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Re: Help using this Group
You can use more than one Email program at a time.
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Michael A. Terrell -----Original Message-----
From: Jeff Urban <JURB6006@...> |
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Re: Help using this Group
There is more than one way to skin a cat. Actually one could just open a new email account just for this group and possibly a very few other things. Problem there is logging out of one and on to the other all the time, I just stay signed in to mine and eliminate the hassle. Having physical control of my PCs simplifies security quite a bit.
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Re: using scope channel 1 and channel 2 invert and add functions vs floating DUT
The battery powered scope is actually floating the scope, but they are designed for that. Even if not they will be quite tolerant of it.
In the case of your LED example, one could only hope that wither one side of the other is anchored fairly well to a constant voltage source. Then it is a simple matter of using AC coupling. If not, the choices are to float the scope say, positive or to float the DUT negative, or vice versa, whichever is needed. Neither are very desirable from a safety or signal integrity standpoint. Let's say in the case of a BTL audio power output, common in automotive audio. One amp feeds one speaker wire and the other the same signal in opposite polarity. When measuring such an output, the supply voltage will dip during the peaks, should one be measuring dynamic clipping headroom for whatever reason. This makes simply using AC coupling a poor option, you simply need enough CMRR to handle it at whatever deflection sensitivity needed. Also, one manufacturer actually did not send flat frequency response to both legs of the output so as to act like a biamped unit, reducing certain forms of distortion when at or near clipping, which such amps see on a regular basis usually. In that case you're adding 2 different signals and inverting one. Suppose you wanted to see the actual output's frequency response and feed it a 1KHz square wave, which I use commonly for that purpose. Manually taking one waveform and the other and figuring out the resulting sum is not something I would like to do. These are not extremely important issues, for one the actual power output of the type of amp mentioned is not that important, and a circuit that elevates both ends of an LED to hundreds of volts would be quite rare, in fact it would take some time to just think where such a situation would be necessary. More likely it would be floated itself, like a panel power indicator running directly off the AC mains when the rest of the unit is isolated and Earth grounded. This discussion is rendered mainly academic because the OPer has not revealed the purpose of the question. Perhaps it is academic for him as well. But I'm OK with it if everyone else is. |
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Re: using scope channel 1 and channel 2 invert and add functions vs floating DUT
I see that I need to clarify further. I thought the topic would be enough, but I should have been more specific, from the replies this topic has received. I wasn't aware what scope I was referring to would be important. I thought it was a generic enough topic.
I have a Tek 2246A and a Hitachi VC-6025 hybrid analog/digital older scope. They both have the channel 1 and channel 2 invert and add functions, and I just have basic 1x-10x probes for them. I have in the past floated the DUT, and that topic has been beat here in the past I am aware. I really don't fully understand what the add function and invert function can do for me. I have played with them, but probably don't know how to apply the functions, and I thought add/invert could possibly be an alternative to floating the DUT, but with some unknown limitations without a fair amount of experimentation. So I come here looking for knowledge/experience with add/invert functions, and the application of it that can't be met by other methods. NielsenTelecom |
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Re: Automatic Brightness Control : was it ever implemented ??
Phew, it's available ^^
Thanks for the link. It's nice to see that Tek still keeps copies of its old scopes manuals and make them readily available on their site. It don't understand though, why they don't do it for 100% of their scopes (or various products)... can't believe they "lost" their own manuals ?!... I don't see the point of releasing only some manuals but not all. All or nothing, I would understand, but doing it halfway, I must say I don't see their point... Would be good if they released component level information for their more "modern" stuff like the TDS 300/400/500/600/700 scopes which are so wide spread/popular. Maybe if Tekscope members send a petition, begging them... ^^ It's not like they are making any significant amount of money from this 20/30 year old CRT scopes... Vincent Trouilliez |
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Re: using scope channel 1 and channel 2 invert and add functions vs floating DUT
Hi Dennis / and Nielsen (Telecom).
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@Dennis... Indeed the reference to floating is in the very subject. In fact, Nielsen wrote more about his question on the subject than in the body of the message. @Nielsen... By "vs floating DUT" I assume you refer to float the DUT with the sole purpose of allowing the use of a single channel, non differential scope input, to take a differential measurement on a circuit (a measurement of the voltage between 2 different nodes on a circuit in which none of them is ground on the DUT). What you suggest, using 2 channels of the scope, in ADD mode with Channel 2 inverted is, by all means, a much better approach to taking a differential measurement than floating the DUT and is the very reason why most 2 channel scopes have the option to invert channel 2... To allow for differential measurements. But, when you float the DUT, although dreadful and although inspiring all the caution that Jeff underlined and more (and wait... even more), you accomplish more than just allowing a differential measurement to be taken so, you must be careful in assuming that you can use the differential approach (CH1 + CH2, CH2 inverted) in every case where someone would float a DUT. For example, the 2 nodes from which you intend to take the differential measure are both, at a DC voltage level that's beyond the scope inputs' maximum (say 500V). The difference signal that you intend to measure is small so, you won't be able to handle it using 2 x 100:1 probe. In that case you can't actually solve this problem by using alone CH1+(-CH2), because you are supposed to connect each channel's ground lead to the DUT's ground, and that will subject each of the scope's inputs to a voltage that exceeds their maximum. And no, by no means you can think of not connecting the ground leads and assume the scope would be protected, because if you didn't float the DUT (which is the proposal of the topic to start with), then it's referred to earth's ground as much as the oscilloscope itself is, and even if you don't connect the ground leads, each input will still see 500V. In that case, sometimes, floating (as dreadful as it is and as caution inspiring as it is) is the only solution *at hand* and using CH1+(-CH2) can't solve it. I said "at hand" because there are sometimes other *technically-sound and yet safe* alternatives, such as galvanically isolated probes. There are many other examples of situations that an differential input setup can't accommodate one (or more) of the reasons behind the need to float a DUT. So, answering to your consideration that "it seems ok", the right answer is: It depends... It's OK whenever the technical limitation of your differential setup can be respected, given the circumstances of the DUT, be it (CH1 + (-CH2)), or scope with differential input stage, or differential input probe, or galvanically isolated probes. It's not OK whenever the circumstances of the DUT vs signal being measured would disrespect any of the input's limitations then, sometimes, Floating ***may*** be the only solution at hand. Rgrds, Fabio On Wed, May 16, 2018 at 07:04 pm, Dennis Tillman W7PF wrote:
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Re: Help using this Group
On Wed, May 16, 2018 at 11:09 pm, tinkera123 wrote:
Me too. I have the notifications here set to daily summary, when it was every post it was unmanageable as I use this email for other things. In those daily summaries are direct links so you don't even have to look for them. Arranging those photo album links will be important do that at least we can find out own. I suggest naming the link in bookmarks the same as the name of the album, less confusion that way. |
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Re: using scope channel 1 and channel 2 invert and add functions vs floating DUT
On Wed, May 16, 2018 at 07:04 pm, Dennis Tillman W7PF wrote:
It was mentioned in the title. At least he is talking floating the DUT rather than the scope. At times, rare, but when working on an SMPS with opto coupled feedback I have, on isolation of course, connected hot and cold grounds together. I wanted to see exactly what was happening in the regulation loop for some reason. It worked fine and was no danger when it was on isolation. It could have been floated without isolation but that is of course a shock hazard. I wonder really how much better of a reading one actually gets floating the DUT. The dual trace scopes I've seen, even the not so great ones seem to have pretty well matched channel;s and a fairly decent maximum input level before it causes a problem. How does that stack up against adding a bunch of capacitance to the - input ? Of course it depends on the bandwidth needed but even knowing that, the effect would be quite unpredictable. Until the OPer gets back... (but it has only been one day) |
Michael A. Terrell
Jeff Urban
Vince
Fabio Trevisan