On Fri, May 22, 2009 at 1:14 PM, Peter Restall <pete@restall.net> wrote:

>
>
> I've now decided to bandpass the 60KHz signal and then sample at 26.7KHz
> and run it through Goertzel's algorithm - seems to work alright in the
> Octave simulations, and gives oodles of spare cycles for updating my LCD,
> spitting out the time over RS232, monitoring for push-buttons, etc.  The
> circuit is basically a 3-pole LC bandpass, two quad op-amps to provide
> incremental gain (one of which is wired as a VGA), and all that feeds into
> the dsPIC.  Still need to write the firmware though.  Or perhaps I should
> just
> look at the appnote as it looks far more sensitive and I could probably
> remove yet more external components...
>
> Regards,
>
> Pete Restall
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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:

### GNU Octave code
# Goertzel algorithm to detect 60kHz signal

clc;
clear;

n=1;
s_prev = 0;
s_prev2 = 0;
rate = 120000      # sample rate
f = 60000          # injected signal frequency
ft = 60000         # signal to look for
samples = 1000    # number of samples to test
sig_amp = .01     # arbitrary signal amplitude

fnorm = ft/rate   # normalized frequency (% of sample rate)

# Calculate Goertzel coefficient
coeff = 2*cos(2*pi*fnorm);

# Create signal to examine
t = linspace(0,samples/rate,samples);
x = sig_amp*sin(2*pi*f.*t);


while (n < samples)
         s = x(n) + coeff*s_prev - s_prev2;
         s_prev2 = s_prev;
         s_prev = s;
         n++;
endwhile

power = s_prev2^2 + s_prev^2 - coeff *s_prev2*s_prev


--
Martin K.
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