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