Unknown peak in the FFT-Plot Sink

GNU Radio
1

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 B. 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

2

Use an Intermediate Frequency (IF) and frequency translating filter.
Paul M.

3

Paul M. <opto whidbey.com> writes:

Use an Intermediate Frequency (IF) and frequency translating filter.
Paul M.
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 ?
I read . Is this equivalent to
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 ?

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