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    NE567 as Am Fm demodulator question

    Can someone confirm if my understanding is coorect.
    Inside the Ne567 there are 2 phase comparators I & Q, you can use the loop filter voltage from these to dermodulate Am and Fm signals, Ok.

    Am I correct in saying the Q phase comparator only recognizes shift in frequency rather than a shift in amplitude as its 'looking' at the crest/trough of the wave where there are no changes in time with amplitude mod.
    And for Fm the I phase comparartor is able to detect shifts in amplitude as its looking at the zero cross point where a change in amplitude does create a change in time for the zero cross.

    Its taken me a while to get the 'hang' if I & Q, I'll be pleased if I got this even just part correct.

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    Re: NE567 as Am Fm demodulator question

    You got the function basically right, but you should consider that the chip is designed as tone decoder (on-off switching) rather than linear AM demodulator. The PLL should work as a halfway linear FM detector.


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    Re: NE567 as Am Fm demodulator question

    Ok thanks.
    Yeah I know the '567 is a tone decoder not a demod chip, I was more interested with understanding the concept of the I & Q sections within the chip.



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    Re: NE567 as Am Fm demodulator question

    I think I get this a little better now.
    The Q section of the Pll recovers the carrier, and gives a steady unmodulated version of it.
    The I section of the chip acts as a multiplier much in the same manner as a switching mixer would, it multiplies the phase shifted recovered carrier with the modulated Rf signal, the loop filter then removes the recovered carrier leaving audio.
    Last edited by dr pepper; 10th October 2018 at 20:43.



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    Re: NE567 as Am Fm demodulator question

    I think you are looking for an overly simplistic interpretation of the I and Q signals.
    Can I suggest that you look this up on the Internet - http://whiteboard.ping.se/SDR/IQ seems (at a quick glance) to show what the signals are all about, and even show how AM and FM are represented.
    Susan


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    Re: NE567 as Am Fm demodulator question

    A typical theoretical explanation thanks Sue.
    Interesting to see I & Q as a 3d chart.
    Looking at the Am modulated I & Q signals you can see why the Pll is needed to regenerate the transmitted carrier.
    Mixing it with the modulated signal then demodulates the Am audio.
    W2aew has a couple of good vids on the subject.



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