ttl full can oscillator outputs

Discussion in 'General Electronics Chat' started by petewh, Apr 29, 2016.

  1. petewh

    Thread Starter New Member

    Mar 23, 2016
    11
    0
    When I scope ttl full can oscillators I measure their outputs overshooting the edges more than 0.5 volts above 5v and below ground. The overshoot can be as high as a volt and higher. Isn't this bad for the inputs of logic chips? I'm relatively new to tll and cmos at 1Mhz and higher. One thing I thought of is I can buffer the oscillator output with a cmos 4050 which accepts inputs above the supply voltage. When I improve my probes grounding the overshoots reduce. So far I haven't seen any problems that occurred with the circuits I've built.
     
  2. Marley

    Member

    Apr 4, 2016
    144
    40
    This is probably an effect of your scope and ringing in the test leads. Won't happen with the very short connections between logic chips.

    Make sure the frequency compensation on your scope probes are properly adjusted. Using a small resistance (try 100 ohm) in series at the point of measurement might also help.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,542
    1,251
    Agree. 99% chance it's the scope, not the part. Remember, a scope does not show you what is going on in a circuit. It shows you someone's opinion of what is going on, and that someone is thousands of miles away and has never seen your circuit. For a digital scope, you see the software guy's opinion of the digital guy's opinion of the analog guy's opinion of the probe's opinion of the circuit voltage.

    ak
     
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  4. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,319
    888
    Marley nails it. What's happening is that the tip-to-shield capacitance of your scope probe and the inductance of the probe's ground lead to your circuit form a series resonant LC circuit which causes a ringing or an overshoot to appear on the scope trace. That ringing/overshoot is not actually happening in your circuit without the scope connected, and it's nothing to worry about (other than the annoyance it causes when trying to interpret scope readings).

    To minimize this, always use your scope probe set on 10X (which has the lowest shunt capacitance) whenever possible, and use the shortest possible connection between the probe ground ring and your circuit ground.
     
  5. dannyf

    Well-Known Member

    Sep 13, 2015
    1,820
    363
    it could be due to parasitics from wiring or your scope's probe.

    Generally it doesn't matter. But newer oscillators tend to output clipped sine (=slew-limited) to help alleviate that problem.
     
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