coax and splitter vs probe question - Page 1
Forum section: electronics / testgear

author: Vincenzo
author_rank: Regular Contributor
timestamp: May 17, 2021, 02:36:00 pm
content: Basically, what is the difference between viewing a signal/waveform (of BW less than 100MHz in a scope that is has a 200MHz sticker on it) using a legit probe, and viewing the same signal/waveform with what's in the picture and very short wires connected between the red and black banana females and the signal/waveform source?

author: tggzzz
author_rank: Super Contributor
timestamp: May 17, 2021, 03:43:53 pm
content: You need to understand basic probe theory, which is well explained in many places.You could do worse than look at the references inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/with particular reference to the Tektronix "ABC" primer.

author: radiolistener
author_rank: Super Contributor
timestamp: May 17, 2021, 10:21:17 pm
content: Quote from: Vincenzo on May 17, 2021, 02:36:00 pmthe difference between coax and banana plugs is transmission line impedance which is not constant across banana plug wires. As result it leads to signal reflections and distortions. But this is critical for high frequency. You can notice difference for signals above 1 MHz.If you're deal with low frequency, about 100 kHz or below that, then the difference will be so small that you cannot notice it. Because cable length is too short against wave length.

author: Vincenzo
author_rank: Regular Contributor
timestamp: May 18, 2021, 07:12:46 am
content: tggzzzI wasn't asking about stuff I need. I have some understanding of how probes work. The coax jumper I'm showing does have 50ohm characteristic impedance, and I mostly use a high Z (1Mohm) scope input or I can add a 50 ohm anywhere when needed. Did you ever have problems with an arrangement like I'm showing?thnxradiolistenerDo you mean changing with time? or changing with temperature? or what? by saying "not constant across wires". There shouldn't be reflections if the coax I'm showing is 50 ohm and is terminated by 50 ohms on both ends (one is terminated by two very short wires). Ok, forget about banana, how about this that is sold on ebay just like all "legit" probes

author: tggzzz
author_rank: Super Contributor
timestamp: May 18, 2021, 09:08:37 am
content: Quote from: Vincenzo on May 18, 2021, 07:12:46 amThe stuff you need is knowledge and understanding. That is contained in the references I gave.I suspect you particularly need the references on safety, even if you don't realise it yet.

author: Vincenzo
author_rank: Regular Contributor
timestamp: May 19, 2021, 04:40:32 am
content: Quote from: tggzzz on May 18, 2021, 09:08:37 amso, what do you think about the last pic?

author: mindcrime
author_rank: Unknown
timestamp: May 19, 2021, 05:27:53 am
content: It's basically a 1x probe, and will work for some range of frequencies and some definition of "work". Not sure exactly what the frequency range is where you'll start getting weird noise and stuff. FWIW, Dave has done several videos on oscilloscope probes -this onemight be of interest to you. I watched it when I was trying to figure out if you could do the same thing you're talking about.

author: tggzzz
author_rank: Super Contributor
timestamp: May 19, 2021, 08:08:55 am
content: Quote from: Vincenzo on May 19, 2021, 04:40:32 amIt is a good picture.The references are good too. What do you think of the information in them?I suspect you will also find it profitable to read the bullet pointshere.

author: radiolistener
author_rank: Super Contributor
timestamp: May 19, 2021, 09:30:54 am
content: Quote from: Vincenzo on May 18, 2021, 07:12:46 amacross wire length. For example first 1 cm can have 70 Ω, then 10 cm 130 Ω, then 10 cm 20 Ω, etc. These are not real digits, just for explanation what I mean when I told you about "not constant impedance". And when you move wires it leads to impedance change, so signal distortions and reflections inside wires will be changed.In order to avoid it, you're needs to use proper RF connectors and coax cable. With proper soldering or crimping, this is also very important. The coax cable has constant impedance across cable length. And RF connectors are designed in such way to avoid significant impedance change across connection.Quote from: Vincenzo on May 18, 2021, 07:12:46 amit has the same problem. I have such cables, they works up to 3-7 MHz, above that reflections and distortions are very significant. Also they have high loss at MHz frequency.You're needs to understand that RF signal is transfered not through wires, but through space between wires. So, the size between wires and insulator material is very important here. Wire geometry also very important. At VHF frequency even a drop of solder can significantly affect impedance.This is why banana plugs are not applicable for RF signals. You can use it for low frequency signals such as audio below 20 kHz, but if you needs to work with MHz signals, then you're needs coax cables and proper RF connectors.

author: Vincenzo
author_rank: Regular Contributor
timestamp: May 20, 2021, 12:17:19 am
content: Quote from: mindcrime on May 19, 2021, 05:27:53 amThank you, sir. I remember that video you referenced. I am trying to understand the significance of the a probe's handle and if it's replaceable by a carefully designed and put together coaxial with very short single end wires to touch the testing point and ground to as high a frequency as is possible. Mr. Alan Wolke (W2AEW) made a simple Thing that is very similar (as far as illiterate me can tell) and he's a senior guy at tekBTW, is "FWIW" an acronym for some world war in a foreign language?

author: Vincenzo
author_rank: Regular Contributor
timestamp: May 20, 2021, 12:19:51 am
content: Quote from: tggzzz on May 19, 2021, 08:08:55 amNooo, I totally disagree. There was not enough light, and the kids did not have great smiles.

author: Vincenzo
author_rank: Regular Contributor
timestamp: May 20, 2021, 12:36:24 am
content: Quote from: radiolistener on May 19, 2021, 09:30:54 amNow, I get that what you meant is the fact that any change in Z0 anywhere in the transmission line (the coax or the probe coax wire) reflection/loss happen, and I get from your thinking that in commercially sold probes, prob handles (test ends that you hold when testing) are matched to the probe wire. If that what you are thinking and not the short pieces of wire at the end picking up RF noise (like 50/60Hz pollution and hundreds of kHz SMPSs everywhere), then I apologize if I didn't make myself clear. What I am trying to do is to use a cheap scope (like a siglent 1202X-E) that has high inpot characteristic impedance (>1M) and have the coax (probably 50) matched on both ends (including to the short wires at the ends).I am sure that I will not be happy at the MHz, but what do you recommend to make the number that comes before "MHz" as high as possible.

author: Vincenzo
author_rank: Regular Contributor
timestamp: May 20, 2021, 12:45:58 am
content: QuoteI fully understand the geometry and transmission line and antenna differential equations and banana-not-good-for-rf stuff, but theoretically, you can put a pi or T matching little LC network at every abrupt change in the transmission line material/geometry for matching (like between the coax and the sensing short wires).Thank you so much for replying and educating me

author: Vincenzo
author_rank: Regular Contributor
timestamp: May 25, 2021, 06:57:49 am
content: So, for readers who will see this thread in the future, here are some answers:1. All (1X) probes (even the most expensive ones) are inherently flawed and have a very limited frequency range (usually less than 10MHz) because even a short piece of coax cable (that is relatively immune to picking up noise from the air) has a lot of (distributed) capacitance that affects the point in the circuit its ends touch (e.g. 100pF is 50at 30 MHz).2. There is an easy solution that resembles the design of a 10X probe, i.e. adding about 4 components to the coax (at either end) to isolate the cable capacitance and take care of (compensate) low and high frequency dependence of the effect on the tested circuit. Thus super-pricey scope probes are overrated (except may be some Tektronix ones that are overpriced) and you can make your own “probes” with careful design.3. The picture I used is for what comes with, for example, most Waveform/function/rf generators that are sold these days with (advertised) ranges of output frequencies of tens of MHz and 50 ohm terminations, so they are great substitutes for scope probes if a couple of passive components are added at either/both end if needed.4. The great Alan (youtube channel W2AEW) who, I think, is one of the (if not the) best scope teacher, because he has a video that is investigating the exact same question (in addition to Dave’s video that one of the commenters referenced about how most new/young RF and electronics professionals and hobbyists are unknowing or keep forgetting the very limited nature of 1X probes). This is Alan’s great video from a decade ago:and there is an addition to it several years later: