Adapter: Scope probes -> Logic Hook ? - Page 1
Forum section: electronics / testgear

author: michaeliv
author_rank: Frequent Contributor
timestamp: March 23, 2016, 10:22:59 pm
content: Hello,I'm wanting to have the inputs of my DS1054Z in the form of logic analyzer grabbers, such as the attached pic shows.The main goal is minimum grabber cable thickness / stiffness while also keeping accurate waveform visualization at high frequencies.I realize that the waveform will probably be distorted somewhat if not using the probes directly, the question is what setup will have minimum distortion.What's the recommended way of doing this ? I was thinking of running a ~40cm, thin, shielded(to probe ground) cable from the grabber to the probe tip.Alternative would be to build my own 10x probe with the same grabber + thin cable running for about 2m, but this will probably have much worse performance.Any recommendation on the setup, or what cable to use ? How much distortion should I expect ?Thanks!

author: tggzzz
author_rank: Super Contributor
timestamp: March 23, 2016, 11:36:45 pm
content: Quote from: michaeliv on March 23, 2016, 10:22:59 pm40cm wire => a 400nH inductor. What's the input capacitance and resistance of your circuit? 10pF? What's the step response of such an LCR circuit? If you don't understand the theory, at least do a spice simulation.OTOH, if it is coax or other transmission line then how will it be terminated, and what's the impedance and what effect will that have on the source?

author: tautech
author_rank: Super Contributor
timestamp: March 24, 2016, 12:41:24 am
content: At logic levels at low BW's you don't need anything flash if you're just needing confirmation of function or basic timing.Here's a couple of ideas from old threads:https://www.eevblog.com/forum/beginners/life-hacks-for-the-beginning-ee/msg769430/#msg769430https://www.eevblog.com/forum/repair/suggested-for-a-sticky-part-one-comments-or-additions-please/msg471163/#msg471163

author: mauroh
author_rank: Frequent Contributor
timestamp: March 24, 2016, 12:35:02 pm
content: Using couple of springs and a piece of heat-shrink tube I made something like this:http://www.amazon.com/Pomona-6553-Modular-Oscilloscope-Operating/dp/B005T96JF2The price is ridiculousFor low frequency it works fine. I made 4 of this for my DS1074Z and for I2C, SPI, Serial debugging it is really handy.Since the grabber are not permanently attached to the adapter, you can connect it strait to pin headers male/female on the board under test.If you dont like the DIY, adding an mcx to BNC adapter this is not badhttp://it.aliexpress.com/item/High-Quality-Digital-Hook-Probe-Mcx-for-Mini-Digital-Oscilloscopes-DSO201-DS203-DSO-QUAD/32235397014.html?spm=2114.46010208.4.1.HxdITHThis is another solution I like, not soldering it to a PCB but adding couple of wires to attach the minigrabbers.http://www.newark.com/keysight-technologies/n2768a/mini-probe-socket-vertical-pcb/dp/80K6001Alan made a great video on "How to make a high performance oscilloscope probesocket" Thank you AlanExtending the spring connections with wires you basically ruin the purpose of this technique but again for low frequency it works fine.Mauro

author: michaeliv
author_rank: Frequent Contributor
timestamp: March 24, 2016, 08:33:21 pm
content: Quote from: tautech on March 24, 2016, 12:41:24 amThat looks like what I would like to make, up to what bandwidth have you used them, how is their performance ?If you have 2 wires close together does the signal from one wire jump to the other ?Quote from: mauroh on March 24, 2016, 12:35:02 pmIt's extra funny that it's shipped & sold by Amazon directly (not a 3rd party seller).That's what I would want for connecting my probes, can you post pictures of your springy things ?Are there any other options for ... "female oscilloscope tip plug" ?

author: tautech
author_rank: Super Contributor
timestamp: March 24, 2016, 09:09:39 pm
content: Quote from: michaeliv on March 24, 2016, 08:33:21 pmConnection to a DUT to maintain signal integrity is a practised art, these I've used to a 100 or so KHz when all you're looking for is functionality and timing correlations.Be aware grabbers are mostly LV rated and connection paths and personal safety need be considered but at logic levels there's not too much to worry about.If designing a PCB do insert test points, this can be accomplished in several ways: headers, SMD test points, wire links or bare pads for "bed of nails" test beds.QuoteAt logic levels and for simple checks crosstalk should be of little worry.

author: mauroh
author_rank: Frequent Contributor
timestamp: April 04, 2016, 11:23:42 pm
content: Quote from: michaeliv on March 24, 2016, 08:33:21 pmSorry for the late feedback.See below the pictures of my super simple adapter.As you can see the Rigol probe tip fit perfectly on a simple female pin header and with the spring that came with the probe the connection is stable and reliable.The cons of this solution is that it require to disassemble the probe and to have around the spring.The probe tip is delicate and you can easily bend it/break it.To solve this issues, I made my version of the solution linked by tautech.I'm very happy with it as a Low Speed ProbeMauro

author: nctnico
author_rank: Super Contributor
timestamp: April 04, 2016, 11:42:42 pm
content: Why bother with the board in between? Just cut, strip and tin the wires to grab them with the hook.

author: mauroh
author_rank: Frequent Contributor
timestamp: April 05, 2016, 12:06:04 am
content: 10 minutes of my life wasted just to have a cleaner solutionI also placed 2 extra ground pins on each signals side to use wire pairs and this was really handy to connect the DUT with a flat cable.Mauro

author: michaeliv
author_rank: Frequent Contributor
timestamp: April 05, 2016, 12:42:31 am
content: Thanks all, I think mauroh's solution is similar to what I'm going to build for myself.mauroh - Does LowSpeedProbe_03.jpg have a significant improvement over LowSpeedProbe_05_Multi.jpg in terms of signal quality ?How do they compare to directly hooking up the probes ?Also can anyone recommand some good not-ridiculously-priced grabbers ?I got someTektronix SMT SMG50which are excellent except for the part where the end that you push on is very, VERY wobbly.I also got someAgilent / HP 5090-4833and they are decent but not as good as the tek. They don't have any wobbliness issues.Does anyone have any feedback on theseAgilent 10230-68702? They seem high quality but also seem to have a ridiculously thick shaft and I can't seem to find dimensions for them or a picture of them next to a banana.

author: tggzzz
author_rank: Super Contributor
timestamp: April 05, 2016, 07:40:28 am
content: Try doing a spice simulation of your probe and a simple circuit. Input capacitance can be found in data sheets, and you should model your wires with an inductor with a value of 1nH/mm.Quote from: michaeliv on April 05, 2016, 12:42:31 amI suggest you build something quickly, attach it to a circuit, and then look at thetransitions. Be aware that:the highest frequency present in a circuit depends only on the edge rate (V/s), not on the period (s or Hz)the logic's correct operation depends on whether the setup / hold times and logic threshold / maximum voltages are correct. The scope's speed and any clock frequency are irrelevantattaching any probe will affect the circuit being probedinductor + capacitor + transition -> ringing and voltage overshoot. That will be visible in the measurnig instrument and reflections might affect the circuit's operationa 40cm wire +15pF probe input capacitor will resonate at around 65MHzGeneral principle: keep wires short and ground leads wide.For theory and practice of probing, have a look at the references athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/For practical examples of probes causing ringing, have a look athttps://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/

author: mauroh
author_rank: Frequent Contributor
timestamp: April 05, 2016, 08:17:26 am
content: I don't know the actual performance of the probe adapters since I don't have the proper setup to characterize it, but as tggzzz stated, we can do some math/simulation to have an order of magnitude. For sure this is not meant for high speed without further improvements.As already stated by tautech, "Connection to a DUT to maintain signal integrity is a practised art" and design a proper probe for a logic analyzer with flying leads is not that easy.What is your specific need?Do you have to perform a measurement on a specific circuit?At which frequency it operates?Mauro

author: mauroh
author_rank: Frequent Contributor
timestamp: April 05, 2016, 08:28:32 am
content: Found this:http://blog.weinigel.se/2016/02/28/ghz-differential-probe-2.htmlLooks promisingMauro

author: tautech
author_rank: Super Contributor
timestamp: April 05, 2016, 08:35:42 am
content: Quote from: nctnico on April 04, 2016, 11:42:42 pmNot a good idea if the DUT and the POI has any significant voltages attached, which is why I knocked up my breakout with recessed loops in a Phenolic offcut and quality HV cable.

author: tggzzz
author_rank: Super Contributor
timestamp: April 05, 2016, 09:21:36 am
content: Quote from: mauroh on April 05, 2016, 08:17:26 amNote that whether or not it is suitable for high speed is dictated by the logic family,notby the clock rate of one particular circuit.Note that the operating frequency is dictated by the edge rate or transition time,notby the clock rate of one particular circuit.

author: tautech
author_rank: Super Contributor
timestamp: April 05, 2016, 09:34:19 am
content: Quote from: michaeliv on April 05, 2016, 12:42:31 amYou must have the correct 0.025" square push-on female connectors then you'll have a nice firm connection.Get the correct connectors and you'll have no trouble.

author: mauroh
author_rank: Frequent Contributor
timestamp: April 05, 2016, 10:24:47 am
content: Quote from: tggzzz on April 05, 2016, 09:21:36 amThank you for the correction.Just to visualize the different response with different accessories I was thinking to use my version of the 74AC14 "TDR" Pulse Generator as source.I'll try to post few oscilloscope screenshot asap.Mauro

author: nctnico
author_rank: Super Contributor
timestamp: April 05, 2016, 10:47:25 am
content: I think there is not much use in characterisation of these way of probing. It is a bad way for high speed signals but very useful in many situations.

author: mauroh
author_rank: Frequent Contributor
timestamp: April 05, 2016, 10:57:52 am
content: Agree, but even though the theory will just be confirmed, quoting you :"There are small lies, big lies and then there is what is on the screen of your oscilloscope."It will be just for fun...Mauro

author: michaeliv
author_rank: Frequent Contributor
timestamp: April 05, 2016, 06:57:46 pm
content: Quote from: mauroh on April 05, 2016, 08:17:26 amIt's just general SPI/I2C and such timing and sanity checks.It's not for a specific circuit, it's for general use.Frequency up to 4MHz.Thanks all for your answers!

author: tggzzz
author_rank: Super Contributor
timestamp: April 05, 2016, 07:29:26 pm
content: Quote from: michaeliv on April 05, 2016, 06:57:46 pmIf you implemented that in 74LVC1G* gates, that same circuit operating with the same clock will have frequencies up to at least 700MHz - because the risetime would be measured in picoseconds.

author: mauroh
author_rank: Frequent Contributor
timestamp: April 08, 2016, 03:28:56 pm
content: Done, see attached the screenshot with the results.Again, no surprise the signal gets worst and worst with the adapters and this is just a bad practice, but for low speed signals it is fine and useful.The Signal with lower ringing is the output of my 74AC14 "TDR" Pulse Generator connected straight to the oscilloscope input whit a T, on the other side of the T I placed the 50Ohm termination.As you can see my pulse generator doesn't have already the world cleanest output (I put it together on a veroboard...)Edit: forgot to mention the most important thing... All the weird and wonderfool behavior introduced by a bad method/probe/adapter... will be also hided by the bandwidth limitation of the scope.100MHz are not enough to appreciate all the bad staff you are adding to the signal with the "LowSpeedProbe"... It will be fun to do the same exercise on a much higher bandwidth scope....BTW I never noticed the DS1104z have just 2 colors for the saved references, all the others are just white.I'll verify this on the manual...Mauro

author: mauroh
author_rank: Frequent Contributor
timestamp: April 08, 2016, 06:12:05 pm
content: Just noticed: Dave is using the "Logic Hooks" on the famous site pictureMauro

author: Howardlong
author_rank: Super Contributor
timestamp: April 08, 2016, 09:36:36 pm
content: If I need a high speed (and often low speed) probe, or for close pitch or tight spaces, for anything more than a quick browse, I get the hot pointy thing out and solder in the test points. Invariably I find the LA probe clips are flimsy, and fall off at the slightest excuse. Add to that, there are fewer and fewer parts with anything to clip onto anyway!A few years ago I bought these beauties from a shop in Akihabara, they go down to 0.5mm pitch QFP, but not on directly adjacent pins :-(It's rare I ever ever use them!Edit: I see they have 0.3mm now.http://goo.gl/fJ6H3y$50 for two

author: tautech
author_rank: Super Contributor
timestamp: April 09, 2016, 01:29:19 am
content: Quote from: Howardlong on April 08, 2016, 09:36:36 pmThose are very nice Howard, let's see if Franky can source them.