Reading in frequency of a modulated sine wave using A/D

I have experience in this particular case. This is what i do:

I use a cross zero detector. A LM393 with 1Mohm resistor positive feedback, this is because when the sine wave cross zero have some crazy peaks and you have to create a little hysteresis of 10 mV above zero. If you dont do this you will see a pulse train and then your square signal. After you have a clean and perfect square signal from the comparator you just have to insert it to the microcontroller and calculate the time of the pulse, this gives you the frecuency (1/T).

This works great and for 50 Hz the micro is going easy. Forget FFT, dont kill a fly with a magnum.

If you need more help let me know.

if you do not have a complex signal and it's rather clean then use a zero crossing method. It's the cheapest on CPU time. Keep a running average on the Peak amplitude, You'll want to generate a Sin/Cos table for speed in the rom image to convert the PCM readings from the base line to obtain the fraction in a linear state.

The only math you may need to do that maybe CPU taxing on an AVR is the scaling of the input signal to match the coefficients in the table.

other than that, it's simple..

If your signal is complex, then FFT is required and that can be taxing on the CPU. Some where around here, I have a simple PASCAL file that you can use to break down a FFT for 8 bit operations for the AVR if you need it.

good idea, I never thought of using a simple comparator which would make life easier on the coding side :) I use that years ago in the DOS days when for Slow scan tv via the serial port.

Interesting problem. What sample rate will you use? Is it constant? How accurate do you need the result to be? How fast can the input amplitude and frequency change? What might the input waveforms look like?

We're working on a similar problem, but we'll do the math in an FPGA. We're considering phase-locking a DDS synthesizer to the incoming sine wave, among other things. That could be done in software, too.

John

Why exactly can't you use the simple comparator and a period/frequency counter? For the ultimate convenience, the AVR does have the comparator on the chip, and this comparator is already connected to the timer input capture.

It depends.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

I've done somthing similar (just not down to DC). I like to amplify the crap out of the signal to straighten up those rising and falling edges, and then feed that to a comparator.

Another use for a phase-locked loop? If you lock a 4046 type PLL chip to the signal, you can read the loop filter voltage through the follower, and get about 1% linearity on the frequency. It only takes one ADC sample to get the frequency (average).