Why are fixed attenuation probes better!? - Page 1
Forum section: electronics / beginners

author: robdejonge
author_rank: Regular Contributor
timestamp: October 21, 2021, 02:25:18 pm
content: Over in the test gear forum I posted a topic on cheap test leads and probes -https://www.eevblog.com/forum/testgear/buying-a-bunch-of-probes-leads-etc-on-a-budget/. I really like the passion of the folks in that forum and appreciate the answers provided. But I didn’t want to bother them with newbie questions for fear of burning up goodwill. I thought I’d try here instead, for the learning.One of the repeated recommendations has been to buy fixed 10x attenuation probes. I’vefound onefor $21.84, which have a dodgy product description, I also see listed as 100x probes and look a bit funny to me. But they’re the only probes listing fixed 10x I can find for a reasonable price.I have very limited purchase channels, AliExpress is basically it for me here in Thailand.I’ve just spent quite a while trying to hunt down fixed 10x probes. But I can’t help but wonder …. All else being equal, how much of a difference does it really make to have a fixed 10x vs a switchable 1x/10x probe? Why the strong recommendations?(I work in low voltage and have a 70MHz scope only)

author: Cerebus
author_rank: Super Contributor
timestamp: October 21, 2021, 03:21:48 pm
content: There's the obvious thing, less to go wrong. Not having moving switch parts makes the probe simpler and thus more reliable. Also good high frequency switches are surprisingly hard to make and the switches in switchable probes aren't always the best.Then, there's the "left it on the wrong setting" and, more often, "didn't realise I'd accidentally put it on 1x" problem with switchable probes - you'd be surprised how often this happens in practice, and how much time you can waste chasing down a "circuit" problem only to realise too late that a wrong probe setting was the problem.Finally, any switchable probe is going to compromise between the best settings/compensation for 10x and the best settings/compensation for 1x. That's an inevitable consequence of making one thing to do two tasks, it can't be optimised to do both.For the most part using switchable probes isn't an issue for most people making most measurements. You're more likely to accrue measurement errors from loading, probing technique and the general laxness of oscilloscope specifications (3% voltage accuracy on a lot of scopes) than from the weaknesses of switchable probes. Once you get into the territory above 100MHz (roughly) then probe infidelities become more important. Using switchable probes at 500 MHz would not be a good idea and, apart from the odd maverick Chinese OEM, nobody makes switchable probes for those kind of frequency ranges for exactly that reason.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 21, 2021, 05:33:05 pm
content: One reason is if you are measuring high-ish voltages. A non-switchable probe avoids accidentally applying high voltage to your scope input. Note that most scopes these days are rated CAT I / 300V which is OK for high voltage but not mains work (something like a vacuum tube amplifier). Some scopes and some non-switchable probes are rated for CAT II-300V and are for working with mains power inside a device. A lot of people here won't recommend measuring mains with an oscilloscope at all or without a dedicated high voltage probe and that is good advice but I have certainly done so when looking at a power supply.If your oscilloscope has 50 ohm input as an option, then fixed attenuation probes will avoid potentially applying 24VDC to your 50 ohm input and destroying it. Of course you won't get a sensible signal if you use a 10 M probe with a 50 ohm input but at least you won't blow up your scope.But the main reason is performance. A fixed attenuation probe will have less capacitance than a similar switchable probe and high bandwidth probes are only available with fixed attenuation.All of this applies to the standard passive 10x/1x probes. This doesn't apply to active probes such as high voltage differential probes. These probes are always high impedance and have a switchable gain stage to optimize the dynamic range for a particular measurement. These are fine but generally much more expensive than passive probes.

author: David Hess
author_rank: Super Contributor
timestamp: October 21, 2021, 11:30:02 pm
content: Switchable x10/x1 probes have slightly compromised performance compared to x10 and x1 fixed probes. But at 200 MHz and below, the difference is insignificant.The old classic Tektronix x10/x1 probes were very nice. Modern ones, or at least the Chinese ones, tend to not be very reliable or long lasting. Good ones are not inexpensive.

author: rstofer
author_rank: Super Contributor
timestamp: October 22, 2021, 12:19:14 am
content: I bought a pair of fixed 10x probes from ProbeMaster.com to protect my Analog Discovery 2 which has a limited range for input voltages. With a fixed probe, there is no chance I will measure a high voltage on x1.I still have several sets of 1x/10x and I doubt I will throw them away but the AD2 deserves special attention. It is my favorite tool.

author: james_s
author_rank: Super Contributor
timestamp: October 22, 2021, 12:40:02 am
content: Quote from: Cerebus on October 21, 2021, 03:21:48 pmAnd how easy it is to damage your scope if you think the probe is in 10x mode and it is actually in 1x mode, you have just exposed your front end to 10 times the voltage you thought you were exposing it to. Probably not an issue for low voltage logic stuff but it can easily blow something up if you're poking at higher voltages.

author: tooki
author_rank: Super Contributor
timestamp: October 22, 2021, 01:56:51 am
content: I’ve never done anything where the 1x setting was superior to the 10x. So for me, the switch is a source of inadvertent user error, in exchange for no benefit.

author: robdejonge
author_rank: Regular Contributor
timestamp: October 22, 2021, 03:29:47 am
content: Thanks everyone for the replies. Summarizing, the main reasons are 'prevents human error' and 'more accurate measuring results'. Although the latter may be less relevant for me, as it is more relevant for higher frequencies. And in general, issues other than the attenuation setting on the scope probes are more commonly the cause of poor measuring. So ... yes they're better, but perhaps I should not sweat it?!If there ARE options out there for me though, why not get them? So ...Quote from: rstofer on October 22, 2021, 12:19:14 amYeah, I've seen those. However, at $40 a piece, with shipping and import duties (I'm in Thailand) I'd spend more on the probes than I didmy second-hand GW Instek GDS-1072b.Quote from: David Hess on October 21, 2021, 11:30:02 pmCan you make any recommendations? Cleqee got "it's not absolute garbage" reviews over in the other (more generic, not just probes)thread, so I am currently looking at buying the$9.49 switchable ones. Although given my experience level (zero) I don't really want to spend a whole lot of money on this, I am open to suggestions. So any thoughts on good probes would be appreciated.

author: magic
author_rank: Super Contributor
timestamp: October 22, 2021, 05:30:15 am
content: Quote from: tooki on October 22, 2021, 01:56:51 amIts usefulness is limited to improving SNR when probing very weak signals (millivolts).At least up to a few MHz, because then the 1x mode stops working well.

author: james_s
author_rank: Super Contributor
timestamp: October 22, 2021, 05:43:34 am
content: I have measured things where 1x is useful, but in those cases I don't use a probe at all, I just solder a coax directly to the node I'm trying to measure. I've never (intentionally) used 1x on a switchable probe.

author: tooki
author_rank: Super Contributor
timestamp: October 22, 2021, 02:58:14 pm
content: Quote from: magic on October 22, 2021, 05:30:15 amYeah, that was my understanding of when it’s useful. Just nothing I normally do.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 22, 2021, 03:14:09 pm
content: x1 offers a weird small improvement in vertical (value) measurement accuracy and noise, yet usually not enough if those are actually important. You still have uncalibrated, noisy, +/- 2% or so, 8-bit instrument. Scope is fundamentally accurate when measuringtime(x axis). For higher accuracy, you'd been using a specialized, expensive scope (those do exist, for example with 12-bit ADCs), or something completely different.I have never used x1 position in my probes so I just tape or glue the switch to prevent accidental switching which is PITA and totally ruins the day.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 28, 2021, 03:17:24 pm
content: Besides blowing your instrument input...A 1X probe can put a load on the circuit 10X more than a 10X probe and change the circuits intended function more than a 10X. Any measurement will somewhat affect the circuit.What you see on the scope is not the signal that should be there because you have loaded the circuit.I have used 1X probes in the past and do not see doing it again. maybe they are better for some triggering.....Sometimes it is nice to feed a signal directly from another instruments output into the scopes 50 ohm input with a BNC coax. Like looking at you signal generators wave form.But 1X probes are not really 50 ohm input, so direct BNC to BNC is better and easier also

author: David Hess
author_rank: Super Contributor
timestamp: October 28, 2021, 10:46:32 pm
content: Quote from: Siwastaja on October 22, 2021, 03:14:09 pmThere is a more important advantage x1 probes have over almost all x10 probes related to accuracy. If the probes are being used with a differential input, then accurate attenuation is required to maintain common mode rejection ratio at low frequencies, and x1 probes are much more accurate than x10 probes.Tektronix used to make special x10 probes that had adjustable attenuation so they could be trimmed to match for differential measurements, but probes like that are long gone. Now if you wanted to do the same thing, the differential amplifier would need to have trimmable gain on one or both sides.

author: james_s
author_rank: Super Contributor
timestamp: October 29, 2021, 01:39:49 am
content: If you want to make differential measurements, the proper way to do that these days is IMO using a differential probe.

author: magic
author_rank: Super Contributor
timestamp: October 29, 2021, 02:36:02 am
content: I think one could add something like a feedthrough terminator with variable resistance on the order of 10~100MΩ to trim ordinary 10x probes at DC and then compensate each one for AC by the usual means.

author: David Hess
author_rank: Super Contributor
timestamp: October 30, 2021, 03:38:51 am
content: Quote from: magic on October 29, 2021, 02:36:02 amThe way Tektronix did it was to include a trimmable shunt resistance in the probe termination, and the series resistance at the probe tip was lowered to keep the attenuation at x10. These probes have an input resistance of like 8, 5, or 1 megohms instead of 10 megohms, which is how they can be identified.