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Phase locked loop FM demodulator

Discussion in 'General Electronics Discussion' started by dayanj, Sep 24, 2010.

  1. dayanj

    dayanj

    15
    0
    Sep 16, 2010
    I have made the phase locked loop in FM demodulating mode, using NE/SE565.
    I am new to electronics and have very little background in making circuits.

    I plan to use this to detect frequency of a oscillating AFM tip(tuning fork) and to convert it to a voltage signal.

    However, I am not sure how to check the performance of this circuit,
    as shown in the data sheet i get the output of pin 4 and 9 square wave of free running frequency and a triangle wave.
    it says out put is a dc level?

    when I check the output between v7-v6, what kind of output waveform i should expect to see in oscilloscope and how should it be changed when i change the input signal frequency?

    please help
    Thanks

    datasheet :http://www.classiccmp.org/rtellason/chipdata/ne565.pdf

    DJ
     
  2. Militoy

    Militoy

    180
    0
    Aug 24, 2010
    Pin 9 should have a negative-polarity triangle wave; and pin 4 around 6V peak-to-peak positive pulse - but offset a bit under zero volts - so dipping a bit negative. Neither should change with input frequency - the VCO center frequency is set by the resistor and cap on pins 8 and 9. What I'd like to know is where you found the IC you're working with? I thought these parts went obsolete in the late 1980s - and they are somewhat scarce these days.
     
  3. dayanj

    dayanj

    15
    0
    Sep 16, 2010
    Thanks a lot for your reply!
    I just had to continue someone else's project! well i think he found it from some old book! May be i should go for LM565 or some other new one( do you have any recommendations?)

    However I guess there should be an "expected output and how it should change according to the input" fact in generally to any PLL in FM demodulation mode? can you explain that to me?
    Thanks :)

    DJ
     
  4. Militoy

    Militoy

    180
    0
    Aug 24, 2010
    In FM demod mode, pins 4 and 5 will be shorted together - and the phase comparator output (pin 5) will force pin 4 to a DC level that will change with the input frequency. The feedback from pin 5 to pin 4 shifts the VCO frequency so that it tracks the input.

    I don't currently use PLL chips - so any recommendation I would make would be from reading the datasheets - which you can probably do as well yourself. Unless you already have the SE565 though - I would look on Digikey or Mouser for a more readily available part.
     
  5. dayanj

    dayanj

    15
    0
    Sep 16, 2010
    Thanks again Militoy!!
    (sorry I am going to ask very basic level question)

    can you please tell me how to measure that DC level. I already built the circuit with NE565. I did connect pin 7 and pin 6 to the scope and thought that is the output, and changed the input frequency. I see a waveform which changes its frequency when I change the input frequency. So how do I know whether my circuit is working fine or not?
     
  6. Militoy

    Militoy

    180
    0
    Aug 24, 2010
    Voltage measurements on IC pins should always be made with the highest impedance instrument you have availlable - so as to affect the circuit as little as possible by minimizing parasitic loading. That means a good scope, if you have one. With your PLL circuit connected as an FM demodulator, if the input and output frequencies track - then your circuit is working. If the output drifts with the input steady; or the output stays stable while the input shifts - then your circuit needs attention. A dual-trace scope will clearly show you if the input and output are tracking.
     
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