Hi All, i tried out to delete some antialiasing and the middle signal of the FFT-Plot. Therefor i use this filter: ### VARIABLE ### #At adc_rate=64MS/s and decim=64 we get an usrp_rate of 1 MS/s self.decim=64 self.usrp_rate=1e6 ### FILTER ### self.firdes_filt_coeffs = gr.firdes.low_pass(1, self.usrp_rate, self.usrp_rate*0.5, self.usrp_rate*0.001, gr.firdes.WIN_RECTANGULAR) self.chan_filt = gr.freq_xlating_fir_filter_ccf (self.chanfilt_decim, self.firdes_filt_coeffs, self.usrp_freq, self.usrp_rate) ### SOURCE ### As source i use: self.usrp_source = usrp.source_c(which=0, decim_rate=self.decim) ### SINNK ### And as sink i use an FFT-sink-Plot: self.wxgui_fftsink2 = fftsink2.fft_sink_c( self.GetWin(), baseband_freq = self.usrp_freq, y_per_div = 10, y_divs = 8, ref_level = 0, sample_rate = self.usrp_rate, fft_size = 512*2, fft_rate = 30, ref_scale=11885000.0,# 11885000.0 oder 32768.0 ? average = True, avg_alpha = None, title = "FFT Plot", size=(1024, 600), peak_hold = False, ) ### CONNECT ### Then i connect this all together: self.connect((self.usrp_source, 0), (self.chan_filt, 0)) self.connect((self.chan_filt, 0), (self.wxgui_fftsink2, 0)) But the antialiasing still exist and the middle signal of the FFT-Plot too. There comes the next problem, that my spectrum in the FFT-Plot will be shifted. Can i use the usrp_rate in the source as well as for the xlating element ? Or do i have to use another decimation value ? Which rate must i use for the FFT-Plot ? Why is the low cut off frequency limited to usrp_rate/2 ? From which point starts the low cut-off frequency and to which direction ? Why must i use the transition width>0 by the WIN_RECTANGULAR ? How can the shifting of the spectrum in my FFT-Plot corrected ? Any Hints ? Regards Markus

on 2009-03-23 14:37

on 2009-03-23 17:55

See 'usrp_am_mw_rcv.py' for an example. Look for the code relating to these lines in particular: if self.use_IF: # Turn If to baseband and filter. self.chan_filt = gr.freq_xlating_fir_filter_ccf (chanfilt_decim, chan_filt_coeffs, self.IF_freq, usrp_rate) Paul Mathews

on 2009-03-23 19:43

Paul Mathews schrieb: > See 'usrp_am_mw_rcv.py' for an example. Look for the code relating to these > lines in particular: > > if self.use_IF: > # Turn If to baseband and filter. > self.chan_filt = gr.freq_xlating_fir_filter_ccf (chanfilt_decim, > chan_filt_coeffs, self.IF_freq, usrp_rate) hI pAUL, i used this example as possible as i can, but this frequency translating filter has one important difference to my application, the FFT-Plot-sink which is connected to this xlating filter is staticly. I have a slider to change my baseband to the wished spectrum, so my xlating filter it is dynamically. I still doesn't know whether to use a low_pass is usefull or a band_pass. The low_pass doesn't delete the middle signal in the FFT-Plot in my tryings. Further on i doesn't know which conditions have to be made to let my application work fluidly ? Which decimation ? Which rate is useful ? Should it use the same rate, than the element before ? Which is the centerpoint from which the low_pass_cutoff and the high_pass_cutoff frequency will be counted ? I tried it out, but still doesn't know how it works. Regards Markus

on 2009-03-23 19:45

on 2009-03-24 18:34

As in the example, a common practice is to signal process like this: USRP tuning w/ hardware decimation --> freq translating channel filter --> demodulator --> additional filtering AFAIK, you haven't told us what sort of signal you're trying to receive, so it's not possible to recommend specific values for anything. Here's the flowgraph backbone that I use for 900 MHz ASK signals with important modulation products in the low kHz range (adapted from 'usrp_am_mw_rcv.py'): ################# Build main flowgraph ################## self.u = usrp.source_c() # usrp is data source adc_rate = self.u.adc_rate() # 64 MS/s usrp_decim = 64 self.u.set_decim_rate(usrp_decim) usrp_rate = adc_rate / usrp_decim # 1000 kS/s chanfilt_decim = 32 demod_rate = usrp_rate / chanfilt_decim # 32 kHz audio_decimation = 1 audio_rate = demod_rate / audio_decimation # 32 kHz chan_filt_coeffs = optfir.low_pass (1, # gain usrp_rate, # sampling rate 10e3, # passband cutoff 12e3, # stopband cutoff 1.0, # passband ripple 60) # attenuation self.chan_filt = gr.fir_filter_ccf (chanfilt_decim, chan_filt_coeffs) if self.use_IF: # Turn IF to baseband and filter combo. self.chan_filt = gr.freq_xlating_fir_filter_ccf (chanfilt_decim, chan_filt_coeffs, self.IF_freq, usrp_rate) else: self.chan_filt = gr.fir_filter_ccf (chanfilt_decim, chan_filt_coeffs) self.am_demod = gr.complex_to_mag() audio_filt_coeffs = gr.firdes.band_pass (1, # gain demod_rate, # sampling rate 4e3, # low pass cutoff 10e3, # high pass cutoff 1000) # transition width # WIN_HAMMING) # window type self.audio_filt = gr.fir_filter_fff(audio_decimation, audio_filt_coeffs) # Wire it all together. self.connect (self.u, self.chan_filt, self.am_demod, self.audio_filt) #, self.volume_control, audio_sink) I chose an IF value of 32 kHz, which puts the IF well above the signal spectrum. Works for me. Paul Mathews