Re: [Discuss-gnuradio] FFT of FFTs

[James Cooley]

Yes, I was looking for something a little faster, but that's a good thought.

Actually, I have one of those things, but haven't pointed the usrp at it 
yet. As a side note, I was thinking about writing a demodulator for 
those.. I found the FCC filings for them which described how they worked.

-j



Weber, Michael J. (US SSA) wrote:

> Most wireless weather sensors transmit a half-second burst or so every
> 40-60 seconds around 433MHz... you can pick up a remote temp/humidity
> sensor from Radio Shack if you or your neighbors don't have one already.
> I've also seen wireless thermometer setups in Home Depot, Target,
> K-mart, and the like.
>
> Or, were you looking for something a little faster?
>
> Mike
>
>  
>
> > -----Original Message-----
> > From: 
> > address@hidden 
> > [mailto:discuss-gnuradio-bounces+michael.weber=baesystems.com@
> > gnu.org] On Behalf Of James Cooley
> > Sent: Sunday, July 10, 2005 4:18 PM
> > To: James Cooley
> > Cc: Eric Blossom; address@hidden
> > Subject: Re: [Discuss-gnuradio] FFT of FFTs
> >
> >
> > Any more suggestions on spotting periodic content? (as below?)
> >
> > Chiefly, can anyone think of something that I might be able to 
> > definitely pick out within TVRX range?
> >
> > -jamie
> >
> >
> >
> > James Cooley wrote:
> >
> >    
> >
> > > OK, I'm just getting around to trying these suggestions... First, re
> > > Eric's suggestions below.
> > >
> > > A few quick questions about this.
> > >
> > > I'm interested in tuning to a frequency (with usrp/frontend) then
> > > examining a single stream for periodic content. I've tried this but 
> > > not exactly sure what I'm seeing.
> > >
> > > The questions are, have I set this up properly? I have intended to
> > > grab whatever signal is at 50kHz (arbitrary) above what 
> > >      
> > we're tuned to 
> >    
> >
> > > and analyze that.
> > >
> > > How do I best select the feed forward and feed back filter
> > > coefficients? (I ended up using the tool as written in the 
> > >      
> > comments).
> >    
> >
> > > Also, any suggestions on things to look at within the TVRX 
> > >      
> > range that
> >    
> >
> > > should definitely test this out? (I think that GSM (TDMA) 
> > >      
> > bands are a 
> >    
> >
> > > bit out of range... up around 900MHz). I wondered if I 
> > >      
> > could pick out 
> >    
> >
> > > the periodicity of NTSC scans perhaps (maybe I'd need like a test 
> > > pattern, eh? to guarantee that the scan is periodic and non 
> > >      
> > changing 
> >    
> >
> > > long enough for me to try it).
> > >
> > >
> > >          # xlating filter
> > >           adnl_decim = 1
> > >           taps = [1.0]
> > >           shift = -50e3
> > >           capture_rate = usrp_rate
> > >           channel_coeffs = gr.firdes.low_pass (1.0,        
> > >      
> >   # gain
> >    
> >
> > >                                                capture_rate,   # 
> > > sampling rate
> > >                                                200e3,      
> > >      
> >   # low 
> >    
> >
> > > pass cutoff freq
> > >                                                200e3,      
> > >      
> >   # width 
> >    
> >
> > > of trans. band
> > >                                                
> > >      
> > gr.firdes.WIN_HAMMING)
> >    
> >
> > >                     xlate_filt =
> > > gr.freq_xlating_fir_filter_ccf(adnl_decim,
> > >                                                       
> > >      
> > channel_coeffs,
> >    
> >
> > >                                                       shift,
> > >                                                       capture_rate)
> > >
> > >           # complex to magnitude
> > >           cplx_to_mag = gr.complex_to_mag()
> > >
> > >           #
> > >           # Filter Coeffs correspond to butterworth iir 
> > >      
> > low pass filter
> >    
> >
> > >           # passband 0 - 1000 Hz
> > >           # Order 1
> > >           #
> > >           #
> > > http://www.dsptutor.freeuk.com/IIRFilterDesign/IIRFilterDesign.html
> > >           #
> > >                     fbtaps = [0.29289326, 0.29289326]
> > >           fftaps = [1.0, -0.41421357]
> > >
> > >           iir_low_pass = gr.iir_filter_ffd(fftaps, fbtaps)
> > >                     # fft
> > >           occ_fft = fftsink.fft_sink_f (self, panel, 
> > >      
> > title="Occupancy
> >    
> >
> > > FFT",
> > >                                         fft_size = 512, 
> > > sample_rate=usrp_rate,
> > >                                         baseband_freq=0)
> > >
> > >           self.connect (src, cplx_to_mag, iir_low_pass, occ_fft)
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > Eric Blossom wrote:
> > >
> > >      
> > >
> > > > On Sat, Jun 18, 2005 at 08:18:38PM -0400, James Cooley wrote:
> > > >
> > > >
> > > >        
> > > >
> > > > > Hi all,
> > > > >
> > > > > I'm trying to take an FFT of an FFT.... Basically, I want 
> > > > >          
> > to tune to
> >    
> >
> > > > > a signal, and for a given RF frequency, try to spot 
> > > > >          
> > periodic usage 
> >    
> >
> > > > > of that frequency. Is this possible? What I have now is 
> > > > >          
> > hopelessly 
> >    
> >
> > > > > slow, so I'm not really sure if I've got it right.
> > > > >  
> > > > >          
> > > > Hi Jamie,
> > > >
> > > > Here are a couple of suggestions.  If there are relatively few 
> > > > frequencies that you want to observe for periodic 
> > > >        
> > occupancy, I would 
> >    
> >
> > > > suggest extracting the frequency bands of interest using a 
> > > > gr.freq_xlating_fir_filter for each one.  If there are 
> > > >        
> > lots of them, 
> >    
> >
> > > > and they are evenly spaced, then the dft filterbank is 
> > > >        
> > what you want 
> >    
> >
> > > > to split them out (blksimpl/filterbank.py).
> > > >
> > > > Once you've got your individual streams of signals, for each one I 
> > > > would compute an estimate of whether it is occupied.  You could do 
> > > > this by computing the magnitude of the stream 
> > > >        
> > (gr.complex_to_mag) and 
> >    
> >
> > > > then low pass filtering that with a gr.iir_filter, 
> > > >        
> > possibly followed 
> >    
> >
> > > > by a limiter (which would need to be written).  At this point, for 
> > > > each of your input streams, you have an output stream that is 
> > > > effectively a stream of 1's and 0's, where 1 means "is occupied".
> > > > Then run each of those streams into it's own FFT.   Point 
> > > >        
> > this whole
> >    
> >
> > > > pipeline at some kind of TDMA input (GSM basestation?) and 
> > > >        
> > you ought 
> >    
> >
> > > > to see the slots (assuming the basestation isn't driving all the 
> > > > slots all the time).
> > > >
> > > > Eric
> > > >
> > > >
> > > >
> > > >        
> > _______________________________________________
> > Discuss-gnuradio mailing list
> > address@hidden 
> > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> >
> >    
>
>