2009/5/29 M.L. : > On Thu, May 28, 2009 at 1:10 PM, Peter Restall wrote: > >> >> On Thu, 28 May 2009 11:13:40 -0400, M.L. wrote: >> >> > Pete, >> > You can't detect a 60kHz signal with a 26.7kHz sample rate. I just ran= an >> > Octave simulation of the Goertzel algorithm and I was able to get good >> > results with something like this: >> > >> > [snip] >> > >> > -- >> > Martin K. >> >> Thanks for the input Martin - I'll have a play about with your script a >> little >> later. =A0For the time being though, I've stuck my own octave script at >> > > > Very simply, sampling theory says that you need to sample at least twice = as > fast as the maximum frequency you want to look at. So if you sampled at > 10kHz the maximum frequency you can see is 5kHz (with much distortion). Y= ou > will get numbers if your Goertzel code is trying to detect a frequency > > sample speed, but they won't be correct. > - > Martin > -- Actually, sampling theory relates to the bandwidth of the sampled signal, not the maximum frequency, So, provided that the bandwidth of the 60kHz carrier is less than 13.35kH, there isn't a "theoretical" problem. This is a typical case of undersampling, where you are using a band limited signal and utilising the information in what would otherwise be described as an alias. RP -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist