Oscilloscope Z input

Discussion in 'The Projects Forum' started by dlp4341, Aug 29, 2011.

  1. dlp4341

    Thread Starter New Member

    Aug 29, 2011
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    I use my Oscilloscope to make XY plots of audio LR signals.When the input signals go off the trace becomes a bright spot, and will burn the CRT.
    Any suggestions on a Spot Killer circuit that uses the Z input to adjust the intensity down ,or off ?
    The audio input signals are are as low as -10 dBv, when the signals are lower or off: I would like the Z output to switch from zero volts to either B+or B- ( +/-10v ) when blanking . And return to zero when the audio returns.
    Don
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Interesting idea.

    It might be helpful to go one step further; have the Z input be a function of how rapidly the X and/or Y inputs are changing; as the faster they are moving the beam around, the greater the intensity (up to the point where focus is lost/beam flood) that you will need in order to see the trace.

    It might be as simple as using a couple of opamp channels as precision rectifiers, their outputs acting as a charge pump with a controlled decay (like a constant current discharge circuit), that will control the Z-axis voltage. If neither the Y nor X axis have inputs, the charge pump output will bleed down to the point where the Z axis turns the beam so low as to be invisible.

    The Z-axis input requirements might vary between O'scopes.
    Does yours require that it be -10v or +10v in order to turn it off?
    Can it be either?
    Is it a sudden or gradual turn-off?
     
    Last edited: Aug 30, 2011
  3. someonesdad

    Senior Member

    Jul 7, 2009
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    I haven't seen a z input on a scope in a long time...
     
  4. SgtWookie

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    You've succumbed to the power of the dark side? :eek: ;)
     
  5. someonesdad

    Senior Member

    Jul 7, 2009
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    Probably the most logical explanation is I haven't seen all that many scopes in the last 30 years... :p
     
  6. dlp4341

    Thread Starter New Member

    Aug 29, 2011
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    SgtWookie
    Thank you for your response
    My scope requires up to +10 V for blanking and it takes less than a half second to blank. When the Z input returns to zero, the trace returns.I can Manually test this with a 9 Volt battery .
    Don
     
  7. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    IIRC mine use negative voltages.
     
  8. SgtWookie

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    Jul 17, 2007
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    Well, it would be a good thing to make certain of the polarity and amplitude required for blanking before trying to build a circuit, wouldn't you agree? ;)

    It would be a simple enough experiment - a 9v battery and a 10k or so pot.
     
  9. dlp4341

    Thread Starter New Member

    Aug 29, 2011
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    SgtWookie

    I found a Tektronics forum and they were consistant on a positive voltage for blanking. Different models had different specs. TTL levels would work for all the models checked. So ... +3 to +5 for blanking, and 0 to 0.5 no blanking .Any ideas for a prototype circuit ?
    Thank You for your help
    Don
     
  10. SgtWookie

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    Jul 17, 2007
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    OK, so far, so good. ;)

    Next, what V/div are you using for your X and Y inputs? Or in other words, at what point do you want the threshold to be?

    I'm thinking something like a precision rectifier circuit for both inputs, followed by a peak detector. If both channels are below the threshold level, the Z-axis is blanked.
     
  11. SgtWookie

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    Jul 17, 2007
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    See the attached for a preliminary schematic. The goal was to provide a reasonably simple circuit using readily available components, while accomplishing the functionality desired.

    To keep the schematic less cluttered, 100nF bypass capacitors for the TL082 and LM339 IC's supply rails are omitted; they are not optional.

    X-in and Y-in are simply providing 1v p-p sine waves at low frequency for 10 cycles and 100 cycles, respectively; representing line level audio input.

    The X and Y channels are identical, so I'll just describe the X channel.

    C1 blocks the input DC level, but allows the AC signal to pass. R1 keeps the signal averaged at 0v. V(in-x) is potted in green on the right.

    A TL082 dual opamp is used as a 2-channel peak detector. R2/C3 provides the time delay for the blanking when there is no signal input. D1 provides a path for the TL082 output to charge C3, but prevents it from discharging the cap. Feedback to the noninverting input removes the varying Vf of D1 due to If (forward current) or temp from the equation. This output is shown on the plot in pink as V(peak-x), as the inverting input to the comparator.

    The comparator needs a threshold level. Due to the likely low signal level from the input, the threshold needed to be established at a fairly low voltage. R6 limits the current through D3 to ~480uA; this results in ~550mV across D3. This provides an adjustment range for the threshold level from ~0v to ~550mV. VR1's wiper is shown set to 0.1, which corresponds to 10%; the wiper output measures ~55mV, plotted in grey as V(threshold).

    If either of the comparator's inputs exceeds the threshold, the open-collector output of the comparator will be turned on, pulling the output to near-0v potential, enabling the beam.

    Room for Improvements:
    1) The inputs, being line level, will cause the threshold level to be difficult to set. A preamp stage consisting of TL072/TL082 opamps with a fairly high level of gain will alleviate this situation.

    2) The comparators have no hysteresis. While this may or may not be a drawback, there will be flickering of the beam when audio is at a low level. If the flickering is annoying, then hysteresis will need to be added.
     
  12. dlp4341

    Thread Starter New Member

    Aug 29, 2011
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    SgtWookie
    Thank You

    I love your circuit. I made a parts layout drawing to start , and
    I have ICs on order from Mousier . I have started building the circuit on my prototype board .I will let you know how it works out.
    Thank you
    Don
     
  13. dlp4341

    Thread Starter New Member

    Aug 29, 2011
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    SgtWookie

    I got your circuit up and running , thank you. You mentioned a possibility of a preamp it does not seem needed to work with my display ckt. I have not seen any flickering and the timing seem about right. It takes a few seconds of no audio to blank the screen. The un blanking timing is fast and right on. The three transistor circuit this replaces had timing issues...too fast turn off, too slow a turn on.

    http://web.me.com/donaldpatten/dpatten-dbmagazine/A_Four_Channel_Scope_Display.html

    Don
     
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  14. SgtWookie

    Expert

    Jul 17, 2007
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    Hello again Don,
    Glad you got it up and running, and happy that my "wild guesses" turned out to be "close enough" and working just like I expected it to. ;)

    I'd planned on it taking 2 to 3 seconds of no audio before the output was blanked, figuring that would likely be approximately long enough for most music. There really isn't a delay for un-blanking; it's simply how fast that U1a/U1b (the TL082's) can charge up C3 and C5 to exceed the comparator threshold. Since the TL082's can source >15mA current and the caps are 1uF, it takes very little time (well under 1mS) to exceed the comparator threshold; to a human eye it should be instantaneous.
     
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