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  • TI Thinks Resolved

What part would I need on order to demodulate an 18Khz frequency modulated signal (carrier) modulated at 500Hz to 7Khz?

Hi. I am seeking to discover whether TI has an analogue product that is suitable for demodulating an 18Khz frequency modulated signal that is modulated by an audio signal as low as 500hz and up to say 7Khz.. I'm contemplating whether the best approach is to use a Phase Locked Loop (PLL) IC. Thanks.

  • Hello Richard,

    Is this for a future product? Or a one-time project?

    The ideal device is the LM565 or LMC568 PLL decoders - which are now obsolete, but still available on the surplus market.

    They were designed for exactly this application  - decoding narrowband FM audio signals. The LMC568 is perfect for what you are doing...but...

    But beware that a 7kHz max frequency deviation on a 18kHz carrier is a very, very wide modulation index. You may have problems with the loop filter dropping out of lock (ripple). Modulation deviation is normally around 10-20% of the carrier frequency.

    The LM567 tone decoder is still available. You can probably tap the recovered audio off the loop filter pins, but it is designed for a narrow lock range (<15%).

    Regards,

    Paul Grohe

    Low Power Amplifiers and Comparators (LPAC) Applications

  • In reply to Paul Grohe:

    Hi. It's for a future product.  So, I cannot be using LM568.  And, yes, I should keep modulation lower, possibly in 100hz steps. No more than say 1Khz or 2Khz max.   It's looking though, like I may be better off trying to demodulate in software. With Arduino or the like.

  • In reply to Richard Rogalewski:

    Hi Richard,

    Have a look at the CD4046B PLL block.

    There was an nice Application Report written about it:

    www.ti.com/.../scha002a.pdf

    Simple, and it should work if you keep the deviation down to a reasonable level...

    Regards,

    Paul Grohe

    Low Power Amplifiers and Comparators (LPAC) Applications

  • In reply to Paul Grohe:

    Hi. Thanks. Actually I've had a thought. That maybe I should be using a DSP chip. Migrating over to a DSP forum.

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