parser.parse_args() tb = FM_tx_FIFO() tb.Run(True)