Products > Test Equipment
SDS1104X-E Unexpected Behaviour
BillyO:
--- Quote from: bdunham7 on November 25, 2022, 04:19:24 pm ---https://www.eevblog.com/forum/blog/eevblog-1223-whats-all-this-ac-rms-stuff-anyhow/
The scope's RMS measurement will give you AC+DC TRMS, for the AC TRMS you use the STDEV measurement. For a 'mean', I'd have to know what mean you mean want but there's probably a way to do it.
--- End quote ---
You'd expect that for the RMS, but that wasn't the case.
The "mean" I wanted was the one provided for in the SD1104X-E measurements. The one that ideally would be 0V for a perfect AC square wave.
There was a way to do what I wanted and get the results I needed, buy summing my signal with -10V and using the scope with DC coupling. It was simple enough to do and gave results that made sense.
As an example if I do simulation here on the bench using a 1KHz square wave @ 4Vp-p with a 10V DC component, trying the method you suggest gives me a VRMS of 0.22V and a mean of -30mV. Using the method I used I get VRMS of 2.03V and a mean of 6.7mV, which is what I get if I use AC coupling, which I can in this case due to the 1KHz I'm using for just that purpose.
I don't think the scope's internal measurements can cope with the position being set so far off the screen. Something is being clipped.
bdunham7:
--- Quote from: BillyO on November 25, 2022, 07:36:58 pm ---As an example if I do simulation here on the bench using a 1KHz square wave @ 4Vp-p with a 10V DC component, trying the method you suggest gives me a VRMS of 0.22V and a mean of -30mV. Using the method I used I get VRMS of 2.03V and a mean of 6.7mV, which is what I get if I use AC coupling, which I can in this case due to the 1KHz I'm using for just that purpose.
I don't think the scope's internal measurements can cope with the position being set so far off the screen. Something is being clipped.
--- End quote ---
My AWG won't do a 10V DC bias with a 4Vp-p square wave (max output is 10V), so I set it up with 8VDC + 4Vp-p square wave, the result should be the same. If somehow you do this and it works at 8V and not at 10, I'll float the AWG and bridge two channels. But I don't think 8V vs 10V will matter.
So I set up my SDS1104X-E as follows:
Offset/position: -8.00V
CH1: 1V/div, measurements RMS, STDEV and MEAN.
Horizontal 500us/div.
The results I get are ~8V MEAN, 2.00V STDEV and 8.21V RMS. The SDS1104X-E doesn't have a formula editor for math and there's no obvious way to have the math channel do CH1 minus 8 volts, so you'd have to mentally subtract the offset from the mean to get the mean you want. Since you were originally looking at very low frequency stuff, I tried slowing the signal down to 0.1Hz and using a slow enough acquisition speed. I get the same result. Those numbers exactly match what a very accurate TRMS meter gives me for both AC and AC+DC, as well as matching the calculated expectations pretty closely as well. The AC and mean should be obvious, the AC+DC is the square root of 68.
One thing I did notice is that if I used 500mV/div instead of 1V/div, the signal is right at the edge of the screen and the scope displays measurements with ">" rather than an "=". That tells you that you are clipping, I guess. Apparently you need to keep it well on the screen for best results. What symbol did your scope show you and how did you set it up?
Clearly an actual offset bias circuit like the one you made for this would work as well, but those just don't exist AFAIK on modern DSOs and there's really no need. More advanced models can easily take that offset off of the mean in MATH if you want to display the value without having to think about it.
BillyO:
Thanks for your efforts bdunham7.
I get similar results, but notice the RMS? It should be around 2V if you are just interested in the AC component.
I'll post some screen shots tomorrow.
Just for information, what versions are you at (hardware, FPGA, OS, Software/firmware)?
Mine:
Hardware: 09-06
FPGA: 2021-11-08
OS: 8.3
Firmware: 6.1.37R9
bdunham7:
--- Quote from: BillyO on November 26, 2022, 03:02:29 am ---I get similar results, but notice the RMS? It should be around 2V if you are just interested in the AC component.
I'll post some screen shots tomorrow.
Just for information, what versions are you at (hardware, FPGA, OS, Software/firmware)?
--- End quote ---
No, that's what I was nattering on about and what the DJ video I posted is about. RMS = AC+DC, STDEV = AC. And by that I mean the STDEV reading is the RMS of the AC component, if that wasn't clear.
Hardware 01-04
OS 8.2
Firmware 6.1.37R6 (I don't think any further updates apply to mine)
FPGA 2021-7-12
I wouldn't think there'd be any difference that would matter here. Post your screenshots and we'll know.
BillyO:
--- Quote from: bdunham7 on November 26, 2022, 04:25:47 am --- RMS = AC+DC, STDEV = AC. And by that I mean the STDEV reading is the RMS of the AC component, if that wasn't clear.
--- End quote ---
Yeah, I do realize that the DC component is a part of the RMS value (it's not simply AC+DC though), however I am talking about the measurement I was taking. I was just interested in the RMS value of AC component. Doing the measurement this way (by changing position) rather than by removing the DC component you don't get the RMS reading of just the AC. In retrospect the STDEV didn't cross my mind. I guess it would have saved me a bit of time. :palm: Thanks for pointing that out. Now it'll always be a consideration.
I'll post my shot a bit later, but it's almost identical to yours.
Edit: Screen shot added.
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