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Hi everyone out there...

I am working on a circuit that has the ability to measure the fundamental frequency of an incoming periodic signal. I do not want to use a microcontroller.
Initially i had the idea of using peak detection and comparison... The distance between two max. peaks gives the period. But what if the signal has multiple peaks in the same period???
I REQUEST EVERYONE TO SUGGEST CIRCUITS FOR THIS PROJECT... PLEASE... PLEASE

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One way to ignore higher-than-fundamental harmonics is with hysteresis. In the picture you can see an input waveform with multiple peaks, and two levels A and B. When determining a rising edge in the input, we compare with level A - we assume the cycle starts when the waveform is determined to be greater than A. Then we switch to comparing with level B, and assume the mid-point of the cycle to occur when the input drops below B. The cycle is complete when the waveform rises again above A.

This hysteresis can be achieved with positive feedback as shown using an opamp as a comparator. The comparator's output is always high or low, +Vs or -Vs (or nearly so, as many opamps' outputs don't swing all the way to the supply rails). A known fraction of that output is fed back to become a reference voltage that the comparator will compare the input signal to.

For example, assume the amplifier is supplied with +12V and -12V. Also assume R1 and R2 equal, such that 50% of the output is fed back to the non-inverting input. When the output is high (+12V), the non-inverting input (+) sees 50% of it, or +6V. When the input signal on the inverting input (-) exceeds this +6V, the output will swing low. Now of course the non-inverting input sees half of this new ouput, or -6V. When the input signal drops below -6V, then the output swings high again. And so on. You end up with a rectangular wave of the frequency of the fundamental of your input signal, and from there you can easily measure its frequency.

You can change R1 and R2 to change the levels A and B that determine the rising and falling edges, but ultimately this simple circuit will only work if the input waveform is known to slightly exceed A, and only slightly drop below B.

The two obvious ways to achieve this condition are: (a) Build an AGC (automatic gain control) which amplifies or attenuates the signal to have the approximate amplitude required by the hysteresis levels, or b) use peak and trough detectors to determine the levels A and B, and feed the non-inverting input of the comparator with these derived values.

You would normally want more than 50% feedback in a frequency measurement application, to avoid erroneous transistions due to input noise. Perhaps 90% would be appropriate, requiring R2 = 9 x R1

post-20531-14279143012585_thumb.png

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yes sir.... i am sure that i don not want to use a microcontroller...

i have attached a picture of a periodic signal... will your method be applicable to such signals??? i will not be using a CRO at any stage... so i cant view the signal also...

say this is my input signal... and my output should be the frequency or period of the signal mentioned on a bcd display unit....

how to do???

post-11090-14279143015981_thumb.jpg

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yes sir... it is an echo...

i am trying to get help from as many sources as possible... i am a student from india.. and most of our teachers are not able to guide us in this project... they suggest the use of dsp processors... but i have no idea about them.... i am a second year undergrad student... just learning the basics of signal processing... so i am on the look out for circuits using components i know...

moreover i do not even know about this PLL.... but after it was suggested i have begun learning about it through the net... my sole aim is to get help from as many sources as possible....

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I'm afraid you are going nowhere with this. Any design needs numbers. Get numbers. Get details. Without them your project is only a little less vague than "make a kind of thing that sometimes does some stuff, perhaps, and some other stuff."

It is just not feasible to start off with "any range" or "as high as possible". You might end up with something perfectly good for measuring microwave frequencies in the millivolts, but completely useless for tuning a guitar. Or vice versa. If you want a frequency counter for all occasions you'll be needing to visit your local Fluke rep.

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