Products > Test Equipment
SDS1104X-E Unexpected Behaviour
bdunham7:
--- Quote from: BillyO on November 26, 2022, 04:05:03 pm ---Yeah, I do realize that the DC component is a part of the RMS value (it's not simply AC+DC though)
--- End quote ---
"TRMS AC+DC" is just a common way of referring to a TRMS measurement where the DC component is not suppressed by AC coupling. It is calculated by your scope quite literally by simply taking the root of the mean of the squares of the samples. Since your square wave is 12V half the time and 8V the other half, the mean is (144 + 64)/2 or 104, the RMS is 1040.5 or ~10.2 and your measured result should match that give or take some for noise and offset.
The STDEV reading is calculated much the same way, except that the scope first calculates the MEAN value and then subtracts that from all the samples before doing the rest. Or at least that's how I expect they do it, perhaps with mathematical shortcuts to speed the process up.
2N3055:
--- Quote from: bdunham7 on November 26, 2022, 04:45:55 pm ---
--- Quote from: BillyO on November 26, 2022, 04:05:03 pm ---Yeah, I do realize that the DC component is a part of the RMS value (it's not simply AC+DC though)
--- End quote ---
"TRMS AC+DC" is just a common way of referring to a TRMS measurement where the DC component is not suppressed by AC coupling. It is calculated by your scope quite literally by simply taking the root of the mean of the squares of the samples. Since your square wave is 12V half the time and 8V the other half, the mean is (144 + 64)/2 or 104, the RMS is 1040.5 or ~10.2 and your measured result should match that give or take some for noise and offset.
The STDEV reading is calculated much the same way, except that the scope first calculates the MEAN value and then subtracts that from all the samples before doing the rest. Or at least that's how I expect they do it, perhaps with mathematical shortcuts to speed the process up.
--- End quote ---
Very nice. simple and concise!!
BillyO:
Okay, sorry to revive this (if anyone gets upset at such things) but it seems most people that contributed to this thread did not understand or get what I was talking about.
Here is a video that explains the difference between offset and position. BTW, I still think an offset control would be a handy thing to have. Siglent (and others) should consider adding it. There is even a menu item for it already in the SDS1104X-E. They just have to make it work properly.
2N3055:
--- Quote from: BillyO on October 24, 2023, 04:54:08 pm ---Okay, sorry to revive this (if anyone gets upset at such things) but it seems most people that contributed to this thread did not understand or get what I was talking about.
Here is a video that explains the difference between offset and position. BTW, I still think an offset control would be a handy thing to have. Siglent (and others) should consider adding it. There is even a menu item for it already in the SDS1104X-E. They just have to make it work properly.
--- End quote ---
What is called Vertical position in that video is also called Vertical position on SDS. That is part where front end injects DC voltage and combines it with signal.
What on Tek is called Vertical Offset, is controlled on SDS by option Reference position in Utility menu.
You can choose if screen expands around the 0V, or around Vertical offset value.
How is Reference position in Utility menu set?
BillyO:
I think you are still missing my point. My point is that on the SDS there are both "Position" and "Offset" controls. "Position" on the front panel and "Offset" in the vertical menu (2nd page) However, rather than working like an offset (as in the video) it works the same as "Position".
The reference position can only be set to "Fixed Position" or "Fixed Offset" and is not adjustable. It does not do the same thing as the offset control on the Tek in the video. Mine is currently set to "Fixed Position". That's where I want it.
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