Dealing with transmission line solder joints and measuring probes - Page 1
Forum section: electronics / beginners

author: NaxFM
author_rank: Regular Contributor
timestamp: December 21, 2021, 08:38:16 am
content: Hi everyone, my intention is to upgrade my very old 70MHz oscilloscope to a 200MHz one, so i started to dig deeper in transmission lines and probing techinques for optimal signal integrity.There are a couple of things i never fully understood regarding transmission lines.Suppose we have a nice, long 50 ohm impedance line on a pcb, sooner or later this line has to end somewhere, suppose on some chip, maybe an amplifier.The thing that always left me with doubt is: isn't the solder joint (and even the internal bond wires) between the line and the amplifier an abrupt change in impedance? Why I've never seen anyone concerned with this? It is my understanding that it's really not a problem, but why exactly?Another similar case is when we use those bnc to crocodile clips probes. In a video Dave used a 50ohm coaxial cable wich terminates in two 15-20 cm long leads with crocodile clips to measure the switching noise of a power supply. He mentiones that the line, being 50 ohm impedance, has to be properly terminated at the oscilloscope, but what about those two long leads flapping around in the breeze which are obviously not 50 ohm? Why that doesn't matter in this case (20 MHz measurement)?The only explaination i could give myself is that if we apply the transmission line rule of thumb*, those lenghts (solder joint and flying leads) are so small that it really doesn't matter, however i'm really not sure if one could apply this rule even at the end (or in the middle ) of an already existing transmission line.* Treat everything as a transmission line if line is longer than 1/6 of wawelength

author: tggzzz
author_rank: Super Contributor
timestamp: December 21, 2021, 09:11:19 am
content: The key points are the physical length versus the wavelength of the highest frequency component in the signal. The highest frequency component is determined by the signal'srisetime; basic Fouier theory indicates the signal's period is completely irrelevant. Naturally the scope's frequency response is also irrelevant, except when it comes to measuring a signal.For many amateur circuits you can ignore impedance changes due to the lead length and solder. Not so for RF and high speed digital circuits, where they become critical. Examples are RF microstrip filters and manufacturers' IBIS models (a packaged version of Spice models) of their chip's interconnections.A useful introduction to all that is Bogotin's rules of thumb on EDN.https://www.edn.com/category/blog/bogatins-rules-of-thumb/page/4/Putting a probe across a high frequency signal is a completely different story. There the probe's capacitance is the key issue; at 100MHz the 15pF tip capacitance turns a standard "high impedance" *10 probe into 100ohms or so.For basic probe theory, including safety pointers, seehttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/Being concerned about a transmission line and termination with croc clips is unlikely to be valid.

author: NaxFM
author_rank: Regular Contributor
timestamp: December 21, 2021, 09:31:13 am
content: Being concerned about a transmission line and termination with croc clips is unlikely to be valid.https://youtu.be/gpwkiJC5hfUIn this video Dave shows that even with croc clips, properly terminating the coaxial transmission line at the oscilloscope makes a HUGE difference. You can see that the probing is done with simple croc clips at the end of a coaxial line.What I don't understand is "Why probing with croc clips is OK for a somewhat high 20MHz bandwidth?"Is it because the section where the coaxial is split into the two leads is so short with respect to the wavelength (always according to the transmission line rule of thumb) that we don't need to treat it as a transmission line?If it is so, this means that I can have an unmatched section at the end of a transmission line as long as this unmatched section is short with respect to the wavelength, right?

author: tggzzz
author_rank: Super Contributor
timestamp: December 21, 2021, 09:57:29 am
content: I haven't got time to watch a rambling 20min video in detail!Those aren't croc clips, they are banana plugs. That won't change anything.As far as I can see, between his quick changes of the horizontal timebase, the ringing is at ~50MHz.Calculate the inductance of the flying leads; assume 1nH/mm as a rule of thumb.Calculate the capacitance necessary to resonate with that at 50MHz.Compare that capacitance with the capacitance of scope and cable.(Also do that with a *10 probe with a flying lead; seehttps://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/)Caclulate, using transmission line theory, the period of any reflections in a cable that length. Compare with 50MHz.Dave doesn't get everything right, as he notes in the video! Kudos to him for pointing a correction.There are many subtle problems that occur with making any measurement - especially with noise and scope probes.

author: jonpaul
author_rank: Super Contributor
timestamp: December 21, 2021, 02:34:49 pm
content: Bonjour, I suggest that you Read a bit:https://w140.com/tekwiki/images/6/62/062-1146-00.pdfThe effect of a few mm solder joint will be unmeasurable till you go to GHz BW and sub nS risetimes.The scopes you mention (especially if Chinese) often have poor BW and Trise.We use the Leo Bodnar 30-40 pS pulser and TEK PG506 CAL sets.All of these have many solder joints and perform beyond specification, at least on TEK 7104 7A29 1 GHz analog scope and Yokogawa TA720 time analyzer, Yokogawa DL7440 500 MHz BW scope.Instead of "clip leads" or alligator, we just solder a RG174/U direct to the board.Finally the excellent TEK P6156 Zo probes are designed for such use to 3.5 GHz.https://w140.com/tekwiki/wiki/P6156Bon Chance,Jon

author: tggzzz
author_rank: Super Contributor
timestamp: December 21, 2021, 05:02:24 pm
content: Quote from: jonpaul on December 21, 2021, 02:34:49 pmPut a 500ohm (preferably 450ohm!) between the board and the coax conductor, and you have just made a P6156. The details are left to the student, but there are many advantages over *10 "high" impedance probes

author: jonpaul
author_rank: Super Contributor
timestamp: December 22, 2021, 09:47:42 am
content: Thanks to TGGZZZS, yes I forgot the series R!Easy to find 450 Ohm 1% but must be low series L and shunt C.Still you cant beat the P6156, occasionally you can get one with all accessories in new cond on fleapay for $40-80.Jon

author: David Hess
author_rank: Super Contributor
timestamp: December 22, 2021, 07:54:41 pm
content: Quote from: NaxFM on December 21, 2021, 08:38:16 amAt higher frequencies it does become a problem, and ICs and parts intended to operate at those frequencies take steps to maintain the transmission line environment through the package and bond wire to the chip. It can become an issue even at 400 MHz in some cases.One trick you used to see is IC and transistor packages with two external pins going to the same bond pad, so the signal goes in one pin and comes out the other to meet the termination. This removes the parasitic inductance which would otherwise limit performance.

author: CaptDon
author_rank: Super Contributor
timestamp: December 23, 2021, 12:29:57 am
content: And then some noob following Dave's "terminate with 50 ohms at the scope" does that and connects the gator clips to some circuit of 50 volts or so and "TERMINATES" the whole front end of the scope complete with that expensive smoldering smoke smell. I always thought EEVBLOG should also have a section to mate with 'REPAIR' called 'IMPAIR' where all the beginners who decided to service some very delicate sophisticated equipment that they bought at a bargain price on EPAY, the typical "AS-IS We don't know how to test it" or the popular "We don't have a power cord to test it with" which you can read as "It's 100% totally f--ked up", tried to service it themselves with no knowledge of electronics and finished blowing the unit to smithereens. I can think of about three posters fitting that description. The 'IMPAIR' category could be where noobs share their smoking hulk of once nice equipment stories. Back to our original story, Those flying leads that are unshielded make fantastic antennas and pick up tons of EMI/EMR especially from SMPS power supplies internally on many of today's circuit boards in all kinds of gear. It is hard to tell what is real and what is magnetic interference when you have flying leads and high impedances.

author: coppercone2
author_rank: Super Contributor
timestamp: December 23, 2021, 03:11:32 am
content: The two wires dangling form a ladderline/twin lead of some kind, so its not exactly so bad. It has its own characteristic impedance.

author: Electro Fan
author_rank: Super Contributor
timestamp: December 23, 2021, 03:33:18 am
content: Quote from: tggzzz on December 21, 2021, 09:57:29 amDoes this rule of thumb assume or care what the diameter of the wire is?

author: jonpaul
author_rank: Super Contributor
timestamp: December 23, 2021, 09:29:03 am
content: Bonjour, Inductance of a loop (in this case one turn) depends on the loop area and the u of the enclosed material, for air or vacuum or non ferrous u=1formulashttps://interferencetechnology.com/understanding-inductance-real-world/calculatorshttps://www.eeweb.com/tools/loop-inductance/https://www.qsl.net/in3otd/ind1calc.htmlThe wire thickness is usually irrelevant as the current path can be decimated to a single loop at the wire center.For lab setups and probing with clip leads etc. treat the wires as perfect zero diameter.Instead of 1 nH/sq mm I recall 1 uH per sq foot....Jon

author: NaxFM
author_rank: Regular Contributor
timestamp: December 23, 2021, 10:16:11 am
content: Quote from: CaptDon on December 23, 2021, 12:29:57 amMaking something explode is part of the learning experience. I'm sure every single person who works regurarly with electronics has destroyed something when probing or connecting wires toghether, it happens to everyone, and it is also a lesson one will never forget. It is the best kind of lesson.I really don't understand what are you trying to say. 50V is an high voltage, and anyone who works with such voltages and does know he needs a termination also knows that he can't put 50V through a 1W 50 ohm resistor unless the goal is to create a fire, my question was related to something else.I just wanted to know why there are cases where we can ignore transmission line effects even at relatively high frequencies

author: tggzzz
author_rank: Super Contributor
timestamp: December 23, 2021, 02:15:34 pm
content: Quote from: Electro Fan on December 23, 2021, 03:33:18 amClearly not!It is an approximation suitable for back-of-the-envelope calculations.If you want something more accurate, then there are online calculators, and then you can use field solvers for even more accuracy.

author: CaptDon
author_rank: Super Contributor
timestamp: December 23, 2021, 03:28:37 pm
content: I can't say I have ever been able to ignore a transmission line effect at the higher frequencies. To some degree it seems to depend on the impedance mis-match end to end. With a TDR unit you can often easily see even a small aberration, for instance two coax feed lines joined together with a double female barrel to marry a pair of PL-259 ends or a pair of BNC ends. Although you can see it, it has a very minimal effect. On the other hand, when examining 'eye patterns' on Ghz DDR memory data lines the tiniest of aberrations or trace length mis-match create huge data error problems. Even the attachment of the probe leads creates a disturbance. If the probe attachment points are mis-matched by even 1/4" the eye pattern becomes nearly useless. Memory and CPU speed is so fast that matching trace length is important and in cases where they add the little 'trace squiggle' to match length they create an inductor in series. X-Band radar work was fun where the transmission line is waveguide. At microwave frequencies tuning or matching is often done with dents, bumps or an adjustable screw protruding into the waveguide. When your test leads are almost exactly 1/4 wavelength of the operating frequency of your U.U.T. weird things can happen. An open at the far end looks like a short at the feed end and a short at the far end looks like an open at the feed end. Always best to not have resonant lead lengths in your coax jumpers!!

author: Electro Fan
author_rank: Super Contributor
timestamp: December 23, 2021, 05:32:11 pm
content: Quote from: tggzzz on December 23, 2021, 02:15:34 pmGlad to hear you say that. The calculators I’ve seen show that to make your rule of thumb work a 1 mm length of wire would need to be .01 mm in diameter. Or maybe I didn’t understand the rule (ie, whether it was referring to wire length, diameter, area, or maybe something else).At about .6426 mm diameter (22 AWG solid wire that might be used in a breadboard) for a 1 mm length of wire the inductance would be about .275 nHAt about .6426 mm diameter (22 AWG solid wire) for a 2.4 mm length of wire the inductance would be about 1 nH(1 inch length of 22 AWG wire should be about 22 nH.)Alternatively a 1 mm diameter wire about 3 mm long would be a little more than 1 nH (1.14 nH).These calcs depend on some other variables including wire composition and I think they can be useful in helping to visualize inductance units like a nH if we have a common and correct understanding of the particular measured values.Simple calculator:https://www.allaboutcircuits.com/tools/wire-self-inductance-calculator/

author: tggzzz
author_rank: Super Contributor
timestamp: December 23, 2021, 05:42:31 pm
content: Quote from: Electro Fan on December 23, 2021, 05:32:11 pmDon't rely on a single source.https://www.eeweb.com/tools/wire-self-inductance-calculator/andhttps://chemandy.com/calculators/round-wire-inductance-calculator.htmgive 100mm of 1mm diameter wire as 105nH.Note that I wrote 1nHpermm.

author: Electro Fan
author_rank: Super Contributor
timestamp: December 23, 2021, 07:30:08 pm
content: Quote from: tggzzz on December 23, 2021, 05:42:31 pmYour first calculator link gives the same results as what I listed above; using all the default values your second calculator link shows results that are very close to the same. So the question is 1 nH per mm in what dimension?

author: tggzzz
author_rank: Super Contributor
timestamp: December 23, 2021, 08:14:47 pm
content: Quote from: Electro Fan on December 23, 2021, 07:30:08 pmLength of course. Look at any set of inductors, and you will see longer wire = more inductive.Try plugging in different diameters, and you will see that inductance isn't astrongfunction of diameter.