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[Discuss-gnuradio] FIR filter , Clock recovery and Quadrature demodulati

From: M. Awais Arshad
Subject: [Discuss-gnuradio] FIR filter , Clock recovery and Quadrature demodulation
Date: Sun, 26 Apr 2009 22:30:01 +0500

     The following are some code segments of gsm-scan.py basically used for gsm data reception. I have a few questions about the difference between input_rate and sps,

clock_rate = 64e6
 #set the default input rate, we will check with the USRP if it is being used
input_rate = clock_rate / options.decim
gsm_symb_rate = 1625000.0 / 6.0
sps = input_rate/gsm_symb_rate

# configure the processing blocks
# configure channel filter
filter_cutoff = 145e3 #135,417Hz is GSM bandwidth
filter_t_width = 10e3

#Only DSP adjust for offset on datafile, adjust tuner for USRP
#TODO: see if we can change this offset at runtime based on freq detection
if options.inputfile:
offset = self.offset
offset = 0.0

filter_taps = gr.firdes.low_pass(1.0, input_rate, filter_cutoff, filter_t_width, gr.firdes.WIN_HAMMING)
self.filter = gr.freq_xlating_fir_filter_ccf(1, filter_taps, offset, input_rate)

               # configure demodulator
# adjust the phase gain for sampling rate
self.demod = gr.quadrature_demod_cf(sps);

#configure clock recovery
gain_mu = 0.01
gain_omega = .25 * gain_mu * gain_mu # critically damped
self.clocker = gr.clock_recovery_mm_ff( sps,
0.5, #mu
0.3) #omega_relative_limit,

self.burst = gsm.burst_ff()
self.connect(self.filter, self.demod, self.clocker, self.burst)

. Can anyone please give me an idea why " gr.freq_xlating_fir_filter_ccf " is used here and 'input_rate' is used as an argument in stead of 'sps'. Whereas, in quadrature demod and Clock_recovery_mm 'sps' is used for sampling rate and why clock recovery is used after demodulation block ? 
Are I and Q samples are flowing out of clocker block or are they the encoded bits ?  



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