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My wave generator has a 50 ohm output impedance. I'm using a 10kohm series resistor and a 100nF capacitor. I calculated the RC time constant to be (10050)(100E-9)=1.005ms. A safe value for charging and discharging should be 5*(time constant), so 5.025ms. My function generator has a square wave with a duty cycle of 50%, so I want the full period to be 2*5.025ms=10.05ms. This corresponds to requiring a frequency < 1/10.05ms, or f < 99.5Hz.

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So I set my wave generator to: Square, 50% Duty, 99Hz, 2.5V Amp, 2.5V Offset:

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Now when I look at the voltage across my capacitor on my oscilloscope, instead of it starting at 0v and charging up to 5v, it shows that it starts at -2.5v and charges up to 2.5v. The circuit simulator I compared it against, however, shows that it goes from 0v to 5v like I expected, with the same source of 2.5v amplitude and 2.5v offset.

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So why is my o-scope showing -2.5v to 2.5v across my cap instead of 0v to 5v?

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    \$\begingroup\$ Did you set that 'scope channel to AC-coupled or DC-coupled? \$\endgroup\$
    – brhans
    Commented Oct 25, 2016 at 1:51
  • \$\begingroup\$ It's set to AC coupled \$\endgroup\$
    – Austin
    Commented Oct 25, 2016 at 1:55
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    \$\begingroup\$ AC-coupled means it essentially puts a capacitor in series with the 'scope's input, and the average voltage of the signal measured at the sampler will be 0 V. So you got exactly what you asked for when you selected that setting. \$\endgroup\$
    – The Photon
    Commented Oct 25, 2016 at 2:18
  • \$\begingroup\$ But if I set it to DC coupled then I just see 5V DC. What do I do to see both the waveform and the offset? \$\endgroup\$
    – Austin
    Commented Oct 25, 2016 at 2:29
  • \$\begingroup\$ oops, looks like that was unintended behavior and I just needed to reset my O-scope. Works now, thanks. \$\endgroup\$
    – Austin
    Commented Oct 25, 2016 at 2:57

1 Answer 1

Reset to default
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The most likely explanation is that you are using a function generator that has a bipolar output. That is: the output signal is symmetrical above and below ground.

Try connecting the scope directly to the output of signal generator and see what the signal is doing.

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  • \$\begingroup\$ Hmm you're right. Even though the output graph is showing 0V to 5V, measuring directly with the O-scope shows -2.5V to 2.5V. Is there something I can change or is this just a property of my USB function generator? I thought the DC offset setting was there to change this, but I guess not? \$\endgroup\$
    – Austin
    Commented Oct 25, 2016 at 1:57
  • \$\begingroup\$ I can look over and play with the settings for a while, but should any function generator have a way to do this, or is it a possibility that I can't do anything about the symmetry? \$\endgroup\$
    – Austin
    Commented Oct 25, 2016 at 2:04
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    \$\begingroup\$ Please read OP's comments on the question and think if you want to revise your answer. \$\endgroup\$
    – The Photon
    Commented Oct 25, 2016 at 2:19

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