Voltage reading on o-scope/DMM

Discussion in 'General Electronics Chat' started by mbxs3, Apr 20, 2011.

  1. mbxs3

    Thread Starter Active Member

    Oct 14, 2009
    141
    3
    I have a tektronix 2215A and a DSO Nano(Seeed Studio) and a Fluke 73iii DMM. I also have a cheap function generator, a Mastech 2MHz. So I was messinga round with the function generator and viewing some wave forms on both of my scopes. At the same time, I had my fluke hooked up to measure the voltage output. The function generator I have only displays the frequency so I had the DMM hooked up to compare the voltage to what my scopes were displaying. Both scopes were hooked up to the same points at the same time and displayed the same waveform with identical frequency and voltage amplitude. However, the voltage level on the scopes would not match what my fluke was reading. I also switched everything over to a dc output and again, the scopes would show the same constant dc signal but its magnitude would differ from the DMM reading.

    I don't recall the exact numbers when looking at sine waves and ac amplitude but I do recall the dc readings. I had the function generator outputting 10vdc per my scopes display, but my DMM would only read 9.3 or so. I thought maybe my fluke was a little off but I hooked it up to my dc power supply which has its own voltmeter and the readings on the DMM and PSU voltmeter were identical.

    Any idea/insight as to why the readings would differ from my scopes to my DMM?

    I will get some pics up soon to give you a visual of what I am talking about/asking.

    Also, a side question...my tektronix 2215a trace is flashy. I can see the waveform fine but is there a setting to allow the trace to be solid and not flashy? It might just be my unit, it is old and purchased on ebay. I am quite thrilled that it displays correct waveforms but was just hoping I could get that trace to not be so choppy.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    When is the last time that the O'scope was calibrated - or any of the rest of your test equipment?

    In the electronics industry, test equipment is calibrated on an annual basis. This ensures that the readings obtained are accurate within the limitations of the particular instrument.

    Hobbyists seldom calibrate their equipment; as a result they drift out of calibration over time. The more time passes since it's been calibrated, the less accurate your readings will be.

    There is a service and calibration manual for the Tek 2215 at the Bama edu Boat Anchor archives; from this page:
    http://bama.edebris.com/manuals/tek
    click on the 2215 link, and you can then download the Tektronix_2215_ServiceManual.pdf file, 13mb.
     
  3. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    What frequency are you using?

    I'd be inclined to believe that the multimeter has an internal low pass filter and you are approaching the limit. This causes the measured drop in voltage.

    Also, scopes aren't very accurate at voltage but they do a decent job with it. A typical digital scope will beat ±2% over full range, and an analog would be a bit wider. However, oscilloscopes aren't really designed to measure voltages alone; they are for waveforms. There's not much point in making a super precise oscilloscope (with regards to voltage - time base is usually very accurate.)
     
  4. mbxs3

    Thread Starter Active Member

    Oct 14, 2009
    141
    3
    I had it set at 60hz. And Sgt, no I have not calibrated either of my scopes. I was leaning toward my DMM being the issue since both of my scopes had identical readings.
     
  5. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    Were you measuring true RMS voltage on both the scopes and the DMM? What waveform were you using?
     
  6. mbxs3

    Thread Starter Active Member

    Oct 14, 2009
    141
    3
    As far as true RMS goes, I honestly don't know. I don't think my DMM is a true RMS DMM. I'll have to look more into it. I still don't fully understand true RMS. Also, I was measuring a sine wave.
     
  7. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    To make a fair comparison, set the function generator frequency to, say, 100 Hz and make the waveform a sine wave. Then compare the measurements of the meter to the scope.

    You're running into the old situation of "A man with one watch knows what time it is. A man with two watches is never sure."

    Also, you need to know what your scope measures. If it's a digital scope, then they typically measure the actual RMS voltage of the waveform. We have the DMM makers to thank for screwing up the nomenclature. A true-RMS DMM typically does not measure the real RMS value of a waveform -- typically, it's the AC-coupled value. So the DMM makers then called the real RMS measurement DC+AC RMS (actually, it goes back to at least the HP 3456 digital voltmeter introduced in the early 80's, which has that nomenclature on the front panel). Anyway, if your DMM was measuring RMS, but it read low compared to the scope, you could also be missing the DC offset. Set the DMM to DC and measure the function generator's output. Then add the DC and RMS values measured by the DMM in quadrature (take the square root of the sum of their squares) and you should have the same reading.

    My guess is the instruments are reading correctly -- you just need to learn what "correctly" means. :p Again, we thank the marketers for screwing things up.

    Ultimately, as SgtWookie mentioned, the only way to be sure is to get your instruments calibrated. Nowdays, a good calibration can cost a substantial fraction of the instrument's cost (or more) -- which is why most of us hobbyists don't bother.
     
    mbxs3 likes this.
  8. Jotto

    Member

    Apr 1, 2011
    159
    17
    Every scope has a 5v cal to check unit. Hook your probe up to that, also make sure your cal is all the way clockwise and clicks. Your calibration is with volts/division,

    Yes calibration is important for you scope if your doing precise adjustments of some gear. Seems everyone passed up the basics on using a scope, an went right to PMEL calibration. I have seen to many people adjust their scopes by turning the calibration on the scope and not realizing that its not in the proper position. Lets troubleshoot problems that don't exist.

    I have one of the best scopes ever made, Sencore SC3100.
     
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