Frequency comparator

[akis]

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!

[DF96]

Pulse count discriminator plus a comparator will do it.

[PChi]

Another way is to use a frequency to voltage converter like a Texas Instruments LM2907 or an Analog Devices AD650.

[Elvee]

Originally Posted by akis

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?

[akis]

Originally Posted by Elvee

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.

[jcx]

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)

[Elvee]

Originally Posted by akis

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.

[dangus]

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?

[Frex]

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

[gmphadte]

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