Analog Period Detector

Can the OK frequency vary over a very wide range?

If the range of OK frequency is fairly narrow, a phase locked loop, like a CD4046 with a very slow error filter might have just about everything you need. Its in lock logic output may be able to drive the indicator, directly.

You get to define hoe fast a frequency change is too fast by how fast the error filter can change the tracking VCO.

LM567 comes to mind.

Best regards, Spehro Pefhany

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You beat me to it. That's what I'd consider first, assuming that the problem fit the part.

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If distortion is a real issue as well as frequency, you will probably need to make a simple distortion analyzer. Is it a sine wave, triangle, or more complex? If frequency is low, you could use a PIC to trigger off the zero crossing or other known point, and take several measurements over the time period. For higher frequency, you may need to generate a reference waveform phase locked to the signal you are checking, and measure the difference. A DSP might be able to perform an FFT.

Paul

A PIC, of course.

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Opps I should have mentioned the average signal frequency.. It's 120Hz... I don't think high speed electronics are required. Also... The tolerable f error is high...I'm guessing 10%.. For example.. A signal repeating at 120Hz+/-12Hz should give the ok light.

I remember playing with the 4046 years ago.. I like the idea and I'll check it out.. Thanks D from BC

You can set the voltage controlled oscillator so that its total range is the range you want the signal to be tracked, successfully. Are you really concerned if the frequency changes above some rate within that range?

John Popelish

At first glance on:

it looks like it'll do the job... I'll stare at the data sheets for awhile... :)

Thanks

D from BC

Hmm. you didn't say if the signal is sine,square etc.. so I assume you're looking for a nulling effect. How about an OP-Amp utilizing both - and + inputs ? as soon as either signal shifts in difference, the output will change. In other words, applying the exact signal to both inputs (common mode) or no signal at all to both inputs should yield a constant state on the output. This state being 0 volts for Dual +/- circuits or 50% of your supply rail for single supplies.

just a thought..

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The repeating signal is around 120Hz.. This should be a piece of cake for PIC speed. It's been 2 years since I did PIC programming... I'm a little afraid of how much time it take me to recall everything.. I don't know if it's a bicycle thing :) That's why I'm gravitating toward a mixed mode circuit or dedicated IC.

Maybe I might bite the bullet someday and blow the dust off the PIC programmer and get back into it. D from BC

Now it depends on how much control you want ? using the LM 567 works how ever, I find that in some circuits if your reference spans double or more that 120 hz it can cause issues. how about a active bandpass filter? with that, you can determine the phase shift of which direction it's going for control etc..

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A 4046 will run that slowly. But if you want to track a funny waveform you might run your VCO say 64 times faster, and compare 64 successive digitised samples of your waveform with a stored digitised reference version - even a PIC should be able to work out the sum of the squares of the deviations between sample and reference at 64x120Hz (7.68kHz or

130usec per sample). A DSP chip could do the arithmetic a lot faster and support many more samples per cycle.

-- Bill Sloman, Nijmegen

How about this:

A multiplying PLL makes, lets say 8 times the frequency.

A 4051 constructs something like a sine wave that is fed to one pin on a window comparitor.

The other pin of the comparitor is run from the signal.

A cd4017 is reset if the window comparitor indicates an out of bounds signal. The clock of the 4017 runs from the divided VCO output and it is disabled if it hits state 10.

Ok...I understand that the 4046 "lock" range can be set.. (IIRC 4046 terminology..I haven't reviewed the data sheet yet..I'm getting to it..) I think I understand your question.. I think your asking what happens if the signal gets a little squirrely in the "lock" range.. I don't know yet... I think that's ok for now.. I'll still working on the design..

If the input signal keeps landing periodically within a v range, then the green light lights.

It's not a picky circuit but it is a PICky circuit.. Somebody put that in the PUN post...!! :)

D from BC

Ooops ..there's another detail I should have put in my OP.

The circuit has to work on a bridge rectified sine wave signal. I understand the off phase detect method. However, I think that's best for sine waves. D from BC

The edge triggered phase detector in the 4046 can work with any wave shape or duty cycle as long as it passes through the reference voltage of the chip. That reference is roughly half of the supply voltage for the chip. You could capacitor couple the full wave rectified wave to the phase detector and it will work fine, as long as the rectifier is loaded with enough resistance to follow the wave to zero twice per cycle. If the unrectified wave is 120 Hz, the VCO will have to be designed to have its center frequency around

240 Hz.

Wow....That's like comparing a signature with a signature..

Using a PIC or DSP are great hardcore solutions.. Arrrff! Arrfff! :) Sometimes I think I should force myself to use micros more often and learn DSP programming just be skilled in those areas..

If I keep using the old stuff like the 4046, I don't think I'm getting the best experience..I just stay dum with the dum stuff... However... I'll have the 4046 working wayy before the time it'll take me to learn DSP programming. :)

D from BC

Yeah..I know..sorry about that.. It's a full wave rectified sine waveform at about 120Hz+/-12Hz. The sine has some harmonic distortion but I can't say how much..

Interesting... A signal reference and a differential amplifier solution.. If the op amp output is 0V then the signals are identical..Green light.. If a resultant signal exists, then the signals are different and no green light. If I create the reference waveform by analog methods I'll need to full wave rectify a sine oscillator... (The source signal is like that..) It'll need to synch up too. Or I could use a DAC and RAM but if I'm going to go that far I'll drop that and start programming a PIC.

mmmmm..... D from BC

I wrote that in response to using an active bandpass filter for phase detection. That's best with sine waves.

But besides... It's good to know the 4046 can work on any wave shape..I last used it

10 years ago..Forgot everything about it..

Thanks for the tips. D from BC