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Old 29th April 2012, 10:22 PM   #1
akis is offline akis  United Kingdom
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Default Frequency comparator

I would like to monitor a frequency and if not in the right range then light up an LED or something similar. The frequency will be a sinusoidal at 100 KHz, 150KHz and 200 KHz (one of these three).

I have so far tried to convert the sinusoidal to a square wave (eg using a comparator, or a simple transistor) and that seems to work but that's as far as I have been.

Would anyone know how best to approach this ? Thanks!
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Old 29th April 2012, 10:25 PM   #2
DF96 is online now DF96  England
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Pulse count discriminator plus a comparator will do it.
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Old 30th April 2012, 02:44 PM   #3
PChi is offline PChi  United Kingdom
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Another way is to use a frequency to voltage converter like a Texas Instruments LM2907 or an Analog Devices AD650.
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Old 30th April 2012, 04:36 PM   #4
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by akis View Post
I would like to monitor a frequency and if not in the right range then light up an LED or something similar.
What is for you a "right range". When out of range, do you want to know if the frequency is above or below the target?
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Old 30th April 2012, 05:59 PM   #5
akis is offline akis  United Kingdom
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Quote:
Originally Posted by Elvee View Post
What is for you a "right range". When out of range, do you want to know if the frequency is above or below the target?
I will have 3 frequency selections: 100 KHz, 150 KHz and 200 KHz. I suppose 5% on either side is OK, but anything more should light up a LED, or somehow else indicate the error, even audibly, and maybe even switch the device off.

( I think audibly is the best as it needs to alert the user )

Why would the frequency NOT be 200 KHz? Well this will certainly be an error condition, maybe some internal fault in the device, maybe the oscillator has stopped functioning properly.
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Old 30th April 2012, 06:27 PM   #6
jcx is offline jcx  United States
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there's also "the really big hammer" approach: today uC with timer/counter, even some with fast ADC can do this for $20 inc dev tools (TI's eZ430)
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Old 30th April 2012, 07:10 PM   #7
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by akis View Post
I will have 3 frequency selections: 100 KHz, 150 KHz and 200 KHz. I suppose 5% on either side is OK, but anything more should light up a LED, or somehow else indicate the error, even audibly, and maybe even switch the device off.
Then, a PLL like the CD4046 with a window discriminator on the VCO input would do.
CD4046 + LM393 is ~£1 components, you just have to add the frequency selector and the warning device of your choice.
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Old 4th May 2012, 12:05 AM   #8
dangus is offline dangus  Canada
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You could probably do that with a $5 microcontroller like the ATtiny13. Atmel AVRs can often be programmed with minimal hardware (a few resistors and a parallel printer port).

And just brainstorming here: Is it possible to make it work for multiples of 50 kHz so all three frequencies can be handled at once?
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Old 4th May 2012, 04:37 AM   #9
Frex is offline Frex  France
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Hi ,

If you need a good frequency detection with narrow bandwidth, probably the easiest way
is o use three tonality decoder, like LM567.
It's an analog IC available in 8 pins DIP or SMD SOIC case.
Very low cost and easy to find.
Just few parts need to get an digital signal saying if your are or not in the defined frequency windows you have choose.
Regards.

Frex
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Old 9th May 2012, 06:23 AM   #10
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When u change the frequency of interest, change the reference point of the error detection. For example, if u use F/V, then just change the Vref.

Gajanan Phadte
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