New scope calibration help

Discussion in 'Electronics Resources' started by djiang83, Aug 1, 2011.

  1. djiang83

    Thread Starter New Member

    Jun 14, 2011
    Hi everyone, I just bought a Tektronix TDS3024 scope and could use some help knocking out some unwanted behaviors on the scope readings.

    First off, all the channels on the scope seem to have a -40mV offset when I first turn it on. This is with no probes plugged in yet and the 'offset' option for each channel zeroed out.

    I can get rid of the offset by connecting a probe to each channel and attaching the probe to the 5V square wave calibration output on the scope. But this gets annoying after a while and doesn't solve the problem permanently as the offset comes back the next time I power up.

    Also, it seems like there's some capacitance affecting the 5V square wave output when I attach a probe to it. On each rising and falling edge, there's a bit of overshoot and a parabolic decay to steady-state. (There's an attached picture of this.) There's no ringing, just overshoot and what looks like a capacitance discharging at each edge.

    Any help would be greatly appreciated. I can't wait to get this scope running smoothly so I can use it on my projects.
  2. TBayBoy


    May 25, 2011
    Your probes might have a variable capacitor that you can adjust to match to the scope.
  3. someonesdad

    Senior Member

    Jul 7, 2009
    If it's not a digital scope, then there's probably an internal pot that can be adjusted to get rid of this offset. Unless someone here is familiar with this scope's adjustments, you'll probably have to find a service manual to find the pot.

    For the overshoot, you need to learn about probe compensation. Find a non-conductive screwdriver and adjust the small capacitor's screw that should be visible in a hole near the BNC connector of your probe. Google "oscilloscope probe compensation" and you'll learn all you need to know.

    Note that any time you change a probe (even moving it to another channel), you should adjust the compensation to get the best measurement performance.
  4. steveb

    Senior Member

    Jul 3, 2008
    Are you using x10 probes? If so, it looks like you need to adjust the compensation on the probe. There should be an adjustment screw on the probe. Turn that until you have nice square waves.

    If you can't adjust it, then the probes need to be fixed or replaced.
  5. djiang83

    Thread Starter New Member

    Jun 14, 2011
    Thanks everyone! All the responses were dead on. I indeed had a 10X probe that had a capacitance adjuster that I needed to dial to achieve impedance matching with the scope. My scope also had a calibration pot in the back which helped with the offset issue.