Osciloscope Help PWM Signal input - Page 1
Forum section: electronics / beginners

author: tpruszin
author_rank: Newbie
timestamp: September 07, 2018, 02:45:31 am
content: Hey everyone so I've bought myself a Rigol DS1054z and today I thought I'd have a look at the PWM signal coming to my servos on my RC plane and this is what I found this clearly isn't what a PWM signal is supposed to look like so I'm not sure what is going wrong with my scope to be showing thisThanks in advance if anyone is able to shed some light on this

author: boB
author_rank: Frequent Contributor
timestamp: September 07, 2018, 03:25:34 am
content: Looks like a lot of 50Hz ripple from the mains to me.Do you see any sparks when connecting the probe ground to the circuit reference ?Servo motor drivers are bridge-connected a lot of times so it may be floating.One sanity check is to connect the probe tip to the probe ground clip and then to thenegative reference in your circuit. If you still get this signal only when the probe is touching that negative,you may have to isolate your scope with an isolation transformer or get an isolated probe amp.boB

author: tpruszin
author_rank: Newbie
timestamp: September 07, 2018, 06:10:57 am
content: Thanks boB!,It is indeed the mains!Solved by simply the ground probe wasn't connected to the ground of the circuit. Or anything for that matterconnecting it fixed the problem but ill have to investigate longer ground cables as there's not much length from the probe clipThanks so much for the help!Tpruszin

author: tooki
author_rank: Super Contributor
timestamp: September 07, 2018, 09:46:44 am
content: Quote from: tpruszin on September 07, 2018, 06:10:57 amWell obviously the ground must be connected for the scope to work.As for longer ground leads: they're short on purpose. The ground lead is a little antenna, picking up noise. The longer it is, the less accurate the signal on the screen gets.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 07, 2018, 11:22:37 am
content: Quote from: tpruszin on September 07, 2018, 02:45:31 amLet me guess... you're a novice at scoposcopy, right? Permit me to make a few suggestions, which may help you to avoid certain pitfalls in the future.1. RTFM. There is a lot of useful information in the User's Manual for the DS1054z. Like for instance how to do screensaves that are perfect in resolution and much smaller in filesize than your nice photograph. And how to do basic connections to a Device Under Test. And how to turn off protocol decoding when it is irrelevant to your desired measurement.2. Read the various oscilloscope training threads on this forum. The oscilloscope is the King of Test Equipment and the more knowledge you have the more useful it will be for you.3. Watch Dave's video on how not to blow up your scope. I'm not kidding, you can get yourself into real trouble if you poke around where you shouldn't.4. Don't even consider "scoping the mains" until you fully understand the dangers to life and property.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 07, 2018, 12:44:39 pm
content: Quote from: tooki on September 07, 2018, 09:46:44 amIndeed, but short leads combined with poorly designed alligator clips make it easy to accidentally pull the ground off whilst probing.Although the OP doesn't say so, that sounds like what probably happened.