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LM567C: is designed to provide a transistor switch to ground output when the input signal matches the center frequency, but I am seeing the oposite

Viktorija Cecil
Mastermind 18491 points
Part Number: LM567C

Team,

My customer is observing behavior that contradicts datasheet:

The datasheet for the LM567C states that the “device is designed to provide a transistor switch to ground output when the input signal matches the center frequency”. I have a device hooked up with a microphone and preamp as an input and my output is showing a low until I turn on the tone generator and then it switches high. As I read the datasheet this is reverse from the description as well as the example waveform shown. My first thought was that there was sufficient ambient noise to give me a true reading, but after turning on my tone generator and seeing the output switch it leads me to believe I’m getting a reverse behavior. Is there something I’m missing here?

Thanks!

Viktorija

  • 0
    TI__Genius 11900 points

    Hello Viktorija,

    I'm not very familiar with the part but will try my best to help you here.

    Based on Figure 13. of the datasheet it looks like the output is expected to stay high until it detects a frequency around the center frequency. This seems to match the description in words.

    What happens when you change the frequency farther lower from your center frequency does it go back low?

  • 0
    Guru 316780 points

    And how much noise is there when the tone generator is off? If it's more than 10 mVrms, it might detect the center frequency.

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 18491 points

    Thanks.

    Update from customer:

    I did a little work looking at this circuit after sending my email and it looks like the tone generator that is using a square wave to generate the tone, has enough harmonic content to throw off the detector. Once I used a sine wave generator, the detector behaved as expected. Since my input is a microphone, it would make sense that there is enough ambient noise to give false readings when the tone generator is not active.