Help with PLL circuits

Discussion in 'General Electronics Chat' started by Geeky, Oct 7, 2013.

  1. Geeky

    Thread Starter New Member

    Oct 7, 2013
    Hi guys,

    How are you all? I am trying to build to a PLL circuits. As part of the circuit, I will need an offset adder, integrator, rectifier, and another stage of offset adder.

    I havent done circuits for very long time. So please bear with me if Im crapping.

    Appreciate your help.

    I have designed the first stage offset adder and integrator.

    Please see attached diagrams. Schematic 1 is the circuit for offset adder which basically a summing amplifier, and then the second stage op-amp is the integrator.

    The AC source with 1 Vac, is passed through the R3 and R1, R3 will later be a variable resistor when I transfer this to PCB.

    From the diagram, I dont understand how the 5V added by V2 becomes 2.5V at the output of U1 (first Op-amp). I tried calculating it out, but could not get the exact point. Please advice.

    Then, at the second stage. If you see the frequency response diagram. The 3dB is approximately at 200 kHz. But by using 1/2\prodRC formula, R=R3 and C=C1, which are 1 kohm and 50 pF, respectively, the cutoff should be approximately 3.1 MHz. So I dont really get it. Please advice if Im in the right direction, or the equivalent circuit calculation is wrong.

    Please point where Im going wrong. I have also attached the time domain graph.

    Thank you so much.
  2. LvW

    Well-Known Member

    Jun 13, 2013
    Geeky, a first and short look onto your diagram reveals that the last stage (integrator) is in saturation because of the dc part that appears at the integrator input.
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    just use a ready made ic NE567
  4. Geeky

    Thread Starter New Member

    Oct 7, 2013
    Hi guys. Thanks for your feedback.

    With regard to Dodgydave comment, I have a found XR-2211A as a PLL IC, see attached datasheet.

    The datasheet provides the circuit construction for tone detection, and I need that. But however, it is unclear how could I tap out the VCO output, cause that is what I need, I need the locked VCO output.

    Please guys, advice me, I really dont understand.
  5. w2aew

    Active Member

    Jan 3, 2012
    absf and eyesee like this.
  6. Brownout

    Well-Known Member

    Jan 10, 2012
    You don't need to tap the VCO. The Locked Detect and FSK outputs are pined out.
  7. radiohead

    Distinguished Member

    May 28, 2009
    Here's a few things that may help you get started with PLL. You could do a google search for specifics of what knowledge you seek.
    absf likes this.
  8. Geeky

    Thread Starter New Member

    Oct 7, 2013
    Hi w2aew, brilliant video! Helped me a lot! Thanks for that.

    Thanks Brownout and radiohead.

    I have building the circuit in practical as well. Its working fine. But I have a small problem, please see the attached schematic.

    So the signal goes through a summing amp, integrator, full wave rectifier and final summing amp. All the sections are working fine. But after full wave rectifier (Op amp U4), there a is a DC offset. I dont want that DC offset. Because I would like to add DC through the final stage of summing amp from 0V to any arbitrary +V. So how can I remove the DC, I dont want any inline capacitor. Im thinking of buffer, will that be the correct move?

    Please advice.
  9. bertus


    Apr 5, 2008