Newbie question on Oscilloscope's Probes - Page 1
Forum section: electronics / beginners

author: cyb3rfly3r
author_rank: Contributor
timestamp: December 21, 2020, 09:24:10 pm
content: Hi everyone,I am learning electronics and have an Analog Discovery 2 (AD2) with the BNC extension and related probes. I like this portable tool and learning its possibilities and limitations. I was wondering how interchangeable probes are between various scopes.- Can one use AD2 probes with a (for instance) Siglent/Rigol/Tektronix scope?Obviously, there may be limitations in bandwidth when using an inexpensive probe designed for the AD2 in a better scope, for a frequency range or Voltage. I also understand that there is probably the need to adjust probe compensation.- Is there something else to consider? Would it be OK to use a probe from a more performant scope with the AD2 provided it has the same impedance?- Are most probes standard "50 Ohm" probes?- What does "50 Ohm probe" means anyway? Impedance depends on frequency, so at what frequency is the probe 50 Ohm?- What about performance at any other frequency? So, are there probes specialized for a frequency range?Thanks in advance for any answer or resources to understand this topic better.C.

author: bob91343
author_rank: Super Contributor
timestamp: December 21, 2020, 10:51:01 pm
content: Tektronix and, I think, HP have published guides on probe use. Consult those first.However, for the most part a probe is a probe. They are rated for impedance, attenuation, and bandwidth. There are FET probes with high input impedance. There are direct probes for low frequency work. There are current probes as well. Some can be switched for various purposes.For higher frequency work, impedance is important. By that, I mean if you are making measurements on 50 Ohm systems you need to terminate any transmission lines with that figure. Some scopes offer that from the front panel, in which case you don't use any probe at all, just a coaxial cable.

author: TimFox
author_rank: Super Contributor
timestamp: December 21, 2020, 11:01:13 pm
content: For "normal" passive x10 high-impedance (10 megohm) scope probes, there is a specification for the input resistance (usually 1.0 megohm) and a range of parallel capacitance required at the front panel connector of the oscilloscope. This should be specified in the documentation on the scope probe, and on the front panel of the scope. You must adjust the "compensation" (a variable capacitor) on the probe connector (sometimes the probe body) to get a good square wave when the probe is connected to the calibrator output on the scope front panel. After that, you should have 10 megohms in parallel with a specified capacitance (which is not trivial compared to the 10 megohms, even at 1 MHz) at the probe tip.For example, the Tektronix P6139A is rated up to 500 MHz, with x10 +/- 0.5% attenuation at DC and low frequency. It can be used with 1.0 megohm in parallel with 8 to 18 pF, and will have 8 pF in parallel with the 10 megohms at the tip when properly compensated. The reactance of 8 pF at 2000 Hz is 10 megohms, so above that frequency the probe impedance is capacitive.

author: CaptDon
author_rank: Super Contributor
timestamp: December 21, 2020, 11:41:20 pm
content: All good probes will have an adjustable 'compensation' whichyou tweek to make 1khz square waves look square. No overshoot,too much high frequency gain, and no undershoot, high frequencyroll-off. This must be adjusted to suit the scope that the probe willbe used with. It is usually a tiny trimmer capacitor with maybe 2 to15pf of capacitance range. Best adjusted with a plastic non-conductivetool!! Tek supplies the tool with their probes as well as a little bagwith colored rings to mark the probe if you use it with a multi-channelscope. Best wishes!!

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 21, 2020, 11:48:02 pm
content: Here is a good thing on scope probes

author: 0culus
author_rank: Super Contributor
timestamp: December 22, 2020, 12:16:04 am
content: https://download.tek.com/document/02_ABCs-of-Probes-Primer.pdfAs Bob mentioned above. Great resource.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: December 22, 2020, 12:24:19 am
content: Find a PDF of "High Speed Digital Design: A Handbook of Black Magic" on the Internet and look at Chapter 3. Chapter 4 has some good stuff too. Well the entire book is good stuff.

author: cyb3rfly3r
author_rank: Contributor
timestamp: December 22, 2020, 12:27:17 am
content: Thanks a lot to all who answered! I will take time to understand and digest the excellent answers.C.

author: james_s
author_rank: Super Contributor
timestamp: December 22, 2020, 12:49:26 am
content: What probes come with the AD2? When you can get a pair of halfway decent 100MHz probes for ~$14 there aren't too many reasons to cut corners and go with something less.

author: cyb3rfly3r
author_rank: Contributor
timestamp: December 22, 2020, 03:01:14 am
content: Quote from: james_s on December 22, 2020, 12:49:26 amI have linked the probes on DigiKey, which also has the specificationshttps://media.digikey.com/pdf/Data%20Sheets/Digilent%20PDFs/460-004P_Web.pdf. In any case, there is nothing wrong with the AD2 probes: I was just asking out of curiosity, to understand how it works and what makes some probes much more expensive than others.C.

author: rstofer
author_rank: Super Contributor
timestamp: December 22, 2020, 03:07:41 am
content: The AD2 probes will be just fine, at X1 they are rated to 6 MHz and at X10 they are rated for 100 MHz. No reason not to use them.https://store.digilentinc.com/bnc-oscilloscope-x1-x10-probes-pair/HOWEVER: The AD2 has a very limited input voltage range, +-25V. It would be pretty easy to have a bench project that exceeded this range if the probes were on an X1 setting. So, I bought some ProbeMaster fixed X10 probes # 3904-2. They're kind of pricey but they can prevent an "oopsie" with the AD2.https://probemaster.com/3900-series-scope-probes-100-mhz/

author: james_s
author_rank: Super Contributor
timestamp: December 22, 2020, 08:13:49 am
content: On the lower end, the name makes some probes far more expensive than others. If they say something like Tektronix, Agilent or LeCroy on them they're going to cost a bundle, they're probably somewhat better quality but this is not guaranteed. Perhaps most importantly you can believe the specs, if it says they'll be 3dB down at 100MHz then you can trust that, the specs on low cost probes tend to be a bit optimistic, but that doesn't mean they're bad probes.Above about 100MHz the cost starts to rise very rapidly, a passive probe that can approach 500MHz is gonna cost you a few hundred bucks, a 1GHz active probe is probably a few thousand. Specialized stuff like current probes are also spendy.

author: radiolistener
author_rank: Super Contributor
timestamp: December 22, 2020, 12:39:11 pm
content: Quote from: cyb3rfly3r on December 21, 2020, 09:24:10 pmno, most of the osclilloscope probes designed for 1 MOhm impedance input.Such probe will works incorrectly with 50 Ohm input.Quote from: cyb3rfly3r on December 21, 2020, 09:24:10 pmit means it have 50 Ohm impedance at any frequency.Well within some range, let's say from 1 MHz till 100 MHz.For frequency below 1 MHz it may have high impedance error, but low frequency is not sensitive to impedance error.Quote from: cyb3rfly3r on December 21, 2020, 09:24:10 pmyes, usually oscilloscope probes are rated for 100-200 MHz bandwidth and 1 MOhm BNC impedance

author: cyb3rfly3r
author_rank: Contributor
timestamp: December 22, 2020, 04:44:09 pm
content: Thanks, @radiolistener.Having read some of the resources linked here, I understand better how the probes are supposed to work. I would then have another question.The AD2 has an extender board that allows one to connect the BNC probes in lieu of the fly-wires. On that board, in the input channel, right "behind" the BNC connectors there is a jumper that you can set to add a 50 Ohm resistor in series with the signal input (or just the ~0 Ohm trace). What is the purpose of that 50 Ohm resistor? Is it for when one is getting the input signal directly from 50 Ohm Coax Cable?I am assuming that I should NOT use the 50 Ohm resistor with the probes, given it already has its calibrated 1 MOhm (1x) or 10 MOhm (10x) impedance.Thanks again,C.

author: rstofer
author_rank: Super Contributor
timestamp: December 22, 2020, 05:10:14 pm
content: You're looking at the wrong pair of BNCs. The scope input pair have jumpers to select DC or AC coupling. In AC coupling mode, there is a small capacitor in series with the signal.The 50 Ohm resistor gets in series with the function generator output to provide a 50 Ohm (or 0 Ohm) source for the external circuitry.The schematic:https://reference.digilentinc.com/_media/reference/instrumentation/bnc-adapter-board/discovery_bnc_sch.pdfSee Features here:https://store.digilentinc.com/bnc-adapter-for-analog-discovery/

author: tggzzz
author_rank: Super Contributor
timestamp: December 22, 2020, 05:15:11 pm
content: I have a suspicion you are confusing oscilloscope inputs with AWG outputs. They are different.The AWG outputs will have an impedance <<50ohms. In many systems a 50ohm output impedance is required, and that is achieved by a 50ohm resistor connected in series with the output. That is "source termination".The scope inputs have an impedance of 1Mohm (in parallel with ~20pF), and they are used directly with standard 10Mohm *10 probes. Many systems require that the load impedance is 50ohms, and that is achieved by a 50ohm resistor connected in parallel with the 1Mohm input. That is "load termination".Where a 50ohm input or output is required, you connect a 50ohm coax cable directly to the input/output.You cannot use a 10Mohm *10 probe with a 50ohm input, nor as an output.There is a type of scope probe (Z0, resistive divider) that is used with a 50ohm input, but they are only relevant at several hundred MHz, i.e. outside AD2's capabilities.

author: cyb3rfly3r
author_rank: Contributor
timestamp: December 22, 2020, 05:22:23 pm
content: Yes, I had confused the AWG jumpers with the scope input jumpers, sorry and thanks for the clarification.C.

author: radiolistener
author_rank: Super Contributor
timestamp: December 24, 2020, 08:37:30 pm
content: Quote from: cyb3rfly3r on December 22, 2020, 04:44:09 pmYes, 50 Ohm resistor is needed when you feed signal directly from 50 Ohm coax cable. Such resistor prevents wave reflections and distortion in the cable.

author: TimFox
author_rank: Super Contributor
timestamp: December 24, 2020, 09:26:34 pm
content: Proper use of a 50 ohm termination (with 50 ohm cable) is one or both of these:50 ohm resistor in parallel with a high impedanceinput.50 ohm resistor in series with a low impedanceoutput.