Tektronix 2445A what read-out should I trust?

Discussion in 'General Electronics Chat' started by CurlsOnKeys, May 20, 2015.

  1. CurlsOnKeys

    Thread Starter Member

    Jan 3, 2015
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    Hi,

    I've recently bought a second hand Tektronix 2445A analog scope. I like it, have been playing around with it for a while now, read the manual and tried out different functions, but after a couple of measurements I got a bit confused.

    Sometimes the read-out level the Tek is giving me (A1 in the upper left corner) differs from the level I measure with the Delta-V-measurement-knobs.
    I've attached a picture as example: I measure an incoming alternating signal, triggered at lowest level (level-knob all the way to the left), division set to 500 mV/div and my Tek tells me the signal magnitude at trigger is .66V
    However, if I measure it myself and put my delta-reference line (dotted line) on 0V, I get to read .5V if I put my delta-V-measurement line on the first division (which let's me assume these measurements are accurate, since division is set to .5V) IMG_20150520_144909.jpg , but when I move the measurement line up all the way to the actual signal , I get to read .78V. IMG_20150520_144938.jpg Which is something else than the trigger of the Tek is telling me in the upper-left corner (you can even see the difference quiet clearly here: my dotted measurement-line is at .66V, which is below the actual signal line, which, according to the trigger of my Tek, should also be .66V... IMG_20150520_144925.jpg )

    So what IS the actual magnitude of my signal? What read-out should I trust? .66V as my trigger level tells me or .78V as I seem to measure? More important: can I "fix" this or what would be the correct workflow? Because when I have tow signals, both triggered and I'm interested in the difference between those signals, I'm confused, because when I align my measurement reference line to one of these signals, I get a different value to start with...

    Thanks!
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    I think you're trying to use the trigger information for something it wasn't intended to do. There's a fixed delay line in the trigger path... On my Tek scopes that have readouts, they don't give me an option to display trigger info...

    I'd trust what the waveform indicates. If you require precise voltage measurements, you should check the calibration on your scope.
     
  3. CurlsOnKeys

    Thread Starter Member

    Jan 3, 2015
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    Thanks for your insights. So what does the value in the upper left corner mean then? (the .66V in this example?). Something like "Value at trigger level" but different from actual level (which can be found through measuring like I did?)
     
  4. dl324

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  5. MrChips

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    Trigger Level is the voltage threshold at which the digital scope aligns its time = 0.
    This is not the amplitude of your signal.
    In order to determine the signal amplitude, use the scale grid marks or use the adjustable cursor set for voltage reading.
     
  6. CurlsOnKeys

    Thread Starter Member

    Jan 3, 2015
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    Ok, watched a few more youtube tutorials and I think I'm getting it a bit better now... @dl324: I have the manual, but sometimes it's hard to find something if you don't know what you're looking for :)

    Anyway, I'm still a bit confused. Trigger level is the voltage threshold at which the digital scope aligns it's time = 0, I get that. Let's say you have a square wave with 5V amplitude (0V minimum, 5V maximum). If you'd set the Tek to auto level mode with the level knob in the center half of it's range, according to the Tek manual, the trigger point would sit midway between signal peaks, so at time 0, you're looking at the waveform somewhere at 2.5V.
    If you turn the level knob to - or +, this mode, again according to the manual, initiates triggering near the 10% or 90% point between signal peaks. So you could move the trigger point to a voltage threshold of 0.5V (10%, all the way to the left) or 4.5V (90%, all the way to the right). Obviously if you'd set the voltage threshold manually (no auto level) and put it lower than 0 or higher than 5, no triggering would take place. Correct so far?

    So: to go back to my example: if I measure with the adjustable cursors, the minimum signal peak I measure is .78V. How is it then, that the voltage threshold for the trigger is LOWER than the minimum signal peak? (.66V). Doesn't this mean that no triggering should take place, since the trigger level is BELOW minimum signal peak?
     
  7. MrChips

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    What do you mean by "minimum signal peak I measure is .78V"

    A signal has a minimum value and a maximum value. For example, a logic level square wave might have a minimum value of 0V and a maximum value of 5V. In order for the scope to trigger properly, the trigger level must be adjusted to be somewhere between 0V and 5V. No triggering should occur if the trigger level is outside of this range.
     
  8. MrChips

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  9. CurlsOnKeys

    Thread Starter Member

    Jan 3, 2015
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    That's exactly what confuses me. I'm measuring a pulse wave with a duty cycle of 87.5%, so it reaches its high level (maximum value) 7/8 of the time and its low level (minimum value) the remaining 1/8. Measured with my scopes measurements tools (delta-V) and with a multimeter, my low-level is .78, maybe .8V. However, according to the scope the trigger level is .66V. Which means, as you say, no triggering should occur. Yet the picture I took in my original topic is a picture of a perfectly stable, triggered waveform.

    What am I missing? :)
     
  10. MrChips

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    1) Duty-cycle will not affect triggering of the type we are discussing.
    2) delta-V is not important at this point.

    What is important is the absolute value of the minimum and maximum voltages in your signal and the trigger voltage level.

    Make sure your input channel is set to DC.
    Make sure your trigger selection is set to DC.
    If the input and/or trigger level is set to AC then that requires an entirely different discussion.
     
  11. dl324

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    Have you tried experimenting with your trigger setting and display on a waveform that isn't a step function? E.g. a sine wave?

    BTW, your scope is digital.
     
  12. CurlsOnKeys

    Thread Starter Member

    Jan 3, 2015
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    I know duty-cycle will not affect triggering, I was just giving all the information I had about the wave I was trying to measure.

    Delta-V are my "adjustable cursor settings" to which you referred a few posts earlier. Which I used to determine the amplitude of my minimum value. Which I confirmed after double checking with a multimeter.

    Input channel is set to DC, trigger selection is set to DC.

    Ok, did some further experimenting. Measured a sine-wave of which I knew the amplitude was 5V. As you can see here, triggering works as expected. My trigger level is set maximum negative (-1.9V) and with my adjustable cursors I measure -2.4V as minimum level. IMG_20150521_125116.jpg Same goes for maximum level.
    Back to my pulse wave: if I put my trigger level to maximum positive, trigger level is 2.4V. With my adjustable cursors, I measure as maximum level 2.8V IMG_20150521_125817.jpg . As expected. However, with my trigger level to maximum negative, trigger level is .3V (yesterday .6) and with my adjustable cursor I measure as minimum level .8V IMG_20150521_125853.jpg (which is the same as yesterday).

    So this leads me to believe there's something in the signal which activates triggering, but which I cannot see on my scope? If I manually trigger, no triggering occurs as long as the trigger level is below .3V, from where triggering starts to happen. But if I measure, the lowest part of the signal I can measure (minimum level) is higher (.8V) than the trigger level. Which doesn't make sense, does it?

    It's my first scope, but I'm pretty sure it's not? Doesn't look like a digital scope to me, but you're comment made me doubt that.
    Not that one should always trust wikipedia, but it's just one of the 29300 results you get when you search for "2445a analog oscilloscope":

    If we are talking about non-storage, non-digital analog scopes in the 2000 series, the main differences are these:
    the 2200 series is 2 channel. The 2335 and 2336 are 2 channel, ruggedized versions mostly made for the military.
    The 2400 series are 4 channel, 2 of the channels having full attenuators. Otherwise, within each the main difference is bandwidth.
    The 2200 series is 20 - 100 MHz. The 2400 series starts out at 150 MHz (2445,
    2445A) but goes up with the 2445B;
    150 MHz for early units, 200 MHz for late units. The 2465 has a bandwidth of 300 MHz, the 2465A 350 MHz, the 2465B 400 MHz.
     
  13. dl324

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    Outwardly, a digital scope doesn't have to look any different than an analog. Both are going to use similar displays and controls.

    When I browsed the manual, I saw some specs that mentioned LSD and there were references to DAC (12 bit IIRC). The scopes I've used that had delta V/T measuring capability were all digital. If you think about it, it's much easier (and more efficient) to digitize a waveform if you're going to try to take measurements from it. It would also explain the apparent discrepancy you see in trigger voltage on a step function vs a sine wave.

    When you're trying to measure voltages on a scope, keep in mind that they're usually calibrated to within 2-4%. There was some Poster who was trying to resolve milivolts on a 20V signal. No scope has that kind of precision.
     
  14. MrChips

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    What you have is an analog scope.
     
  15. dl324

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    I've seen a lot of references to Tek 24xx scopes with sampling rates in the 100-500MS/sec range. I guess if I really cared, I could ask on the Tek Scopes Yahoo Group...

    Whatever you have, it's a nice scope.
     
  16. MrChips

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    You are using the ΔV controls incorrectly.

    Do the following:

    1) Set the input channel to GND. (Assume all adjustments are for CH1 only.)
    1B) Make sure all the VAR knobs are turned fully clockwise until it clicks.
    2) Adjust the VERTICAL POSITION for CH1 until the trace is in the centre horizontal line of the screen. Don't touch VERTICAL POSITION again.
    3) Set the input channel to DC. Apply test input signal.

    4) Set TRACK/INDEP to INDEP
    5) Adjust ΔV until the cursor is at the same centre line as (2) above.
    6) Verify that adjusting Δ brings the second cursor to the centre line while ΔV1 reads 0.00V

    7) Don't touch ΔV again.
    8) Use the Δ cursor adjustment to read of voltage of the signal as ΔV1

    Edit: Sorry, I didn't look at your screen shots closely enough. I seems that you are using the cursors correctly. I will have to review your notes again.

    Can we see the full front panel of the scope so we can see how the triggering is set up?
     
    Last edited: May 21, 2015
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