RE: [Discuss-gnuradio] FFT of FFTs

[Weber, Michael J. \(US SSA\)]

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
> >>
> >>  
> >>
> >
> 
> 
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