Probes: the good, the bad and the ugly - Page 1
Forum section: electronics / testgear

author: nctnico
author_rank: Super Contributor
timestamp: January 26, 2015, 11:31:45 am
content: Several times I have stated that a high impedance passive divider probe is useless to measure signals over 100MHz and higher frequency high impedance probes are a waste of money. Last week I had the chance to play with a 1GHz scope and felt it was time to put my money where mymoutherr.. keyboard is.I used an inexpensive Testec 150MHz 1:10 high impedance probe (bottom trace), a 1:10 Agilent 10073C 500MHz high impedance probe (middle trace) and a 1:10 Tektronix P6156 3.5GHz low-Z passive divider probe (top trace). I measured a 26MHz clock signal in a real circuit using one probe at a time. I used the flying ground lead with each probe. I think the results speak for themselves.

author: leppie
author_rank: Frequent Contributor
timestamp: January 26, 2015, 11:36:46 am
content: Is the reference level the same for all traces?

author: Pjotr
author_rank: Frequent Contributor
timestamp: January 26, 2015, 11:49:07 am
content: Quote from: nctnico on January 26, 2015, 11:31:45 amYou have proved that a flying ground lead is not done with such signals anywayThat doesn't mean that the result is not usable, depends on what you are trying to find.

author: EEVblog
author_rank: Unknown
timestamp: January 26, 2015, 11:52:17 am
content: Quote from: nctnico on January 26, 2015, 11:31:45 amThere's your problem.Repeat with proper high frequency probing techniques and I think you'll find the results quite different.

author: babysitter
author_rank: Frequent Contributor
timestamp: January 26, 2015, 12:15:17 pm
content: Reserving my business idea: Dont you dare to sell probes with different flying lead lengthsEnforced by the flying lead idea protection monster, rawwwr!

author: nctnico
author_rank: Super Contributor
timestamp: January 26, 2015, 12:20:11 pm
content: Quote from: EEVblog on January 26, 2015, 11:52:17 amThe screendump shows there is a considerable current flowing through the ground lead if the probe capacitance is high. The low-z probe (for which I also used the flying ground lead!) is much more likely to produce an accurate signal shape representation.

author: Pjotr
author_rank: Frequent Contributor
timestamp: January 26, 2015, 12:43:09 pm
content: Then it's not the fault of the probes but probably the fault of physics....

author: Howardlong
author_rank: Super Contributor
timestamp: January 26, 2015, 12:57:53 pm
content: I am still confused why you didn't use a spring clip ground? We all know those 4" flying ground clips are useless for HF+ for Hi-Z probes, that's why they supply them. I'd re-do your tests like the ones I did a few weeks ago with the spring clip ground instead.https://www.eevblog.com/forum/testgear/oscilloscope-probes-41026/

author: nctnico
author_rank: Super Contributor
timestamp: January 26, 2015, 12:59:04 pm
content: I see now I should have use a 100MHz signal to make the point more clear. Next time when I have the time & chance...@Howardlong: my point is that real signals at higher frequencies are very likely to be distorted by the capacitance of a probe tip. If you use a generator with a low enough output impedance everything will look just fine.

author: macboy
author_rank: Super Contributor
timestamp: January 26, 2015, 02:45:20 pm
content: Quote from: nctnico on January 26, 2015, 11:31:45 amDo it right and repeat the test. No ground leads.A Hi-Z 1:10 probe can do a good job up to several hundred MHz if the ground is low inductance,andthe circuit can tolerate the capacitive loading of such a probe. Inductance in the ground causes waveform distortion due to the use of capacitive divider at HF. This is where Lo-Z probes shine: they depend on a resistive divider even at GHz and present very little capacitive load.

author: c4757p
author_rank: Super Contributor
timestamp: January 26, 2015, 02:50:19 pm
content: Quote from: macboy on January 26, 2015, 02:45:20 pmThat'swhyhe used the ground lead, if I understand correctly - to accentuate this capacitive loading to demonstrate that it is quite significant and will almost definitely have an effect on the DUT. Without the lead, the loading is still there, and it's something to be well aware of as many devices willnotcheerfully drive a few ohms' worth of capacitive load without a significant waveform distortion.

author: Pjotr
author_rank: Frequent Contributor
timestamp: January 26, 2015, 03:00:47 pm
content: Quote from: c4757p on January 26, 2015, 02:50:19 pmThis only comes into account with analogue HF circuits or high speed digital circuitry where timing and board delays are critical (which in fact also belongs to the HF analogue category) . But evaluating a simple microcontroller board clocked at say 10 - 20 MHz or LF audio circuitry, a flying lead gnd is usually not much of a problem.

author: macboy
author_rank: Super Contributor
timestamp: January 26, 2015, 03:05:16 pm
content: Quote from: c4757p on January 26, 2015, 02:50:19 pmIf the point of the test is to illustrate probe capacitive loading effects, then don't throw the inductive ground lead into the mix. Isolate the capacitive loading by doing everything else (i.e. grounding) right. Then we see the effect of the probe capacitance loading the circuit, rather than the inductance of the croc ground clip. We don't see the Hi-Z probes at their best here; to the contrary, this is them at their worst (you can't do worse than a croc clip ground). Use the probe properly, and show is how badly they fail, even at their best. I am not defending Hi-Z probes; they will fail compared to Lo-Z or active probes, but this test does not illustrate that at all.

author: Howardlong
author_rank: Super Contributor
timestamp: January 26, 2015, 03:05:58 pm
content: Quote from: c4757p on January 26, 2015, 02:50:19 pmIf that's the case, a good way to demonstrate that would be to probe the output of a high speed unity gain opamp where excess capacitive loading can be disastrous.

author: Co6aka
author_rank: Unknown
timestamp: January 26, 2015, 04:31:00 pm
content: Quote from: nctnico on January 26, 2015, 11:31:45 amSaturday I was checking a 30.72 MHz clock in a real circuit, using a Hi-Z 500 MHz Rigol 10:1 probe, and the traces resembled your top trace. I was referencing the probe at the ring via a springy-clip.

author: georgd
author_rank: Regular Contributor
timestamp: January 26, 2015, 04:33:31 pm
content: Using long GND probe lead is bad/dangerous practice not only from view of singal integity.Spikes from switching DC/DC converter can pollute meassurement on the whole board.

author: tggzzz
author_rank: Super Contributor
timestamp: January 26, 2015, 07:37:06 pm
content: Quote from: nctnico on January 26, 2015, 12:59:04 pmWhile that is true, yourntest didn't demonstrate it. Your test demonstrated that tip capacitance resonates with ground lead inductance. No news there!Remove the inductance, and then and only then you can look at the effect of tip capacitance.

author: nctnico
author_rank: Super Contributor
timestamp: January 26, 2015, 07:40:29 pm
content: Or remove the capacitance. See the top trace...

author: davorin
author_rank: Unknown
timestamp: January 26, 2015, 07:48:55 pm
content: Evenin' (o;I've read a couple of threads here regarding probes...seems to be a never ending discussion (o;Also discovered that my probes are mentioned here...with really bad results....some chinese blah-dunno-6500 models going to 500mhz....As it turned out they are really crap when I had to capture some simple ROM access timing on a 16500B CPU board....Which model would you recommend for a 9354AL scope with 500MHz bandwidth?Budget around US$ 100 - 200 for 4 probes...

author: andrija
author_rank: Regular Contributor
timestamp: January 26, 2015, 07:49:11 pm
content: Isn't it obvious from your pictures that the higher spec'd the probe, better results you get? The most expensive probe (with least capacitance, I presume) produces the perfect square wave. So are you saying your assumption that high spec passive probes are a waste of money was wrong? That's how I read graphs you presented.

author: tautech
author_rank: Super Contributor
timestamp: February 04, 2015, 10:25:44 am
content: Stumbled across a link buried in an old thread (2012) and looked for an appropriate thread to link it from.IMO here is a good place.Probing High-Speed Digital Designshttp://www.sigcon.com/Pubs/straight/probes.htmEnjoy.

author: tautech
author_rank: Super Contributor
timestamp: February 04, 2015, 10:36:57 am
content: And another....lets put a bunch of them all together.I'll leave it to you to judge the best ones.DC to 1GHz Probe Construction Planshttp://www.emcesd.com/1ghzprob.htm

author: tautech
author_rank: Super Contributor
timestamp: February 04, 2015, 11:20:03 am
content: Quote from: davorin on January 26, 2015, 07:48:55 pmHave a look at BK Precision PR500B 10x 500MHz 10pF passive probes.Customer wanted to push my demo SDS2304 waaay past what it was comfortable with the supplied 300 MHz probes.Sourced ex Digi-key.

author: Pjotr
author_rank: Frequent Contributor
timestamp: February 04, 2015, 01:31:36 pm
content: Quote from: tautech on February 04, 2015, 10:36:57 amStrictly it is not necessary to terminate the cable at the probe tip. 50 ohms at the scope end suffices to dampen reflections. I have had good results with home-brew mini probes with just a 2K 0805 thin film resistor at the probe tip with RG316 coax up to 1 GHz.

author: Howardlong
author_rank: Super Contributor
timestamp: February 04, 2015, 04:19:07 pm
content: Quote from: tautech on February 04, 2015, 11:20:03 amIf you want readout, the Texas TX6150 isn't bad. I bought four from the ebay ad below, they took about three weeks to arrive though.http://www.ebay.co.uk/itm/220306076099Also here:http://www.ezprobe.com/probes/TEX500RA.html

author: w2aew
author_rank: Super Contributor
timestamp: February 04, 2015, 04:21:26 pm
content: I did a video about 2 years ago showing some examples the affects of ground lead length on signal integrity - both on a nice clean low impedance function generator output, as well as from the output of a CMOS oscillator:

author: Howardlong
author_rank: Super Contributor
timestamp: February 04, 2015, 04:23:22 pm
content: Quote from: w2aew on February 04, 2015, 04:21:26 pmIndeed, essential watching on the subject IMHO.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 04, 2015, 05:06:40 pm
content: i will take the advice if its from someone who can build, compensate and tune a passive probe at required application needs. much like a jedi who can build his own light saber. otherwise for those who only know how to "buy" one, i agree with the OP, most likely it will be just a waste of money or either as a matter of buying a cheap one or expensive one. i think what those pictures are speaking about is about comparison between a "well compensated" and "poorly compensated" probes, or ringing in the play as others have noted. but i think the OP theorem is correct if one is to measure a quite high source impedance signal because it will throw the probe's build in compensation network out of the way.

author: DanielS
author_rank: Frequent Contributor
timestamp: February 04, 2015, 10:40:23 pm
content: Quote from: Mechatrommer on February 04, 2015, 05:06:40 pmOP's images show the difference between misused probes and how a low-Z probe is less susceptible to gross distortion from misuse. There is no point in talking about "poorly compensated" probes since proper compensation requires proper use of the probes - use of proper grounding - in the first place. The best compensation in the world does sweet f-all if you are going to use a 4-6" alligator ground clip to probe high-speed edges.