Cheap wideband 1:21 oscilloscope probe for logic signal applications - Page 1
Forum section: electronics / projects

author: jahonen
author_rank: Super Contributor
timestamp: December 11, 2009, 05:13:14 pm
content: Hi everyone,I made alittle studyon the DIY-probe mentioned in book by Howard Johnson and Martin Graham, High-speed digital design. I have found it very useful on several occasions, as it costs next to nothing compared to commercial active probes. Just a BNC-connector, some coaxial cable (RG-174 is quite suitable), 1 k? resistor and 50 ? inline terminator if scope doesn't have internal one. Hope that you'll find this useful too.With some work and front-end design, this makes excellent probe for a DIY logic analyzer.Regards,Janne

author: Mark_O
author_rank: Frequent Contributor
timestamp: March 16, 2010, 11:11:16 am
content: Quote from: jahonen on December 11, 2009, 05:13:14 pmJanne,I just wanted to comment that this was a very nice project, and say "thanks" for sharing it. It's impressive the kind of results that can be achieved by DIY, if you know the principles involved. I really liked how you validated your results (great VNA graphs), and how well it compared against the super-expensive Infinimax probe.Just a couple suggestions, that you may already be aware of... instead of using a 1 k? sense resistor, use two 470? units in series. This will serve 2 useful purposes: a) cut your parasitic capactance in half (extending your high end), and b) give you a 20x probe, instead of 21x.Also, it's a good idea to make the probe tip fairly long, to insure that the sense resistor always stays a cm or more (half an inch) away from the ground plane. Failure to do so can result in picking up an extra 0.5 pF of parasitic capacitance from ground plane proximity, and degrading your performance curves by a factor of 4!Please keep us informed of your results, if you continue experimenting. I really enjoy seeing these.- Mark

author: tesla500
author_rank: Regular Contributor
timestamp: March 16, 2010, 07:54:09 pm
content: Thanks Janne, very good info! Would you be able to repeat this using a small (1/8W through hole, or SMT) resistor and see if that's any better (or worse)?You mention that you can't set arbitrary attenuation on your Agilent MSO6034A. Actually, you can! Just set the probe scaling to dB, and you can set it to anything you want between 0.1 and 1000. If you don't have the dB option, install the latest firmware. Took me awhile to figure that one out, they don't mention it anywhere.One other thing, using an inline attenuator on a standard 1M/14pF scope input will only give you ~200MHz bandwidth and will cause reflections (50Ohm // 14pF = 200MHz low pass filter).Regards,David

author: jahonen
author_rank: Super Contributor
timestamp: March 16, 2010, 09:40:08 pm
content: Thanks for your input. I already thought that nobody was interested about this simple thingI can try the SMD resistor out, but what I remember from previous undocumented experiments, it didn't affect the scope measurements very much. Original description on the HSDD book used 1/8W resistors. VNA measurements are probably going to show more dramatic effect. Don't hold your breath waiting for the results, though.Low-pass effect is probably visible, but I think that most scopes having enough bandwidth to show this effect, have internal terminators thus no external terminator is necessary. I have Tektronix TDS3034 at work (it has a switchable internal termination option), I can check this effect too in practice. Comparing the results against the internal termination is easy too.Yes, there are some reflections (even with the very good matching VNA, as you can see on the frequency response plot), but usually they can be neglected for most digital applications. This is because they have quite low amplitude and the probe cable is usually long enough to delay the reflection quite a bit from the actual signal edge.Actually now when I think about it, MSO6034 really has the dB option for each channel, but I haven't used that very much. Thanks for reminding me about that, I'll check that too.Regards,Janne

author: Unknown
author_rank: Unknown
timestamp: March 18, 2010, 09:38:04 am
content: I'd also like to thank Janne for this investigation. I wonder how significant the choice of resistor is. I estimated the inductance of a 1/4W metal film resistor as 26nH max. (depending on the number of turn of the spiral cut). A 1/8W resistor would probably be slightly less. This would introduce a 15% error at 1GHz. I believe this is why Doug Smith recommends an SMD thin film resistor, and Howard Johnson recommends against using metal film (although he recommends carbon film, which is just as inductive as far as I know). I think that Doug Smith's design is generally acknowledged to be the best design (Howard Johnson also refers to him).A comment about the comparison with the $10K active probe. While it's useful to compare it to a brand-name product, the suggestion that you need to spend $10k for equivalent performance in a commercial product is somewhat misleading (I know you don't actually write this). I'm sure a significant part of the $10k price tag is because it's a differential probe. A single-ended >1GHz FET probe will be much cheaper ($2k list for a 1.5GHz model). You can get a used FET probe for a few hundred dollars on ebay (depending on the bandwidth).All major oscilloscope brands also sell resistive probes. The Tektronix (I'm the most familiar with their products, I'm sure Agilent has something equivalent) P6158 (20x Zo probe, 1kohm, 1.5pF, 3GHz) is $500 list. The Tektronix P6150 (1x/10x Zo probe, 50ohm/500ohm, 0.15pF, 9GHz) is $2k list. Both (and the older P6156, which is slightly better than the P6158) can sometimes be picked up on ebay for less than $100. Still a lot more than a meter of RG-174, a connector and a resistor, but no $10k either. The commercial probes do give you mechanical rigidity and compatibility with some accessories (these two are the weak points of most DIY probes in my opinion), and better performance with less aberrations.My intent is not to criticize DIY probes, I think it's a useful technique, and it teaches us about high-speed signal acquisition.

author: jahonen
author_rank: Super Contributor
timestamp: April 05, 2010, 08:36:27 am
content: Yesterday after spending a few hours to make up a measuring system with a LabView, I prepared a measurement for measuring the probe with an oscilloscope. Here is a baseline plot with just a coax (and external terminator):There seems to be some peaking at the low end of the VNA signal source, like you see on the probe measurements. Some steps seems to result where the scope timebase is stepped. Next, I'll measure with the probe, and perhaps I'll make a comparison with an "ordinary" 1:10 probe.Regards,Janne

author: Mark_O
author_rank: Frequent Contributor
timestamp: April 05, 2010, 08:02:39 pm
content: Janne,>Here is a baseline plot with just a coax...<I'm glad to see that you're still pursuing this. But what kind of coax is down 10 dB at 400 MHz?Ah, I see. You've got the Tek TDS3034B in the test loop. A different kind of testing. Cool!- Mark

author: alank2
author_rank: Super Contributor
timestamp: October 19, 2014, 12:27:03 am
content: I've been doing some reading on z0 type probes and I saw 450ohm for 10x and 950ohm for 20x, but what about using a larger resistances for less loading? 2450ohm for 50X, 4950ohm for 100X. Would the only issue with that be that you wouldn't be able to go smaller than say 100mV per div (assuming your scope can go to 1mv per div).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 19, 2014, 12:43:08 pm
content: Large values are impractical due to capacitance dominating at high ratios. Think of it this way: the resistor should be arranged over nearby grounds as a transmission line, with characteristic impedance comparable to the resistor value. Needless to say, it's hard making transmission line over 500 ohms.For the same reason, high ratio attenuators are generally multi-stage for best results.Tim

author: David Hess
author_rank: Super Contributor
timestamp: October 19, 2014, 02:13:12 pm
content: Quote from: alm on March 18, 2010, 09:38:04 amI have seen a lot of old time-domain RF designs which use 1/8th watt carbon composition resistors for this very reason. You will find loads of these in old Tektronix sampling heads and circuits operating at 1 GHz and higher.QuoteI would be satisfied with a comparison to modern low-Z probes. Considering how simple these are in comparison to high impedance passive probes, they sure are expensive.

author: edavid
author_rank: Super Contributor
timestamp: October 19, 2014, 04:51:01 pm
content: Quote from: T3sl4co1l on October 19, 2014, 12:43:08 pmWell, you have to add a compensation cap, as on a high impedance probe. For example, the Tek P6057 is a 100X 5Kohm probe with 1.4GHz bandwidth.

author: eurofox
author_rank: Unknown
timestamp: October 19, 2014, 05:29:25 pm
content: For my sampling scope with max 2V input I use the same idea to make a 1/10 probe.A 450 ohm SMD resistor and a coaxial cable to the 50 ohm input of the scope.

author: lapm
author_rank: Frequent Contributor
timestamp: October 19, 2014, 05:45:04 pm
content: Considering how simple this is, im suprised how well it seems to perform over all... I think i better make note of this and make couple when i get a chance...

author: David Hess
author_rank: Super Contributor
timestamp: October 19, 2014, 06:17:38 pm
content: Quote from: eurofox on October 19, 2014, 05:29:25 pmI just have a couple of RG-316 and RG-178 pigtails which I solder into place temporarily with the resistor added because the ground lead connection needs to be equally short anyway. I find that Teflon insulated cable is easier to use for this because it does not melt like polyethylene.I would like to have some of the real probes but they are too expensive compared to this solution.Quote from: lapm on October 19, 2014, 05:45:04 pmThey are also handy for fast oscilloscopes with 50 ohm inputs when a passive high impedance probe will have too much capacitive loading.

author: alank2
author_rank: Super Contributor
timestamp: October 19, 2014, 11:11:03 pm
content: Hi,Quote from: T3sl4co1l on October 19, 2014, 12:43:08 pmI really need to spend some time learning RF stuff.Looking at the picture in the link above, it looks like the resistor wire itself was almost filed into a probe point. Along side it is the ground lead which I can't tell if it is a pin of some type soldered to the braid, etc.Am I correct in thinking that the 50 ohm impedance relationship in the RG174 is created by the core vs the position of the braid? Are you suggesting that the same thing needs to happen with the resistor in the picture and the ground lead coming along side it? How would you adjust it to obtain a certain impedance? Would that impedance be relation to the size of the resistor? Is Mark's message above about using two resistors to cut parasitic capacitance related to this?What I'd really love to do is make a compact style probe like this - where the RG174 ends and there are a couple of wires/leads coming off of it that have the resistance embedded in them and then have a nice pin for connecting them to a through hole probe point, breadboard, etc. Can someone explain to me what happens at this point where the RG174 stops and these two leads begin that would be awesome. I am assuming the shorter the better, how would 2" for the signal and 2" for the ground affect the probes capability?Thanks,Alan

author: Marco
author_rank: Super Contributor
timestamp: October 20, 2014, 12:13:30 am
content: Just google for oscilloscope probe ground lead inductance, there's quite a few articles on that and most of the information is relevant here ... they generally assume something like 20 nH for per inch for a wire, so 4 inches at 1 GHz is about 500 Ohm, so a 10x divider gets turned into a 20x one.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 20, 2014, 01:12:31 am
content: Quote from: alank2 on October 19, 2014, 11:11:03 pmYes, or more precisely (as long as it's coaxial), the ratio of diameters.Consider the facing surfaces of the wires. This gives capacitance. A thinner wire has less capacitance, as does a greater separation between wires.Now consider the space between wires. This allows room for magnetic field around them. More separation means higher inductance, as does thinner wire.A coax cable made with (literally) hair-fine core and regular sized shield could achieve an impedance of a few hundred ohms. Average stuff, obviously, achieves 50-75 ohms. A very thin insulator with a heavy core could go down into the 10 ohm range (which starts to look not so much like coax, as, were you to take two strips of copper, a thin dielectric, and rolled that up).A pair of wires in space have impedance to each other, which for typical insulated wire at the typical diameter of the insulation, this is around 75-120 ohms. A wide separation increases inductance and reduces capacitance, so that common "twin lead" is 300 ohms.If you have no companion wire at all, but just wire traveling through "free" space, it works against the impedance of free space alone, which is 377 ohms; actual impedances still depend on resonance and geometry, so that an antenna (a wire sticking up with no adjacent ground pair) still exhibits a fairly low impedance (50-100 ohms for common types like 1/4 wave whip or 1/2 wave dipole) despite that.Getting impedances much over 300 ohms is kind of hard (for the odd case where you need it). The conductors need to be very thin and distant, while also not coupling to free space (i.e., radiating), while also not resonating (you can get quite high impedances if you reflect the radiation with a shield... but then it only works at certain resonant frequencies!).So, that's kind of a very gross qualitative touchy-feely look at transmission line impedance.When you see advice like "minimize inductance", what they generally mean is not actually minimizing the lumped equivalent inductance of the structure, but reducing the impedance (but not minimizing -- at least, not except for very rare cases) so that the transmission line / lumped line / whatever equivalent more or less sort-of matches the circuit's desired characteristics.In this case, since we're talking high resistance, we should also be talking high inductance -- but not to the point of actually using an inductor, because that forms a low pass filter with parasitic capacitances. Suppose you laid it out so that the transmission line sections are still 50 ohms; they'll look like capacitors, and act to resonate or attenuate the highest frequencies, rather than transmitting them proportionately. So the more pressing minimization, if there should be one, would be capacitance -- but true absolute minimization is rarely the ideal goal, and going for an optimal combination of inductance and capacitance leads to the best result.QuoteSort of. The thing about resistors is, there's a semi-conductive layer on the thing, which therefore acts as a transmission line against nearby conductive surfaces. That's line to ground action. But you also have the fact that the resistor is built from two conductors, and you have coupling between the faces of those conductors. The resistor is, in effect, shunted by a very short transmission line in parallel with its (DC) resistance. Because it's very short, it doesn't play much role at most any frequency, but it becomes more noticeable for larger R and higher F.Using resistors in series effectively connects those parasitic electrode-to-electrode transmission lines in series, increasing the total series impedance. Which is good. But, each of those nodes between the resistors (in addition to the resistors themselves) have impedance to ground, so ultimately you're still not dividing it evenly, and you'll get weird rises and dips in the frequency response.The low frequency story is, capacitance from between-resistors-node to ground causes a droop at some intermediate frequency. If you connect capacitors in parallel with the resistors, you can swamp this effect, and extend the high frequency range, at least until the chain of capacitors itself causes too much loading (or has too much ESR, ESL, weird transmission line effects, etc.) to do its job anymore.The ultimate case, then, one should quite reasonably suspect -- there IS no such thing as a true ideal lumped resistor and capacitor divider, but it's all just restated transmission lines. And in that light, one should suppose the best result will be a 450 ohm transmission line dividing into a 50 ohm transmission line, or whatever -- which looks odd, because that 450 ohm line is connected "line to ground" from source to load, not "line to line". You can't really implement this, because of common mode rejection. But if you can approximate this in some series-parallel equivalent, including resistive materials, you'll be doing very well.QuoteTwo things:1. You get an impedance discontinuity. Which, if it's short enough for the frequencies of interest*, won't matter, but you will want to deal with it so you can match it correctly. Of course, I couldn't tell you offhand just what arrangement of resistors and transmission-line-ey action would be necessary.. you'd have to faff around quite a lot to get it just so. And then it loses its usefulness as a probe, because everything has to be just so: the probe wire positions, the cable, the ground, where it is over the circuit (and whether the circuit is ground-planed or not), etc. (I'd be happy to give it a try figuring it out for you, but I'm not cheap...)*Which is undoubtedly the case -- by which I mean, if you're using RG-174 which is just awful stuff even for short lengths, for frequencies over 100MHz -- you won't have enough bandwidth at the far end of the cable to even care!2. Even if you get the impedances and dividers and stuff laid out correctly so that the frequency response at the feed point (i.e., even ignoring cable losses) is correct, other reasons may pull weight first -- in particular, in the presence of common mode noise (easily present in digital and switching circuits, and careless or poorly terminated RF amps, and ambient noises, and... anything else), any non-coaxial connection is just asking for interference. Try to do a sensitive measurement and you'll just get a mess! See Linear Tech AN-47 for good probe ideas and examples.Tim

author: jahonen
author_rank: Super Contributor
timestamp: October 20, 2014, 11:53:45 am
content: Quote from: alank2 on October 19, 2014, 12:27:03 amThe biggest problem is that resistors have some shunt capacitance which will cause a high frequency boost. For 1206 SMD resistor, that is about 50 fF. Achieving larger attenuations will require that the attenuation is done in several stages. I don't think that single ordinary resistor will do if you want similar bandwidth.Quote from: T3sl4co1l on October 20, 2014, 01:12:31 amThis kind of probe has impedance discontinuity at probe tip by the definition. It relies upon the impedance matching between feed coax and the scope 50 ohm terminator to suppress the reflections. So one can have whatever nasty impedance at the probe end and still get perfect reflection-free response if scope and coax termination is perfect. But in reality there will be some reflections but usually they are small enough not to disturb the measurement too much. Of course, the reflections would be considerably better attenuated if both ends would be matched.RG-174 is perfectly sufficient in my opinion for this kind of construction. For lengths of 1 meter or so and typical relevant scope applications, high frequency losses are negligible. You don't perhaps get the world's best probe but often it gets the job done. That is what matters. Pushing the bandwidth above 1 GHz would require substantially more effort than is meaningful to try to do in a short time. Coaxes with substantially less loss at GHz frequencies are usually considerably more expensive and/or clunky (semi-rigid or otherwise impractically thick), so I think that RG-174 is acceptable. RG-316 has slightly less loss but not by a huge margin, but has much better soldering heat durability. I have a measurement of insertion loss of RG-316 lying around somewhere but I'm not at correct computer right now.Regards,Janne