some help please on more efficient way to detect tones?
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From: jacobfenton on 9 Jan 2010 17:18 Perhaps I need to give some more information. The I-Q data is coming from an AD9874 IC. I can change the rate at which is sends me data. But I have a requirement that I need to have a certain IF bandwidth filter, which is achieved in the AD9874 and is affected by what I tell the AD9874 to set its decimation factor too, which corresponds to how fast I get data. This IF bandwidth is rather large, therefore I cannot set the decimation in the AD9874 to high, so I need to accept the fast data rate. Do I make the correct assumption that I must do the FM demodulation at this high data rate, otherwise if I decimate more inside the DSP before I demodulate, I effectively reduce my IF bandwidth filter? It does seem that I can decimate after demodulation for my tone detection. I also have bandwidth requirements for my tones I must detect. I put a matlab file together which does my Goertzel algorithm, therefore the bin size affected the bandwidth of the tone I could detect along with the threshold I set. So if N gets too large, my bandwidth for detection does down it seems. So there is some compromise between N and bandwith of tone I can detect. Also is there a faster FM demodulation formula I could use, or is the one I posted the best one? Thanks all for your input. -Jacob Fenton
From: Fred Marshall on 9 Jan 2010 17:54 jacobfenton wrote: > Perhaps I need to give some more information. > > The I-Q data is coming from an AD9874 IC. I can change the rate at which > is sends me data. But I have a requirement that I need to have a certain IF > bandwidth filter, which is achieved in the AD9874 and is affected by what I > tell the AD9874 to set its decimation factor too, which corresponds to how > fast I get data. This IF bandwidth is rather large, therefore I cannot set > the decimation in the AD9874 to high, so I need to accept the fast data > rate. > > Do I make the correct assumption that I must do the FM demodulation at > this high data rate, otherwise if I decimate more inside the DSP before I > demodulate, I effectively reduce my IF bandwidth filter? > > It does seem that I can decimate after demodulation for my tone > detection. > > I also have bandwidth requirements for my tones I must detect. I put a > matlab file together which does my Goertzel algorithm, therefore the bin > size affected the bandwidth of the tone I could detect along with the > threshold I set. So if N gets too large, my bandwidth for detection does > down it seems. So there is some compromise between N and bandwith of tone I > can detect. > > Also is there a faster FM demodulation formula I could use, or is the one > I posted the best one? > > Thanks all for your input. > > -Jacob Fenton I get the impression that you're overconstraining your approach. For example, why would N affect the bandwidth of the tones you can detect? If N is too small then I can imagine the filter bandwidth being too large but not the other way around. Fred
From: Jerry Avins on 9 Jan 2010 19:02 Fred Marshall wrote: > jacobfenton wrote: >> Perhaps I need to give some more information. >> >> The I-Q data is coming from an AD9874 IC. I can change the rate at which >> is sends me data. But I have a requirement that I need to have a >> certain IF >> bandwidth filter, which is achieved in the AD9874 and is affected by >> what I >> tell the AD9874 to set its decimation factor too, which corresponds to >> how >> fast I get data. This IF bandwidth is rather large, therefore I cannot >> set >> the decimation in the AD9874 to high, so I need to accept the fast data >> rate. >> >> Do I make the correct assumption that I must do the FM demodulation at >> this high data rate, otherwise if I decimate more inside the DSP >> before I >> demodulate, I effectively reduce my IF bandwidth filter? >> >> It does seem that I can decimate after demodulation for my tone >> detection. >> >> I also have bandwidth requirements for my tones I must detect. I put a >> matlab file together which does my Goertzel algorithm, therefore the bin >> size affected the bandwidth of the tone I could detect along with the >> threshold I set. So if N gets too large, my bandwidth for detection does >> down it seems. So there is some compromise between N and bandwith of >> tone I >> can detect. >> >> Also is there a faster FM demodulation formula I could use, or is the one >> I posted the best one? >> >> Thanks all for your input. >> >> -Jacob Fenton > > I get the impression that you're overconstraining your approach. > For example, why would N affect the bandwidth of the tones you can > detect? If N is too small then I can imagine the filter bandwidth being > too large but not the other way around. The distribution of electric power was delayed for years and thought to be entirely impractical. For maximum power transfer, one matches the load to the generator, and that allows only 50% efficiency in the ideal case. Edison realized that making the load's impedance much higher than generator's avoided that loss. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: jacobfenton on 9 Jan 2010 21:02 >jacobfenton wrote: >> Perhaps I need to give some more information. >> >> The I-Q data is coming from an AD9874 IC. I can change the rate at which >> is sends me data. But I have a requirement that I need to have a certain IF >> bandwidth filter, which is achieved in the AD9874 and is affected by what I >> tell the AD9874 to set its decimation factor too, which corresponds to how >> fast I get data. This IF bandwidth is rather large, therefore I cannot set >> the decimation in the AD9874 to high, so I need to accept the fast data >> rate. >> >> Do I make the correct assumption that I must do the FM demodulation at >> this high data rate, otherwise if I decimate more inside the DSP before I >> demodulate, I effectively reduce my IF bandwidth filter? >> >> It does seem that I can decimate after demodulation for my tone >> detection. >> >> I also have bandwidth requirements for my tones I must detect. I put a >> matlab file together which does my Goertzel algorithm, therefore the bin >> size affected the bandwidth of the tone I could detect along with the >> threshold I set. So if N gets too large, my bandwidth for detection does >> down it seems. So there is some compromise between N and bandwith of tone I >> can detect. >> >> Also is there a faster FM demodulation formula I could use, or is the one >> I posted the best one? >> >> Thanks all for your input. >> >> -Jacob Fenton > >I get the impression that you're overconstraining your approach. >For example, why would N affect the bandwidth of the tones you can >detect? If N is too small then I can imagine the filter bandwidth being > too large but not the other way around. > >Fred > I am not trying to constrain my approach any more then the requirements are set upon me, but perhaps I don't fully understand everything, which is what I am trying to do, learn.
From: Fred Marshall on 10 Jan 2010 12:08
jacobfenton wrote: >> jacobfenton wrote: >>> Perhaps I need to give some more information. >>> >>> The I-Q data is coming from an AD9874 IC. I can change the rate at > which >>> is sends me data. But I have a requirement that I need to have a > certain IF >>> bandwidth filter, which is achieved in the AD9874 and is affected by > what I >>> tell the AD9874 to set its decimation factor too, which corresponds to > how >>> fast I get data. This IF bandwidth is rather large, therefore I cannot > set >>> the decimation in the AD9874 to high, so I need to accept the fast > data >>> rate. >>> >>> Do I make the correct assumption that I must do the FM demodulation at >>> this high data rate, otherwise if I decimate more inside the DSP > before I >>> demodulate, I effectively reduce my IF bandwidth filter? >>> >>> It does seem that I can decimate after demodulation for my tone >>> detection. >>> >>> I also have bandwidth requirements for my tones I must detect. I put a >>> matlab file together which does my Goertzel algorithm, therefore the > bin >>> size affected the bandwidth of the tone I could detect along with the >>> threshold I set. So if N gets too large, my bandwidth for detection > does >>> down it seems. So there is some compromise between N and bandwith of > tone I >>> can detect. >>> >>> Also is there a faster FM demodulation formula I could use, or is the > one >>> I posted the best one? >>> >>> Thanks all for your input. >>> >>> -Jacob Fenton >> I get the impression that you're overconstraining your approach. >> For example, why would N affect the bandwidth of the tones you can >> detect? If N is too small then I can imagine the filter bandwidth being > >> too large but not the other way around. >> >> Fred >> > > I am not trying to constrain my approach any more then the requirements > are set upon me, but perhaps I don't fully understand everything, which is > what I am trying to do, learn. Jacob, I'm sorry I wasn't of more help as I don't fully understand your constraints. But, I trust my gut and that's what it was telling me. I hope I gave you enough of a hint to be able to explore those woods of yours on your own. I *think* I understand the IF issue: You have a very high carrier frequency or center frequency shall we say? Pre-sampling, the tones show up there and are quite close to a multiple of the sampling frequency. In fact, about 7-13 kHz away / close. Post-sampling IQ, the tones are where they're advertised to be and I'd say that the "Intermediate Frequency" or IF is intentionally set to be zero. And, from the information you've given us, the IF bandwidth needs to be around 16kHz. I've not yet taken into account that the samples are IQ so there may be a factor of 2 smaller bandwidth needed as each set of samples only needs to occupy at most -/+8kHz and the sample frequency 16kHz. I'm not able to reconcile this with the reported "tone frequencies". If the IQ sampling were done efficiently then the tones would show up symmetrically around zero with the lower ones (or upper?) below zero and the upper ones above zero frequency. But, this isn't something that I've done so I'd best not go there.... So, now we're really into it.... the AD9874 technical article says: "For example, with a clock frequency of 24 MHz the signal bandwidth can be as low as 12.5 kHz or as high as 250 kHz." Hmmmm .... you say you have an IF bandwidth "requirement" but didn't say what that bandwidth is. So, is it different than the tones given would imply? Otherwise I'd be looking for less input bandwidth or Fs coming into the DSP. Fred |