Help needed reading waveform generator output with scope

[jj5]

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?

[LA7SJA]

Hei jj5
1.) 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.

[jj5]

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.

[LA7SJA]

Hey jj5

HighZ 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-adapters

Johan-Fredrik.

[jj5]

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?

[TimFox]

Coaxial transmission lines cannot be much higher than, say, 125 ohms, in characteristic 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.

[jj5]

So much to learn...

[TimFox]

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.

[LA7SJA]

Hi jj5

Here is one example
https://www.ebay.com/itm/353852144461?hash=item52633b3b4d:g:Y4oAAOSwR9Zh3U3V

Johan-Fredrik