Hello everyone. I am working on Amplitude demodulation. I am using the
attached python code. I have some doubts as to why particular frequencies
are chosen. Firstly, in the code
*channel_coeffs = gr.firdes.low_pass (1.0, if_rate, 8000, 1000,
gr.firdes.WIN_HANN)*
Why a cut off frequency of 8000Hz is chosen?
Any Particular reason why a transition bandwidth of 1000Hz is chosen?
Secondly,
*volumecontrol = gr.multiply_const_ff(.003)*
Why are we multiplying by a factor of 0.003?
Thirdly,
*audio_coeffs = gr.firdes.low_pass (1.0, demod_rate, 9e3, 4e3,
gr.firdes.WIN_HANN)*
Why a cut off frequency of 9000Hz is chosen?
Any Particular reason why a transition bandwidth of 4000Hz is chosen?
Thanks in advance..
#!/usr/bin/env python
#
# Demodulate an AM signal from the USRP or a recorded file.
# The file format must be 256 ksps, complex data.
#
from gnuradio import gr, eng_notation
from gnuradio import audio
#from gnuradio import usrp
from gnuradio.eng_option import eng_option
from optparse import OptionParser
import sys
import math
#
# return a gr.flow_graph
#
def build_graph (file_name, freq_offset_khz):
# Center IF frequency of down converter.
IF_freq = 10.7e6
# Sampling rate of ADC on USRP
adc_rate = 64e6
# Decimation rate from USRP ADC to IF.
usrp_decim = 250
# Calculate the sampling rate of the USRP and capture file.
if_rate = adc_rate // usrp_decim
# Decimate the IF sampling rate down by 4 to 64 ksps
if_decim = 4
demod_rate = if_rate / if_decim
audio_decimation = 2
audio_rate = demod_rate / audio_decimation
# This is a flow graph that has an input (capture file) and output (audio
channel).
fg = gr.flow_graph ()
# Signal source is assumed to be 256 kspb / complex data stream.
if (file_name == "usrp"):
# Select USRP channel 0
mux = 0xf0f0f0f0
# Set the input mux to channel 0, real samples and decimation rate.
src = usrp.source_c (0, usrp_decim, 1, mux)
# Tune to the desired IF frequency. Our down converter inverts the RF
spectrum, so we'll invert the frequency.
src.set_rx_freq (0, -IF_freq)
actual_IF_freq = src.rx_freq(0)
src.set_pga (0, 20)
else:
src = gr.file_source (gr.sizeof_gr_complex, file_name)
# Channelize the signal of interest.
channel_coeffs = gr.firdes.low_pass (1.0, if_rate, 8000, 1000,
gr.firdes.WIN_HANN)
print "len(channel_coeffs) = ", len(channel_coeffs)
# Tune to the desired frequency.
ddc = gr.freq_xlating_fir_filter_ccf (if_decim, channel_coeffs,
freq_offset_khz, if_rate)
# Demodule with classic sqrt (I*I + Q*Q)
magblock = gr.complex_to_mag()
# Scale the audio
volumecontrol = gr.multiply_const_ff(.003)
# Low pass filter the demodulator output
audio_coeffs = gr.firdes.low_pass (1.0, demod_rate, 9e3, 4e3,
gr.firdes.WIN_HANN)
print "len(audio_coeffs) = ", len(audio_coeffs)
# input: float; output: float
audio_filter = gr.fir_filter_fff (audio_decimation, audio_coeffs)
# sound card as final sink
audio_sink = audio.sink (int (audio_rate))
# now wire it all together
fg.connect (src, ddc)
fg.connect (ddc, magblock)
fg.connect (magblock, volumecontrol)
fg.connect (volumecontrol, audio_filter)
fg.connect (audio_filter, (audio_sink, 0))
return fg
def main (args):
nargs = len (args)
if nargs == 2:
filename = args[0]
freq_offset_khz = float (args[1]) * 1e3
else:
sys.stderr.write ("usage: am_rcv_file {file_name,'usrp'}
freq_offset_KHz\n")
sys.exit (1)
fg = build_graph (filename, freq_offset_khz)
fg.start ()
raw_input ('Press Enter to quit: ')
fg.stop ()
if __name__ == '__main__':
main (sys.argv[1:])
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