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Frequency limiting...

Discussion in 'General Electronics Chat' started by raffter, Apr 23, 2008.

  1. raffter

    Thread Starter Active Member

    Feb 28, 2008
    Hello guys,

    How do you DO a frequency limiting circuit, like: when freq reaches a certain limit, the output will result to "no frequency" or a V++ or Gnd...

  2. Audioguru


    Dec 20, 2007
    A lowpass filter passes low frequencies and has reduced output at high frequencies.
    A highpass filter passes high frequencies and has reduced output at low frequencies.

    You can make simple filters that have a gradual slope or make complicated filters that have a very steep slope.
  3. Dave

    Retired Moderator

    Nov 17, 2003
    There is also band-pass filters which filter all but a specified frequency band, and band-stop filter which filter a specified frequency but allow frequencies above and below this frequency band. As Audioguru correctly states, there are means and ways of making the transitions gradual or steep, and you have either analogues techniques at your disposal or digital techniques. The traditionalists amongst us might argue to the contrary, but I am of the opinion digital filter offers the benefit of implementing complex filters with relative ease (that is not to say it is easy), particularly for band-pass and band-stop filtering.

  4. Caveman

    Senior Member

    Apr 15, 2008
    Couple of things. First of all, you cannot have a perfect filter. To do it would require that the filter know about the future. You can do pretty good though and there are tradeoffs with different kinds of filters.

    One other part of filter design that many people forget is the phase distortion that happens as the frequency is close to the cutoff frequency. Basically different frequencies take different amount of times to get through the filter which causes the output signal to be distorted. By the way this is called group delay. To remove this, you want constant group delay. Sometimes it matters, sometimes not. No analog filter can achieve constant group delay, but FIR digital filters can. However, they will add a constant delay through the circuit because it requires a lot of taps.
  5. raffter

    Thread Starter Active Member

    Feb 28, 2008
    Thanks for the input guys.. :)

    I was kinda thinking of a 567 pll... but I dont know if you could use the output to "permanently" trigger something if the input freq is higher than the trip-off

  6. hgmjr

    Retired Moderator

    Jan 28, 2005
    The NE567 is a tone detector. It is used to detect when the frequency is equal to a set frequency +/- a few percent of the set frequency.

    You could consider using a frequency to voltage converter and then feed the signal from the output of the F-to-V to a voltage comparator.

    What frequency are you trying to detect?

  7. raffter

    Thread Starter Active Member

    Feb 28, 2008

    an adjustasble 100Hz - 166Hz detection... +/-5Hz resolution.. cool idea btw, BUT too much circuitry... I was hoping fora single chip approach.. if at all possible :)