Forum: GNU Radio unknown peak in the FFT-Plot Sink

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85ffd97bae0e78056e1d06312744d7d1?d=identicon&s=25 Markus Feldmann (Guest)
on 2009-03-20 13:07
(Received via mailing list)
Hi All,

in all the examples and in my own gnuradio Programm, there
is an unknown Peak in the FFT-Plot sink.

Here is the Screenshot:
http://img257.imageshack.us/img257/8017/nochstrender.jpg

How to avoid this ?
Wherefrom does this come ?
I attach my Program to this mail, but i think it is still ok.

I need this to measure the power of the noise.

Further on you see the noise has a wave form always in
the middle of the FFT-Plot, still when i watch another
spectrum. In the gnuraio example from Eric Blossom this
doesn't appear, but this example only use a sin-signal-source
not a usrp-source.

How to avoid this ?
I tried to change the decimation and the sample Rate,
but doesn't solved this.

Regards Markus
A5bd7ae805ecf3d11fad16fcffee2f5b?d=identicon&s=25 Paul Mathews (Guest)
on 2009-03-20 17:36
(Received via mailing list)
Use an Intermediate Frequency (IF) and frequency translating filter.
Paul Mathews
85ffd97bae0e78056e1d06312744d7d1?d=identicon&s=25 feldmaus (Guest)
on 2009-03-21 14:38
(Received via mailing list)
Paul Mathews <opto <at> whidbey.com> writes:

>
> Use an Intermediate Frequency (IF) and frequency translating filter.
> Paul Mathews
I still use the IF by tune my usrp source with,
usrp.tune(self.usrp_source, 0, self.selected_device, self.usrp_freq)

But i think you mean to tune the "frequency translating filter" ?!
Therefor i want to know what is <tabs> ?
I read <channel filter>. Is this equivalent to <bandpass filter>
or do you mean the IO-Ports from the DA-converters  ?

Sorry but i didn't learned the english translated technical term.

How to fit up this <frequency translation filter> ?
# USRP SOURCE
self.usrp_simple_source_x = grc_usrp.simple_source_c(which=0, side='B',
rx_ant='RXB')
self.usrp_simple_source_x.set_decim_rate(decim)
self.usrp_simple_source_x.set_frequency(freq, verbose=True)
self.usrp_simple_source_x.set_gain(gain_usrp_source)

# FREQU. TRANS. FILTER
self.gr_freq_xlating_fir_filter_xxx_0 =
gr.freq_xlating_fir_filter_ccc(decim,
(1, ), freq, samp_rate)

# FFT PLOT SINK
self.wxgui_fftsink2 = fftsink2.fft_sink_c(
  self.GetWin(),
  baseband_freq=0,
  y_per_div=10,
  y_divs=8,
  ref_level=80,
  sample_rate=samp_rate,
  fft_size=512*2,
  fft_rate=20,
  average=True,
  avg_alpha=None,
  title="FFT Plot",
  peak_hold=False,
  )

When i start this, it seems to be busy !
When i tune the freq with my slider, all 3 elemts will be tuned.
Is this correct?
Or should i set up the baseband frequency of the fft plot to 0Hz ?

Any Hints ?

Regards Marksu
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