Rigol DS1074Z vs Picoscope 2208B Inverter Output Waveform Noise Measurement - Page 1
Forum section: electronics / testgear

author: Headhoncho
author_rank: Newbie
timestamp: July 22, 2025, 09:55:28 pm
content: Apologies if this is the wrong place but this seemed to be more of a oscilloscope/test equipment question than a off-grid/reneweable question.I'm validating the output of a generic chinese 72VDC-120V/240V pure sine wave inerter for harmonic content and noise and I am getting strange measurements when I compare my two oscilloscopes.The inverter is a generic low frequency inverter hard switching some NCEP15T14 150V mosfets at 20khz. It has an torroidal inductor on the output for what I'm assuming is filtering which then leads to a torroidal autoformer with a 2.7:1 turn ratio to get the 120V/240V split phase output.I'm probing the output of the inverter using a Micsig DP1007 HV differential probe and when I look at the trace using the picoscope there is a ton of noise on the positive side of the AC waveform. It doesn't change if I plug or unplug my laptop from the mains.When I take the same probing setup and plug the BNC from the differential probe in the Rigol scope the AC waveform doesn't have near the overshoot and noise. I still see some 20khz content in the 60Hz main wave but I can't think of reason why I get such a large discrepancy between the two scopes.I'm open to any suggestions on why there is such a large difference between the two.

author: 2N3055
author_rank: Super Contributor
timestamp: July 22, 2025, 10:24:25 pm
content: There is no information how pico was setup..

author: Headhoncho
author_rank: Newbie
timestamp: July 22, 2025, 10:29:11 pm
content: When you ask about how it was setup do you mean if it is was in DC coupling or AC coupling?The DPS1007 was in 100X mode, the Picoscope was in DC coupling with the BNC from the HV probe on channel A, 5ms/Div, 200V/div.

author: bdunham7
author_rank: Super Contributor
timestamp: July 22, 2025, 10:36:49 pm
content: Set up the Picoscope again but reverse the two DP connections then observe whether the noisy part is the negative or positive going section of the wave.

author: 2N3055
author_rank: Super Contributor
timestamp: July 22, 2025, 10:38:26 pm
content: Quote from: Headhoncho on July 22, 2025, 10:29:11 pmNo, I mean all the details. Sampling memory, sampling rate etc..How it looks when Pico is at 1ms/div also...?Pico does have slightly higher BW if Rigol is not hacked.

author: Headhoncho
author_rank: Newbie
timestamp: July 22, 2025, 10:57:10 pm
content: Yesterday evening I swapped the connections to between the positive and negative on the inverter and when the top portion of the trace had the noise the positive lead of the HV probe was hooked to the +110V AC output and the negative lead to the ground and when I swapped the positive and negative probe leads the lower portion of the 60Hz wave had all the noise.The Rigol is hacked to 100Mhz so both scopes have the same bandwidth and I was running the picoscope in 8bit mode without upsampling.I'll hook everything up tomorrow evening again and take a couple of traces with different ms/div

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2025, 03:28:56 am
content: Quote from: Headhoncho on July 22, 2025, 10:57:10 pmOK, a couple more things to try.1) When using the Picoscope, ground the shell of the BNC if it isnt' already.2) Use a BNC tee and the necessary cables and connectors to connect the output of the DP to both the Picoscope and Rigol at the same time. I know this gives you an amplitude problem, but you still get a reasonable signal.

author: Headhoncho
author_rank: Newbie
timestamp: July 23, 2025, 07:41:49 am
content: So I took the advice of bdunham7 and put a tee between the rigol scope and the picoscope and I was getting inconsistent results. Sometimes I would have a clean waveform sometimes it would be really messy.Then I saw the problem.I am using my CNC router which has an extruded aluminum bed as a makeshift work table and immediately under the table is the inverter.When the picoscope is on router table the noise is significant. When I pick up the scope off of the table the waveform cleans right up and looks just like the Rigol.I guess somehow the EMI from the inverter is coupling to the router table and then through the chassis of the picoscope.So mystery solved.

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2025, 03:39:56 pm
content: Quote from: Headhoncho on July 23, 2025, 07:41:49 amFixed but IDK about solved!That's a bit strange. What I was thinking is that the DP wasn't dealing well with common mode noise somehow and that when connected to the picosope there was no direct ground connection but probably at least a diode junction somewhere that suppressed that noise on one half of the cycle. The reason I had you reverse the DP connections was to see if the issue was with the Picoscope itself or elsewhere--and what you showed indicated to me that it was NOT in the Picoscope. So I'm not sure exactly what is going on.