FSK modulation and demodulation

Discussion in 'General Electronics Chat' started by JiaZheng, Jan 28, 2013.

  1. JiaZheng

    Thread Starter New Member

    Jan 28, 2013
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    Hello everyone. The followings are problems in my project.
    1. I'm in difficulty to generate two fundamental FSK frequencies around 10MHz. Maybe 900KHz and 1100KHz. I intend to use two active filter (lowpass may be good enough) to filter out high frequency components in square waves to give two sine waves. Later on switch between them to get my FSK signal. However, I found the active filter cannot be effective when square wave frequency over 100KHz. I thought it may be my Op-AMP bandwidth product is not large enough (What I'm using is MCP6292, which has gain-bandwidth product 10MHz). I think 10MHz is large enough for 1MHz signal but I was wrong. The output distortion is very large. My friends suggested me to use simple RC. But what if my load has very small impedance compared to RC filter?

    2. I'm going to demodulate the FSK by PLL circuit. But I also found most of PLL circuit are CMOS logic. I mean the VCO output is square wave. I found an IC, CD4046, that its application sheet states it can demodulate FM signal. So I think it also be feasible to FSK. But its frequency range is only up to 1.4MHz. (my FSK is 10MHz). So I turn to another PLL, CD74HC4046. It doesn't give application reference in FM demodulation. I wonder whether it can works and what external components should I add to this PLL so that I can achieve a good performance in demodulate FSK. Many thanks for your help
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    5,991
    3,737
    Why do you need such a high FSK frequency?

    If you are transmitting, then use a 10M hz broadcast frequency with your FSK signal modulated into the carrier frequency (e.g. 1000 and 2000 Hz). The whole process becomes much easier.
     
  3. JiaZheng

    Thread Starter New Member

    Jan 28, 2013
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    Yes. I thought to modulate the carrier by signal. But all ICs to transmit FM requires inductor (I don't like inductor) or its transmitting frequency is up to 400MHz or 900MHz. Do you know some IC can easily transmit FM?
     
  4. JiaZheng

    Thread Starter New Member

    Jan 28, 2013
    20
    0
    Anybody can help me about using 74HC4046 to demodulate FSK?
    I tried by my self but it can only work for very small signal frequency, at most 100Hz. There are too much noise and ringings in the output. Anybody has done such project before. Please help me !
     
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