Beginner mistake using an oscilloscope? - Page 1
Forum section: electronics / beginners

author: alexander_
author_rank: Newbie
timestamp: November 12, 2024, 04:47:10 pm
content: Equipment:O-scope - Hantek DSO5102PAWG - UNI-T UTG962EBottom line, my oscilloscope is reading two very different voltage levels on it's channel 1 and channel 2 from a sine wave AC signal I'm generating from my AWG though I would expect both signals to read the same.My AWG has two channels and both are producing a 1kHz sine wave, 5V peak to peak. Channel one plugs directly into the Oscope using a BNC connector. Channel 2 from the AWG plugs into a bread board using alligator clips and I read the signal on the oscilloscope's channel 2 using a probe. On the oscilloscope, the channel 1 signal (direct from the AWG) looks as expected; 5Vpp. However, the channel 2 signal is only 10mVpp. I would expect it to be 5Vpp as well.To troubleshoot, measured the AC voltage coming off the alligator clips from the AWG's channel 2 and it reads 5mV. I'm a fresh faced newbie with electronics, but this behavior seem quite unexpected. My intuition would say that that either the AWG's alligator clip cable is the problem. Is there something I'm missing?

author: wasedadoc
author_rank: Super Contributor
timestamp: November 12, 2024, 04:55:25 pm
content: Not the whole story but in the second photo (which you now seem to have removed) you have the generator CH1 set to 50 Ohms and CH2 set to HiZ. That of itself gives a 2:1 difference.

author: ebastler
author_rank: Super Contributor
timestamp: November 12, 2024, 05:09:12 pm
content: Not sure what external cable goes where in your wiring. (Edit: Where are the ground connections?) But isn't the yellow jumper wire right in the middle short-circuiting everything?

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 12, 2024, 05:30:22 pm
content: Quote from: alexander_ on November 12, 2024, 04:47:10 pm(To begin with:) If you want to read the same voltage on both o-scope channels then connect both channels to the same point.(To continue:) Measure generator outputs with the same o-scope channel.This simple procedure will lower number of unknown variables to one or zero instead of four.

author: alexander_
author_rank: Newbie
timestamp: November 12, 2024, 05:49:49 pm
content: That's exactly what the problem was. I've got a lot to learn! Thanks to everyone who replied.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 12, 2024, 05:51:13 pm
content: 1. The breadboard has a short circuit on it, made out of the 3 small wire bridges between the red and the black signal wires. You are measuring the voltage drop on the yellow piece of wire, which should be close to zero, just as your oscilloscope shows.If you remove the yellow piece of wire, you should see 5V on the oscilloscope.Aside from the above, you may want to know these two more things about the generator, to avoid future possible confusions with who knows what other circuits:2. Those 5V are produced inside the generator, indeed, but then there is yet another 50ohms resistor between the produced voltage and the output wire of the generator. That 50ohms is inside the generator, it will always stay in series with whatever is connected outside of the generator.3. It doesn't matter if you set the generator for 50 ohms output impedance, or for high Z output impedance, the 50 ohm series resistor (one on each channel) will still be there, only the numbers on the generator display will change, but the electric circuit will always have a 50 ohms in series with the output of a channel, no matter the generator settings.

author: Mr. Scram
author_rank: Super Contributor
timestamp: November 12, 2024, 06:03:33 pm
content: Quote from: alexander_ on November 12, 2024, 05:49:49 pmPerhaps it helps to understand that voltmeters, which a multimeter and oscilloscope both are, just measure the voltagedifferencebetween two points. What number you get depends on which points you select, both the plus and the minus. You can easily end up with irrelevant voltage readings when you select the wrong points.In this case, you effectively measured the voltage difference between the beginning and the end of a small piece of wire, the difference of which should be near zero. This is exactly what you measured. There are no absolute voltages, a voltage reading is always one point relative to the other.