[Discuss-gnuradio] help on xlating frequency

[feldmaus]

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