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how do bad contacts affect (falling) edges

Discussion in 'Electronic Design' started by Jenalee K., Jun 26, 2006.

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  1. Jenalee K.

    Jenalee K. Guest

    When I set up a simple low-speed combinatorial pulse detecting circuit
    using HC-logic on a breadboard I noticed that especially the falling
    edges were horrible and caused glitches. The edges had some sort of
    oscillation on them (unlike ringing) and I supposed they were due to
    bad contacts of the breadboard but whatever I tried I couldn't get rid
    of them until I finally soldered it all together.

    Now my questions: _how_ do bad contacts do this? And why only the
    falling edges?

    Thanks,
    Jenalee K.
     
  2. Joerg

    Joerg Guest

    Hello Jenalee,

    As Jack hinted typical breadboards such as the white "plug-and-go"
    boards or whatever they are called aren't good enough anymore for modern
    logic. Take a look at VCC, maybe you'll see ringing there as well.

    Anyway, a photo and a schematic would help. I really don't think it's
    bad contacts. Then you should be seeing changes when you tug on the wires.
     
  3. jack

    jack Guest

    Breadboards -- the conventional variety -- are death over a few hundred kHz.
     
  4. Jenalee K.

    Jenalee K. Guest

    Joerg a écrit :
    Thanks Joerg for your reply. I will try to post pictures and schematics
    tomorrow.

    Thanks,
    Jenalee K.
     
  5. Have you placed a decoupling capacitor across each IC (100nF)? It should be
    right on top of the IC's, not at the end of the vertical tracks.

    Meindert
     
  6. Jenalee K.

    Jenalee K. Guest

    Meindert Sprang a écrit :


    C1
    4n7
    || ___ |\ pulse with
    pulse ------||---o---|___|---o--| >O- weird edges
    15Vtt || | R1 | |/
    | 4k7 | 74AC14
    .-. |
    | |R2 --- C2
    | |10k --- 4n7
    '-' |
    | |
    | |
    | |
    === ===
    GND GND
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    Original breadboard circuit without C2. The output pulses had weird
    oscillating rising edges. As Joerg proposed, closer inspection of the
    power lines reveiled some 100MHz or so ringing on the 5V and GND. After
    the addition of C2 the problem went away, the ringing became one period
    of "slow" 20MHz-ish bouncing. I had placed decoupling caps (100n & 10u)
    as close to the 74AC14 as possible but they didn't seem to have any
    influence.

    There were also clamping diodes just after C1 but they didn't influence
    the behaviour so I left them out for clarity. The purpose of R1 is
    protection for the schmitt-trigger and its value doesn't seem to have
    any influence.

    BTW, I made an error, it is on the rising edges only, not on the
    falling edges.

    Thanks,
    Jenalee K.
     
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