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Pulse generator question

Discussion in 'Electronic Repair' started by [email protected], Apr 18, 2005.

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  1. Guest

    I'm trying to help my son with his electronics lab work for school. The
    project is to set up an exclusive or gate, an SN or DM 7486 and apply a
    1.0KHZ square wave at a TTL level, (which I'm assuming is simply
    5.0Vpp) to one input while alternating an either high or low level to
    the other input. We are to observe the output of the gate on the scope
    and then draw up a waveform display depicting the output corresponding
    to the different inputs applied.
    I'm using an old Systron Donner Datapulse 101 pulse generator which has
    two outputs available: a positive and a negative. They are marked
    "output into 50 ohms" I set up a 5.0Vpp 1.0ms positive going pulse with
    a 50% duty cycle as measured on the scope and then attempted to apply
    it to the input of the gate. As soon as I did this though the pulse
    disappeared as though the gate's input was loading it down. I next
    placed a 50 ohm termination on the output and tried it again. I noted
    that the output (before I connecteed it to the gate had decreased
    slightly due to the additional loading the 50 ohms provided, so I
    boosted it back up to the 5.0V level and observed that the pulse once
    again disappeared a soon as I connected it to the gate input. I am
    aware that some generators have a "TTL" output. I suppose that I should
    know this but its been a long time since I've played around with any
    TTL stuff so I have to ask:
    a. Is my 5.0V pp signal level correct?
    b. Would a generator having a "TTL" output provide a higher current
    drive signal into the gate? And is this what is needed?
    c. Is this why my generator is being loaded (I suspect) down?
    d. Can I do anything about this short of trying to find another
    generator?
    (Which would be difficult, if in fact it appears that this is the
    problem).
    The project is due on Thursday so any assistance anyone may be able to
    offer would be most sincerely appreciated.
    Lenny Stein, Barlen Electronics
     
  2. Ross Herbert

    Ross Herbert Guest

    The input current requirements of a DM7486 gate are very small (max
    1mA) so this should not load down the pulse gen. Check the operation
    of the gate/s in the IC using simple hard-wired logic level inputs on
    a bread-board and make sure that the truth table is confirmed. If this
    works as expected then perhaps the pulse gen is faulty.

    SNIP
     
  3. Jerry Maple

    Jerry Maple Guest

     
  4. Guest

    Yes we did double check all the wiring and it is correct. I never
    considered that the pulse generator could be bad though. I suppose if
    there was series resistance to the output and it was exceptionally high
    then the output would display on a scope but might be loaded by the
    input of a gate. It seems far fetched but I'm not sure what else it
    might be. Of course I have no schematic for the pulse generator so I
    can't take a look at the output circuit, and Systrom Donner has been
    sold / bought out at least twice since this model was made. Lenny.
     
  5. DaveM

    DaveM Guest


    It's been several years since I used a Datapulse 101 generator, but it
    certainly should be able to drive a single TTL input. In fact, IIRC, that
    generator has a 50-ohm output, which, even when loaded with a 50-ohm
    termination, will drive quite a load. If the generator's output is being
    killed by a TTL input, there is certainly something wrong.
    1) Make sure that you aren't AC-coupling the generator to the circuit under
    test.
    2) Make sure the pulse is ground referenced, with a maximum of +5V peak
    pulse amplitude
    3) Make sure the TTL gate works when you apply jumpers to Vcc and ground.

    If all the above are good, then the most likely cause is a faulty generator.
    THose things are very hefty instruments, but they do fail, especially if
    someone applies a DC voltage to the output.
    Cheers!!!!

    --
    Dave M
    MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
    the address)

    Never take a laxative and a sleeping pill at the same time!!
     
  6. Eric

    Eric Guest

    why not just get a LM555 timer IC and generate a 10KHz pulse for your
    project, that output signal should not disappear.



    I'm trying to help my son with his electronics lab work for school. The
    project is to set up an exclusive or gate, an SN or DM 7486 and apply a
    1.0KHZ square wave at a TTL level, (which I'm assuming is simply
    5.0Vpp) to one input while alternating an either high or low level to
    the other input. We are to observe the output of the gate on the scope
    and then draw up a waveform display depicting the output corresponding
    to the different inputs applied.
    I'm using an old Systron Donner Datapulse 101 pulse generator which has
    two outputs available: a positive and a negative. They are marked
    "output into 50 ohms" I set up a 5.0Vpp 1.0ms positive going pulse with
    a 50% duty cycle as measured on the scope and then attempted to apply
    it to the input of the gate. As soon as I did this though the pulse
    disappeared as though the gate's input was loading it down. I next
    placed a 50 ohm termination on the output and tried it again. I noted
    that the output (before I connecteed it to the gate had decreased
    slightly due to the additional loading the 50 ohms provided, so I
    boosted it back up to the 5.0V level and observed that the pulse once
    again disappeared a soon as I connected it to the gate input. I am
    aware that some generators have a "TTL" output. I suppose that I should
    know this but its been a long time since I've played around with any
    TTL stuff so I have to ask:
    a. Is my 5.0V pp signal level correct?
    b. Would a generator having a "TTL" output provide a higher current
    drive signal into the gate? And is this what is needed?
    c. Is this why my generator is being loaded (I suspect) down?
    d. Can I do anything about this short of trying to find another
    generator?
    (Which would be difficult, if in fact it appears that this is the
    problem).
    The project is due on Thursday so any assistance anyone may be able to
    offer would be most sincerely appreciated.
    Lenny Stein, Barlen Electronics
     
  7. Mark

    Mark Guest

    make sure the generator is set up so the low is around 0 volts and the
    high is around +5 (Note that is not the same as 5vp-p becuase 3 to 8
    is also 5 volts p-p but would be wrong)

    If the generator can drive a 0 volts to +5 volt pulse into a grounded
    50 Ohm resistor but not into your gate, then you have the gate hooked
    up wrong or the gate is bad.

    Mark
     
  8. Guest

    Perhaps I stated this ambiguously. The 1.0KHZ pulse starts at the zero
    line, or zero volts. One complete cycle is 1.0ms. The pulse starts off
    at t=0. It takes a positive excursion to +5.0V for .50ms. It drops back
    to 0V for .50ms and returns to +5.0V at 1.0ms. One complete cycle.
    Does this sound like what you're describing? In any case I'll reconfirm
    the signal into a 50 ohm load and if that works then I'll have my son
    triple check his wiring. Thanks.
    Lenny.
     
  9. Gerard Bok

    Gerard Bok Guest

    Are you aware of the 7486 pinout oddities?
    There are several flavours and they do not share the same pinout!
     
  10. Guest

    Well we discovered the problem. Actually I'm a bit embarrassed to
    relate this but I will. My son triple checked his breadboard and
    assured me that everything was ok. So I took him at his word. He is
    good but after almost 45 years in this business (is the power off?, ...
    SURE it is....) I should have looked myself.
    The schematic showed one of the inputs going through a 1K resistor to
    5.0V. The junction of the gate and resistor went through a switch to
    ground so that input could be pulled low when needed. The other input
    of the gate has a symbol for a pulse generator to ground. He mistook
    that as a direct connection to ground, effectivly grounding any signal
    applied to that input. The circuit now works fine and my 15.00
    Datapulse generator which I picked up at the surplus store the other
    day is a keeper. I sure would like to have a manual for it though.
    Thanks to everyone for all the help and advice. Lenny Stein, Barlen
    Electronics.
     
  11. Ross Herbert

    Ross Herbert Guest


    So, if you had taken my advice and tested the ic by itself using
    hardwired logic levels (ie, without any of the fruit used by the
    circuit you were building), you would have solved the problem in 5
    minutes. Any experienced tech should learn very quickly never to take
    another persons word on oath when things don't work as they should.
    They always check the circuit themselves.
     
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