Detect specific squared wave frequency

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prprog

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What will be the name (to do a search) of a circuit that detects an specific squared wave frequency an act as a comparator (sending an ON signal)?. I prefer not to used a PLL chip. Read somewhere that a 555 (missing pulse detector) circuit can do this, but no idea how to implement.

Thanks,
 
A missing pulse detector can be used for detecting a square wave above (not missing) or below (missing) a specific frequency. Mark-space periods other than 50% will affect this since the detector uses the time between pulses to determine the missing pulse.

I'd recommend an LM567 Tome decoder, but they're obsolete. There may still be sources out there somewhere. Mouser carries an NTE832, that is a replacement.

Ken
 
Could you not use a DSP, or a type of digital filtering where, the only frequency that IS passed, is your desired frequency?

From there, just use some logic to detect the simple pulse, rather than the more complicated frequency.

Does that make sense?
 
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DSP is way to complex for my current knowledge, but thanks for the reply.

What I trying to get is this-> a comparator can be used to detect an specific voltage (or a range voltage) and ouput an ON signal...so can I used some frequency to voltage converstion to get the same results when a specific squared wave is the input?

Thanks
 
A frequency-to-voltage converter with a comparator "is" a missing pulse detector. And, will only tell you if the fixed duty-cycle square-wave is above or below the set frequency. How accurate does the frequency detection have to be...+/- 1Hz?...10Hz?...100Hz? Two missing pulse detectors, with frequencies set slightly above and below your frequency, could give you a binary logic output indicating if it is above the lower, but not above the upper frequency. This seems to be a very "tweeky" solution when you can buy an 8-pin Dip IC for $5 that will do this.

Ken
 
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Digikey still carries factory-fresh LM567's. A missing pulse detector will not detect a specific frequency, but this will.
 
The circuit you are looking for is known as a monostable or one shot. A 555 will do this,using the standard timer circuit. The trigger terminal is fed via a small capacitor, less than 100pf,say, and a pullup resistor of a couple of kohms. The timer will fire on the negative going edges of the input squarewave ,and produce a train of pulses of constant width, the same frequency as your square wave, and constant voltage, rail to rail. Filter this with a suitable low pass filter and you have dc, proportional to the input frequency. Now add a comparator or two, and you have your frequency cutoff or window. Get a few bits and play some! Hope this helps a bit.
 
Hi.
What is the duty cycle for this? 50%? More or less than 50%? 50% would probably be easiest to deal with. A signal with 50% duty cycle and a fixed peak-to-peak voltage can be fed into a simple RC circuit and produce a known voltage out which can then be fed to a window-comparator.

Later!
kenjj
 
That's the point of it! The pulses are a fixed width, such that they are about 90% at the highest frequency you want to measure. As you reduce the input frequency, there are less pulses, so the output voltage from the filter , the average voltage, drops.
 
The filter output voltage will be independent of frequency, which will make a poor frequency discriminator.
 
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