Help needed reading waveform generator output with scope - Page 1
Forum section: electronics / beginners

author: jj5
author_rank: Unknown
timestamp: April 03, 2022, 04:08:47 pm
content: Hi there. I'm new. Recently I purchased a Rigol MSO5074 scope and a UNI-T UTG962E waveform generator.Today I plugged the channel 1 output from the UTG962E into the channel 1 input of the MSO5074 using a BNC cable and a 50 ohm terminator.The first time I did that and pressed the 'Auto' button on the Rigol the signal was detected and everything worked great. But then I lost the signal and I have been unable to get it back. Now when I press the 'Auto' button (with the cabling unchanged) I get either 'Signal overrange detected' or 'No auto signal detected' on the scope.I've tried various configurations for the UTG962E including e.g. this:What am I doing wrong? I've never used a scope before, but I figured that it would be able to auto detect a simple signal from a waveform generator... no?

author: LA7SJA
author_rank: Regular Contributor
timestamp: April 03, 2022, 04:46:15 pm
content: Hei jj51.) Your generator is sett to HighZ in to a 50Ω so your signal is lower than the 500mV-pp you think you have.2.) You have a +2V offset on the generator.3.) Try a sinus signal!Johan-Fredrik.

author: jj5
author_rank: Unknown
timestamp: April 03, 2022, 05:55:00 pm
content: Hey Johan-Fredrik, thanks for your notes!1) So if it's HighZ I don't need the 50 ohm terminator?2) I put the offset down to zero and it fixed the problem!3) Yep, a sine wave worked, but so does a square wave now too!Thanks again.

author: LA7SJA
author_rank: Regular Contributor
timestamp: April 04, 2022, 04:52:55 pm
content: Hey jj5HighZ means that the voltage from the generator is correct if the load is high impedance, in 50Ω setting the unloaded signal is 2 times the signal on a 50Ω load. Whether you need 50Ω depends on the frequency of the signal. You can just use the barrel/BNC adapter (the metall thing in the little bag of probe accesories) on the tip of your scope probe and set the generator to HighZ (You should measure what the generator is outputing/showing on the display)https://electronics.stackexchange.com/questions/574355/what-is-the-purpose-of-oscilloscope-probe-to-bnc-adaptersJohan-Fredrik.

author: jj5
author_rank: Unknown
timestamp: April 06, 2022, 09:29:54 pm
content: Hey Johan-Fredrik, thanks again. My probe doesn't seem to have come with a barrel/BNC adapter. My probe is a Rigol PVP2350. Do you know if I can buy barrel/BNC adapters for those? I tried searching but not sure what to search for... Also, is there such a thing as a HighZ BNC cable?

author: TimFox
author_rank: Super Contributor
timestamp: April 06, 2022, 09:50:13 pm
content: Coaxial transmission lines cannot be much higher than, say, 125 ohms, incharacteristic impedance(q.v.) due to the logarithmic dependence of the characteristic impedance on the dimensions.However, a short length (without termination) of normal coax (RG-58/U 50 ohm cable, for example) into a high-impedance oscilloscope input (1 megohm in parallel with maybe 20 pF) will look like a high resistance (1 megohm) in parallel with that 20 pF plus the capacitance of the cable (95 pF/m for RG-58/U). When terminated with a 50 ohm resistor, it looks like 50 ohms almost-resistive at the other end.

author: jj5
author_rank: Unknown
timestamp: April 06, 2022, 09:57:13 pm
content: So much to learn...

author: TimFox
author_rank: Super Contributor
timestamp: April 06, 2022, 09:59:10 pm
content: I forgot to mention: In my reply, "short" means that the "electrical length" (propagation time down the cable length) is much less than the period of the sinusoidal signal or rise time of a pulsed signal. An unterminated line that is not "short" will show interesting reflections that should be studied in their own right. In RG-58/U coax, 1 meter would have an electrical length of roughly 5 ns.

author: LA7SJA
author_rank: Regular Contributor
timestamp: April 06, 2022, 11:23:44 pm
content: Hi jj5Here is one examplehttps://www.ebay.com/itm/353852144461?hash=item52633b3b4d:g:Y4oAAOSwR9Zh3U3VJohan-Fredrik