[Aurornis]

> This can be done safely with high voltage differential probes like the R&SRT-ZHD, but we don't have any.

Entry level differential probes are $300. Less if you shop around or buy used. Micsig makes a good starter probe that would be more than enough for 60Hz AC mains testing and it comes in a generic form that would have worked with this scope.

A lot of things can go wrong, some dangerously so, if you incorrectly probe high voltage lines.

I don't know why they got such an expensive oscilloscope and then proceed to cheap out on the most basic tools needed to use it properly.

[SigmundA]

Why are differential probes so expensive and why don't more scopes just have differential port built in?

For about $300 you can buy a Tiepie differential usb scope: https://www.tiepie.com/en/usb-oscilloscope/handyprobe-hp3

[Aurornis]

You can create a pseudo-differential input by combining two input channels on almost every scope. That's not the problem the differential probe is solving, though. The differential probe exists to provide a differential measurement between two voltages that may be isolated or significantly different than the ground voltage of your oscilloscope.

The ground lead on your probes is connected straight to the ground on the power cable. This gets new users in trouble when they're probing power circuits and they don't realize that connecting the ground part of the probe to something will cause a short to ground. If that ground clip pops off and brushes against the high voltage you're trying to probe, you get sparks and maybe a destroyed scope.

The differential probe provides isolation and rejects the common-mode (shared) voltage between the two probe points before it gets to the oscilloscope.

I don't know about that USB probe, but I prefer not to have single-purpose instruments that require their own desktop software to use.

[SigmundA]

I understand what a differential probe does, what I don't understand is why they are so expensive. Again one can get a whole USB differential scope for the same price or less. Seems like just a differential probe should be relatively inexpensive well under a $100.

The Tiepie software actually works very well even though it's Windows only, they do have Linux library, just no GUI on Linux. Its not single purpose its a full oscilloscope that happens to use differential inputs.

They actually do sell one more purpose built for power quality analysis which is new. I would love to have a couple on my home power split phases to view and log power quality in detail: https://www.tiepie.com/en/usb-oscilloscope/handyscope-tp450

[labcomputer]

The thing you posted isn't the same thing as a differential probe. It's a single input low bandwidth scope that floats at the common mode voltage, with an optically-isolated data output.

Now, let's say you want to probe two things at the same time (triggered by a common signal source). You can't. And the reason you can't is because the producer took the expedient of floating the entire scope, and there's no trigger input.

In other words, they took the cheap way out by not actually building a differential probe. Related to that, this thing doesn't appear to have a step attenuator, which is why the effective resolution depends on the volts/div setting of the input.

Also, they don't specify the CMRR, which is one the main figures of merit people look for on differential probes. Any capacitive coupling between the scope and ground is going to degrade CMRR. So who knows if it can actually measure anything useful.

You can buy a scope with multiple optically-isolated channels as well as a trigger input, but those end up costing as much as a differential probe. Because it turns out that achieving good CMRR when you have multiple inputs is as hard a problem as making a good differential probe.

This is not to say the product you linked shouldn't or can't be used for anything, but it is a niche product. That's probably why it is advertised as a "power quality monitor" and not an "oscilloscope".

[SigmundA]

>Now, let's say you want to probe two things at the same time (triggered by a common signal source). You can't. And the reason you can't is because the producer took the expedient of floating the entire scope, and there's no trigger input.

Yes this is very inexpensive single input scope, they make more expensive multi input scopes you can stack as many as you want and have them synced:

https://www.tiepie.com/en/usb-oscilloscope/handyscope-hs6-di...

The difference in price between their single ended and differential scope is not the much so it seems the actual differential part is not the expensive part, back to my original point.

>Related to that, this thing doesn't appear to have a step attenuator, which is why the effective resolution depends on the volts/div setting of the input.

They sell differential step attenuators: https://www.tiepie.com/en/usb-oscilloscope/accessories/diffe...

>Also, they don't specify the CMRR, which is one the main figures of merit people look for on differential probes.

CMMR 60db from thier spec sheet: https://download.tiepie.com/Documents/Datasheets/Datasheet-H...

>This is not to say the product you linked shouldn't or can't be used for anything, but it is a niche product. That's probably why it is advertised as a "power quality monitor" and not an "oscilloscope".

From their spec page: "The tables below show detailed specifications of the Handyscope TP450 high voltage oscilloscope."

and

"The Handyscope TP450 is delivered with the versatile Multi Channel oscilloscope software, which transforms the Handyscope TP450 into an oscilloscope, spectrum analyzer, data logger, multimeter and protocol analyzer."

They make quite a few differential scopes, that just happens to be a inexpensive one physically deigned for power quality analysis.

[Aurornis]

A good differential probe has to use precision components combined with careful factory trimming and calibration to get good common mode rejection ratio.

The CMRR of the Micsig I linked is pretty average, but it's a lot better than the TiePie at low frequencies. Micsig also specifies it at multiple points across the spectrum, while TiePie doesn't even say where they measured it.

It's all the differences like this that make good test gear expensive. The Micsig is not expensive on the scale of these devices. The professional gear will have even better specs, calibration, long-term stability, temperature stability, and many more features.

For playing around, the TiePie thing will do fine.

[SigmundA]

>The CMRR of the Micsig I linked is pretty average, but it's a lot better than the TiePie at low frequencies. Micsig also specifies it at multiple points across the spectrum, while TiePie doesn't even say where they measured it.

It worse at high frequencies and the same in the middle. Again this is a whole scope compared to a probe.

Not sure why you think Tiepies are toys, they are use professionally in Europe, popular for automotive. They have a lot more expensive scopes, and the price difference for the differential versions are not that much.