Confused about Xtals

Discussion in 'General Electronics Chat' started by bumclouds, Nov 21, 2013.

  1. bumclouds

    Thread Starter Active Member

    May 18, 2008
    82
    0
    Hi everyone,

    [​IMG]

    I have two USB hubs on a board which work just fine, but it's only when I plug a USB device in that I can see a sinusoidal crystal signal. And furthermore, it's only on "X2", the "Xtal Out", not on the X1 (in) pin.

    a) How can there be an output when there's no input?
    b) I thought crystals were totally passive devices which produced sinusoidal signals always. Why is the sinusoidal signal appearing and disappearing?

    Thanks!
     
  2. colinb

    Active Member

    Jun 15, 2011
    351
    35
    Probe capacitance is affecting the oscillator circuit when you probe it. One side of the xtal is connectoed.to the oscillator bufferoutput and might be able to tolerate more load capacitance... thus the asymmetry.
     
  3. bumclouds

    Thread Starter Active Member

    May 18, 2008
    82
    0
    I'm a little confused still.

    So pin "1" of the crystal producing a signal, and pin "3" is just simply accepting a signal from the usbhub IC?

    Why does the IC output a crystal signal?
    Also, Pin 1 is producing a signal that I can't see because my oscilloscope interferes with it?
     
  4. colinb

    Active Member

    Jun 15, 2011
    351
    35
    To make an oscillator with a crystal, an active element providing gain >1 and positive feedback is required. The IC has something like a CMOS inverter that drives the crystal and produces a stable is oscillation at the crystal's resonant frequency. The crystal just responds to the drive signal, it doesn't create a signal alone.

    You can buy self-contained oscillators that have 3 or 4 pins, and then you just apply power and ground and get a clock out.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Are you sure that the chip shown is not powered through the 5V on the USB cable?
     
  6. colinb

    Active Member

    Jun 15, 2011
    351
    35
    Also look up Pierce oscillator to see how the crystal is used in an oscillator circuit.
     
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