Should scope probes be 3x "better" than the scope they're used on? - Page 1
Forum section: electronics / projects

author: mmb
author_rank: Newbie
timestamp: December 14, 2017, 04:37:06 am
content: I heard this rule of thumb a long time ago: if you're using a 20MHz 'scope near the limits of its performance, your probes should be rated at least 60MHz... and so on. 100 MHz 'scope? 300 MHz probes--or better.I know that "you're always not measuring everything" and you don't want probes to adversely affect the measurements. Is this a sound rule of thumb, or overkill? Help me, EEV hive mind! And thanks.Apologies if this has been answered here before; my search-fu might be weak.

author: rx8pilot
author_rank: Super Contributor
timestamp: December 14, 2017, 05:11:30 am
content: Probes, in general, get a 3db rating. This means the probe that claims 100Mhz should have a response that is 3db down at 100Mhz. If you were measuring at the full 100Mhz, your voltage would be attenuated a little. If you wanted a flat response through 100 Mhz, you would need a faster probe.Probing is a big fat topic, but I can't see any benefit in going 3x the bandwidth of the scope. Perhaps 15-20% may help a little with linearity at the top end.

author: RobK_NL
author_rank: Frequent Contributor
timestamp: December 14, 2017, 09:18:15 am
content: Simplistically approached:Bandwidth = 0.35 / trA 100MHz scope has a trof 3.5ns, a 100MHz probe also has a trof 3.5ns. The trof the system can be determined by root-sum-square, so theend-to-end trwould be SQRT(SQR(3.5ns)+SQR(3.5ns)) = 4.95nsThe system bandwidth is thus 0.35 / 4.95E-9 = 70.7MHz.Repeating this with a probe that has a trof 1ns (BW = 350MHz) results in a system bandwidth of 96MHz.Things are more complex in the real world, of course, but this shows that there is more than 'some' merit in using a faster probe.

author: Berni
author_rank: Super Contributor
timestamp: December 14, 2017, 09:25:44 am
content: Well it makes sense to use a slightly higher bandwidth probe so that you get the -3dB being right at -3dB of your actual scope.That being said there is more to probes than just the MHz printed on it when going to high frequency. Probes have quite a few pF of capacitive loading on the tip and when you get to 100MHz that is actually a lot of current flowing into the probe and loading your circuit. Additionally the path from the probe to the circuit starts to exhibit a frequency response. That little 5cm ground chip can modify the signal a lot at these speeds. All of this affects how the probe will perform in the high end of frequency. Probing anything above 100MHz becomes a bit of an art due to all this.But you don't really want to be measuring stuff really close to the bandwidth of your scope since what you see on the screen is no longer what is happening, but hey sometimes you don't have a choice, good scopes are expensive.

author: mmb
author_rank: Newbie
timestamp: December 14, 2017, 11:35:05 am
content: Re not getting close: This is of course, to some degree, the attraction of the Rigol hack.And I know capacitance kills as freq goes up but it always bears repeating. As I say when I am coaching utter noobs, "You're never seeing everything, and sometimes your tools make that worse."

author: stj
author_rank: Super Contributor
timestamp: December 14, 2017, 11:46:05 am
content: well the Rigol 50MHz base model still comes with 150MHz probes - so that's all good.

author: borjam
author_rank: Unknown
timestamp: December 14, 2017, 12:06:09 pm
content: The main advantage of better probes can be just better features.For example, I have some four nice LeCroy PP005 probes. 500 MHz is overkill for a Siglent+Rigol user. But the spring loaded tips are great.

author: danadak
author_rank: Super Contributor
timestamp: December 14, 2017, 01:25:42 pm
content: Cascaded two ports, that is cascaded isolated amps, you computethe cascade BW from -http://demonstrations.wolfram.com/SystemBandwidthForCascadedAmplifiers/In case of scope probe it is not isolated, loading affects. But to a first order approximation.Regards, Dana.

author: macboy
author_rank: Super Contributor
timestamp: December 14, 2017, 03:30:42 pm
content: Technically if your scope is 3 dB down at 100 MHz and the probe is 3 dB down at 100 MHz as well, then the combination of them should be 6 dB down at 100 MHz. The -3 dB point will have shifted downward to a lower frequency. You can predict this frequency as Dana suggests, but any bumps and dips in the response of the scope or especially the probes (which are usually not very flat in their highest octave or two) will contribute.But it is not so simple. What is the response of the probe, really?I recently sold a nice Tek 2465A scope with 350 MHz bandwidth. I included several P6133 probes with it, these are rated at only 150 MHz. On the surface this is a terrible combination. But I've measured these probes to have a bandwidth in the 500 MHz region. I've observed the same with P6138 probes, nominally 350 MHz, but measured to ~ 500 MHz. The pulse response of the scope+P6133 probes was only a hair slower than a direct connection (~ 1.05 ns vs. 1.0 ns), and the new owner of the scope trusted the tests over the specs and was happy with the deal.There are probes that do not exhibit this behavior. Cheap Chinese "P6100" probes rated at 100 MHz do not exceed their spec and in fact just barely meet it. I would definitely not recommend those for anything faster than a 50 MHz scope. I'd recommend any of the Tek P613X miniature or sub-miniature probes without reservation, with the primary driver for going for higher spec within the series being getting a lower tip capacitance (see below).Usually the pulse response is used as a performance metric over the -3 dB point. In fact 'scope bandwidth specifications have often listed a -3 dB bandwidth that is explicitly estimated/calculated from pulse rise time, rather than directly measured. The pulse response rise times add as root-sum-of-squares. If the probe is 3x faster than the scope, then it contributes ~ 5% slowdown to the rise time. At >10x faster, the effect is negligible (<0.5%). Here, too, any peaking in the probe response can throw off the calculated estimate.When buying probes, consider the tip capacitance. This is a very important but frequently overlooked specification. A 10 Mohm probe is only 10 MOhm at DC! At AC (above ~ 100 kHz), the load is completely dominated by capacitance, at:\$Xc=\frac{1}{2 \pi F C}\$You can see that >10 MHz or so, the loading is very significant. At high frequencies (>100 MHz) a so-called "Low-Z" passive probe can have less loading than a "High-Z" probe. The Low-Z probe has just 1 kOhm (or 500 Ohm) resistance but ~ 1 pF capacitance, and they can extend to several GHz. Active FET probes also have low input capacitance, but are expensive, limited in voltage, and easily damaged.

author: David Hess
author_rank: Super Contributor
timestamp: December 15, 2017, 04:42:54 am
content: You are all wrong but only because of a misunderstanding about how oscilloscope probes are specified.With specific exceptions which are not relevant here, there is little or no difference between oscilloscope input bandwidth with and without the probe because probes are specified to provide their rated bandwidth at the probe tip with a suitable oscilloscope. The standard root of the sum of squares rule does not apply. Instead, think of the probe bandwidth specification as indicating that the probe will meet its specifications up to that bandwidth.Also note that the combined probe and oscilloscope bandwidth is specified with a 25 ohm source at the probe tip. The value of 25 ohms comes from the shunt termination of a 50 ohm signal source.HP and Tektronix have published articles discussing the above issues and how oscilloscope and probe bandwidth is specified.

author: macboy
author_rank: Super Contributor
timestamp: December 15, 2017, 02:14:48 pm
content: Quote from: David Hess on December 15, 2017, 04:42:54 amYou are absolutely correct. Except those times when you are not. Many probes are in fact intended for a specific scope model/series, and just as you said, their specifications are given in combination with that scope. The P6133 I had mentioned is a perfect example. According to its manual it is "designed specifically for use with Tektronix 2400 Series oscilloscopes having bandwidths less than or equal to 150 MHz.", hence the (very conservative) 150 MHz rating. But not all are designed or marketed this way. Especially now with smart probe interfaces like TekVPI or Lecroy ProLink, probes have become separate products, rather than in-the-box accessories. I recently bought a Tek P6243 whose specifications give the analog bandwidth and rise time explicitly for "probe only". The specifications for any generic probe (e.g. Chinese P6100) should also be considered to be for probe only. In such cases it is left to the operator to determine the scope+probe bandwidth and/or risetime. In such cases, overrating of the probe is usually necessary in order to maintain scope bandwidth to the tip.Tek's "ABCs of Probes" discusses "Bandwidth to the probe tip" which means that a Tek oscilloscope provides its specified bandwidth at the probe tip (not only the input jack) when used with included or specified probes. It fails to mention that even they have trended away from standard accessory or specifically "compatible" (full bandwidth) probes for most of their lineup. In fact if you buy a new Tek DPO5204 2 GHz scope, it comes with a TPP1000 1 GHz passive probe. So "Bandwidth to the probe tip" doesn't always apply. Fortunately, the paper also extensively discusses bandwidth considerations including calculating effective bandwidth from combined rise times.

author: w2aew
author_rank: Super Contributor
timestamp: December 15, 2017, 03:00:10 pm
content: David Hess and macboy have given you the right info. One more point, a wide BW probe might not have enough adjustment range to compensate for the additional input capacitance of a much lower BW scope input, so you do have to be careful about that too.

author: David Hess
author_rank: Super Contributor
timestamp: December 16, 2017, 05:29:49 am
content: Quote from: macboy on December 15, 2017, 02:14:48 pmThe examples you give are part of the exceptions I mentioned which most people will not need to deal with. The old Tektronix 11K series had a couple of those which Tektronix mentioned as examples and they occur at higher bandwidths with high impedance inputs where things get weird. There was a bunch of this with the higher bandwidth 24xx series of oscilloscopes (1) where Tektronix issued A and B probe revisions which were designed to work with specific models of oscilloscope to achieve their full bandwidth. The 150 MHz 24xx models do not have any problems with it or at least mine does not which is to be expected.The oldest versions of the ABC's of Probes document from Tektronix, which has gone through like a revision per year, were much more informative on the subject and pointed out where the bandwidth at the probe tip specification broke down.Texas makes some high impedance probes for operation up to 500 MHz which include 2 or 3 high frequency transient response adjustments which should work well in cases like these but making the adjustments is not a trivial operation as few people have the equipment to do it. A fast enough reference level pulse generator is required along with a coaxial probe tip adapter with the termination built into the adapter. The same probes are sold to some companies with the adjustments preset and covered for their higher frequency oscilloscope models.As a practical level however, at high frequencies it is better to use a low-Z or active probe with a 50 ohm oscilloscope input which will present less loading to the circuit but verifying performance is still a problem. High impedance passive probes will only achieve their rated bandwidth with a 25 ohm source and present considerable loading.(1) Watch out for the Ebay guy selling butchered Tektronix 150 MHz 2445/A/B oscilloscopes as 300/350/400 MHz 2465/A/B models with the low pass filter bypassed. They lack the network used to correct the phase response of the delay line so they can never have good transient response. It might be possible to adjust one of those Texas probes for good transient response but only by limiting the bandwidth.

author: danadak
author_rank: Super Contributor
timestamp: December 16, 2017, 11:59:20 am
content: Some ref material -http://www.ni.com/white-paper/14825/en/https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwikjqeovI7YAhULMyYKHRl4AmQQFginATAB&url=http%3A%2F%2Fwww.tek.com%2Fdl%2F60W_18324_0_0.pdf&usg=AOvVaw3QV86KrdqUN1c1uCAmZWF1https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=13&cad=rja&uact=8&ved=0ahUKEwikjqeovI7YAhULMyYKHRl4AmQQFgjXATAM&url=http%3A%2F%2Fweb.mit.edu%2F6.101%2Fwww%2Freference%2FABCprobes_s.pdf&usg=AOvVaw3jqTqjxTe5xhU8dQaIF70Uhttps://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=14&cad=rja&uact=8&ved=0ahUKEwikjqeovI7YAhULMyYKHRl4AmQQFgjdATAN&url=http%3A%2F%2Fwww2.electron.frba.utn.edu.ar%2F~jcecconi%2FBibliografia%2F06%2520-%2520Osciloscopios%2520de%2520Almacenamiento%2520Digital%2FUnderstanding_Oscilloscope_BW_RiseT_And_Signal_Fidelity.pdf&usg=AOvVaw2F8bNBdJaSApbZVcZKa4QkRegards, Dana.

author: stj
author_rank: Super Contributor
timestamp: December 16, 2017, 01:33:13 pm
content: Quote from: danadak on December 16, 2017, 11:59:20 amthat's nice of you to index it all through the scumbags at google.lets link it PROPERLY this time:http://www.ni.com/white-paper/14825/en/http://www.tek.com/dl/60W_18324_0_0.pdfhttp://web.mit.edu/6.101/www/reference/ABCprobes_s.pdfhttp://www2.electron.frba.utn.edu.ar/~jcecconi/Bibliografia/06 - Osciloscopios de Almacenamiento Digital/Understanding_Oscilloscope_BW_RiseT_And_Signal_Fidelity.pdf

author: ocset
author_rank: Super Contributor
timestamp: December 16, 2017, 03:40:02 pm
content: All good advice, but remember to pick a probe for you r application, and always use the lowest bandwidth probe that you can get away with...because our "friend" mr electrical noise finds it easier to play tricks with us when you use a high bandwidth scope probe.....in fact, i think scope probes, with that horrendous ground tail, and pretty horrific at high frequencies, and there seems to be precious little better offtheshelf probe tips that dont use the ground tail.

author: CopperCone
author_rank: Super Contributor
timestamp: December 16, 2017, 05:36:23 pm
content: I use 500MHz probes on my 100MHz rigol. Only reason I use less if I want something with a higher voltage resistance (some old beefy tek probe) or active probes. Still need to get a low z probe.

author: mtdoc
author_rank: Super Contributor
timestamp: December 16, 2017, 06:00:58 pm
content: Great discussion and info here. Thanks.QuoteActually I don't think that is true. They have been shown to perform well above their specified bandwidth. Seethis thread.

author: CopperCone
author_rank: Super Contributor
timestamp: December 16, 2017, 06:09:46 pm
content: yes, but be careful of voltage rating with frequency.

author: David Hess
author_rank: Super Contributor
timestamp: December 16, 2017, 10:45:12 pm
content: Quote from: CopperCone on December 16, 2017, 05:36:23 pmI do the same thing. I have some Texas 250 MHz probes which I end up using on my 100 and 150 MHz oscilloscopes because they have the x10 readout function. And they are better then the cheap x1/x10 probes that I have where the x1/x10 switches are becoming intermittent.

author: tautech
author_rank: Super Contributor
timestamp: December 16, 2017, 11:01:44 pm
content: I always thought higher BW probes were desirable but reading Perfoma01's review recently he explains and shows why this is not the case.Scroll to the heading on P14: Probe Bandwidth PP510, in this link to his pdf:https://www.eevblog.com/forum/testgear/siglent-sds1104x-e-in-depth-review/?action=dlattach;attach=378192

author: mmb
author_rank: Newbie
timestamp: December 19, 2017, 08:24:59 am
content: I'm gonna try to budget for at least one "overkill" probe from Tek or HP [yeah, Keysight, gimme a break] so I have some standard to judge other probes by.

author: mmb
author_rank: Newbie
timestamp: December 19, 2017, 08:40:26 am
content: I'm guessing the oldest versions of the "ABC's of Probes" might have predated .PDF files. Can you give me a rough idea of how old the "good stuff" is-was?

author: tautech
author_rank: Super Contributor
timestamp: December 19, 2017, 09:04:24 am
content: Quote from: mmb on December 19, 2017, 08:24:59 amI'm not sure that's necessary or wise. Sure there were some old HP miniature probes that are lovely compact probes and those might be quite desirable for some work however if you'd studied and understood the full content of the linked article and all under the heading: Probe Bandwidth PP510 you'd see it's just not that simple unless by some good fortune you come up trumps with perfect compatibility between scope and probe over full BW range of your scope.In short use the manufacturer's recommended (or optional) probe for the scope for best results.

author: Berni
author_rank: Super Contributor
timestamp: December 19, 2017, 10:36:25 am
content: Something like this is also usefull:https://www.eevblog.com/forum/projects/yet-another-fast-edge-pulse-generator/Probing a fast edge that is beyond the bandwidth of the scope can be a nice way of seeing its performance. The rise time of the edge can tell you what your actual bandwidth is while any ringing or bumps after the pulse can show you if there are reflections or impedance mismatches happening. So nicer of a step you get the better the probe+scope combo is.

author: David Hess
author_rank: Super Contributor
timestamp: December 19, 2017, 01:35:33 pm
content: Quote from: mmb on December 19, 2017, 08:40:26 amI scanned and made a PDF from the May 1990 version some time ago however it is way too large to upload to the forum.https://drive.google.com/open?id=1MJ4PjIVnm4OjUVjSBo47mpB97uB2r70N