Square wave distortion seen at lower freq - Page 1
Forum section: electronics / beginners

author: ArnieG
author_rank: Newbie
timestamp: October 09, 2025, 03:23:16 am
content: I am using these equipment: Siglent SDS 1102CNL+ 100MHz scope and Siglent SDG810 arbitrary waveform generator.I want to use the waveform generator to produce a 100 Hz 6Vpp square wave. But when i display the waveform on the oscilloscope, I get the distortion seen here in the attached image.Usually this type of distortion can be alleviated by turning that small screw on the oscilloscope probe, but somehow it didn't work in this situation. Changing the cables just to make sure they are ok , no help. I am suspecting the equipment (compatibility?)When freq is increased to 200Hz and higher, the distortion disappears.Am surprised that the distortion cannot be "tuned out" using the screw on the probe, and why does it go off at higher frequencies?

author: Benta
author_rank: Super Contributor
timestamp: October 09, 2025, 11:30:22 am
content: Either your scope or your AWG (or both) are running in AC mode. Switch to DC and it should be OK-

author: ArnieG
author_rank: Newbie
timestamp: October 09, 2025, 01:18:24 pm
content: Quote from: Benta on October 09, 2025, 11:30:22 amI know about the scope mode DC-AC-GND. Made sure it was DC.As for the AWG, is there such a thing as "AC mode" on function gens? It just generates waves... AC or DC depending on the chosen settings, i.e. frequency, offset voltage and all that.Having said that, this is the first digital AWG that I am using, I might be unfamiliar with the nuances. Read through the manual (SDG 810) but couldn't find any references to modes.

author: Grandchuck
author_rank: Super Contributor
timestamp: October 09, 2025, 01:30:42 pm
content: Try adding DC offset to make sure there is response to DC.

author: ArnieG
author_rank: Newbie
timestamp: October 11, 2025, 05:35:23 pm
content: Quote from: Grandchuck on October 09, 2025, 01:30:42 pmDefinitely tried that earlier and here are the pictures.Output square wave pulse 6Vpp with a +3V DC offset, at 100Hz and 200Hz, with the timebase adjusted to make the 2 traces easily comparable.There's a distortion creating slight curves to the expected flat lines at 100Hz; whereas at 200 Hz, the distortion lessens to the point where it is no longer observed.Is this some kind of incompatibility of the settings between the AWG and oscilloscope?

author: Zero999
author_rank: Super Contributor
timestamp: October 11, 2025, 05:42:20 pm
content: As mentioned above, the distortion appears to be indicative of a high pass filter. Does the oscilloscope measure a pure DC voltage properly?

author: StillTrying
author_rank: Super Contributor
timestamp: October 11, 2025, 05:49:49 pm
content: Check there's no filter on that channel, if the cnl+ has them.Try the scope's channel 2.

author: radiolistener
author_rank: Super Contributor
timestamp: October 12, 2025, 02:03:19 pm
content: it looks like known issue of Signlent oscilloscopes where it has frequency response slightly drop at low frequency < 50 Hz, but since distortion is < 3 dB, it still within specification

author: CaptDon
author_rank: Super Contributor
timestamp: October 14, 2025, 01:16:53 pm
content: A scope that can't display accurately below 100Hz? That's why I only buy reputable stuff. My analog scopes are a Tek SC501 and SC504. They are the modular ones that work in the TM or RM series of chassis. My digital scope is a Tek TDS644B. It claims 500MHz bandwidth but when troubleshooting a Wavetek 3000 R.F. generator I found the TDS644B to be usable to over 1GHz. I bought the two analog scopes used in perfect condition, about $300USD total for the pair. The TDS644B came from the dumpster at work. It had no internal display due to a bad canned oscillator but the external VGA was working. I replaced the oscillator and now the internal display looks sharp!! I adjusted the trace color selection to be the same as the newer Tek scopes, 1 yellow, 2 blue, 3 magenta, 4 green. I got turned off to Rigol years back. The company I worked for bought one but it and the replacement both failed within a week. Finally the 3rd one worked. I always felt like these were Agilents that failed QA/QC final inspection or calibration. GE/WABTEC bought some of those USB PicoScopes. They work o.k. for what they are but at first they were a bit awkward to get used to. My field engineers liked them.