I have an application where I need to find the lowest frequency component in an audio signal (say 500Hz..8KHz) comprised of a number of unrelated tones. This will not necessarily be the dominant amplitude component and I will need to have an amplitude threshold below which minor components are ignored. DSP based solutions are not ideal. Something analog and low powered would be ideal. Suggestions ?. cheers M
Digital filtering, of course.
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Possibly silly ideas:
If you pass the composite signal through a series of integrators, low frequencies are selectively amplified 6 dB/octave per integrator stage.
A cutoff-frequency feedback loop could be closed around a sharp-cutoff tunable lowpass filter, switched-capacitor probably.
John
Look at the signal with a spectrum analyzer that covers the frequencies in question. All will be revealed. ;-)
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How analog/low-powered?
Beat the input signal with step-wise sweep oscillator starting at lowest tone and going upward. Low-pass filter the result into a threshold detector, and stop when triggered. You could store the inputs to your VCO, pre-amp, and comparator in ROMDAC.
-Le Chaud Lapin-
A HC4046.
Take the input and convert it to a PWM signal a some high frequency.
Apply this PWM signal to the XOR phase detector.
Use an op-amp integrator based loop filter. Make the PLL filter have a low enough gain that a nearby strong signal won't yank the PLL up away from the lowest frequency signal. Don't forget the zero to make the loop stable.
Force the integrator output to 0V. Apply a small current to the input to cause the integrator to slowly ramp the VCO input up.
When the PLL comes to the first frequency, it will lock onto it.
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You could use a frequency sweep and a mixer-type phase detector. Feed your signal and the frequency sweep into the phase detector. The mixer output, suitably filtered, would tell you the amplitude at that frequency.
--Mac
Make that, lets say, 512 times the suspected frequency. If not, you may have a lock onto the harmonic beats. ie: the VCO is running 512 times the input.
Also convert the output of the divider chain to PWM at, lets say, 256 times the suspected frequency. You have to do this to avoid subharmonic locking.
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