DIY 1x oscilloscope probe?  - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 14, 2023, 12:50:43 am
content: I remember Dave making a video a few years ago about cutting a regular coax cable and creating a DIY 1x oscilloscope probe which supports 100MHz and greater frequencies. I can't find the source video; does anyone know what it is? I'm wanting to make a similar probe to measure switching regulators.Thanks.

author: BrokenYugo
author_rank: Super Contributor
timestamp: September 14, 2023, 01:53:01 am
content: I sort of remember that one. I cant find it either, but I did find this nice writeup on the subject, not 1x but I think it's what you're talking about.http://jahonen.kapsi.fi/Electronics/DIY%201k%20probe/EDIT: As mentioned by others, "Z0" is the magic word, start at 13:45.

author: metebalci
author_rank: Frequent Contributor
timestamp: September 14, 2023, 05:06:37 am
content: Not 1x and not sure if you are asking about the same thing but there is something called Z0 probe or passive broadband probe. I also tried it some time ago:https://metebalci.com/blog/z0-probe/

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 14, 2023, 08:26:31 am
content: But why not use a 10x probe? 1x in any high frequency circuit is generally a bad idea.

author: Berni
author_rank: Super Contributor
timestamp: September 14, 2023, 08:31:20 am
content: A 1x probe with >100MHz bandwidth means you need an active probe that has a high bandwidth gain 1 amplifier inside it.The easy to make DIY high speed probes are the ones that people already mentioned above of using a resistor on the end of a 50 Ohm terminated coax. They work great for digital work.So why exactly do you need it to be 1x?

author: EPAIII
author_rank: Super Contributor
timestamp: September 14, 2023, 09:14:43 am
content: I never tried to use them for 100 Mhz or anything over 5 Mhz for that matter, but I often made perfectly usable 1X probes with RG-59 coaxial cable, a BNC connector, and a junk box probe from a VOM or an alligator or other type of clip. Compensation is only needed when the probe includes a division, like 10X or 100X. So 1X probes do not require any compensation. And with the high impedance scope inputs, the characteristic impedance of the coaxial cable used is of little importance so 50 or 75 Ohm coax works just fine. And NO, the coax did NOT need to be terminated. Just straight into the 1 MOhm scope input.Why shop made probes, you ask. Tektronix or other name brand probes were expensive and often only two 10X probes were purchased for THE scope in a TV station or other facility. Many situations required both of those probes to be connected to circuit points to display two waveforms while a third probe was needed for the trigger signal which came from a third point. So a shop made probe, often using a scrap coaxial cable, was a perfect third probe. And sometimes even two such 1X probes were used in addition to the two 10X ones. The 10X probes were used where the signal needed to be displayed with high accuracy while the 1X ones were used for lower frequency signals or where the signal wasn't even displayed at all.Due to the type of service I used them for, I never really paid much attention to the exact limitations of such shop made, 1X probes. But then the scopes being used often had bandwidths of only 25, 35, or 50 Mhz so perhaps the probes were better than the scopes.And of course, in today's world, 10X probes are a lot more reasonable in price. I have four or perhaps even five that I personally purchased for two scopes that I own. I am sure I paid less for all of them together than what one Tektronix probe cost back in the day. And at least two of them are rated at 200 Mhz while the Tektronix probes I used were only 50 or 100 Mhz, IIRC.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 14, 2023, 02:09:21 pm
content: Quote from: BrokenYugo on September 14, 2023, 01:53:01 amThat's the probe at 1345 that I was thinking of! Thanks!

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 14, 2023, 02:15:20 pm
content: Quote from: Berni on September 14, 2023, 08:31:20 amI guess I don't need it to be 1x, I just wanted to make one because of its low loop inductance (I think that's the right words?) for measuring high frequency signals from switching power supplies. I guess adding a 9Mohm resistor may be a good idea, if I'm wanting to increase the bandwidth of the probe?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 14, 2023, 03:56:43 pm
content: Quote from: Berni on September 14, 2023, 08:31:20 amWhy would you think it should be 1x?As far as I know there are also 500Ohm probes, which have a voltage divider and conditioning on the "hot end" of the coax, and a 50Ohm termination on the scope side. You can just cut off a coax cable and probe your circuit with the end, but the results will not be great. the coax will have a high capacitive load on the measurement point, and the coax needs to be properly terminated on both ends to work as a proper transmission line.Up to 100MHz or so the conventional 10:1 probes work adequately for most practical purposes, and that is why (nearly?) all scopes come with these probes as standard, but there are a lot of more specialized probes for special purposes.When you go into the really high end scopes, they do not even have a BNC input anymore, because BNC is not good enough. They have the same beefy screw connector as you see on spectrum analyzers, and such scopes aso do not have the 1:10 probes.(I always forget, is it 1:10 or 10:1. anyway they divide the signal by a factor of 10 (together with the 1M scope input) You put in 10v, out comes 1V.)

author: tggzzz
author_rank: Super Contributor
timestamp: September 14, 2023, 04:39:57 pm
content: Quote from: Doctorandus_P on September 14, 2023, 03:56:43 pmMy fastest scope, 200ps risetime, has GR874 connectors. It works really well, given the limitations of having BC107 transistors in the signal path (there are 3 transistors with fT=2GHz, in the trigger)Here it is 200ps/div with a signal that has 250ps risetime.The OP might find the refs athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/useful. They should pay particular attention to the safety and praxis refs, since they are apparently interested in SMPSs. If they are interested in EMI/EMC, completely different classes of probes are necessary.

author: Gyro
author_rank: Super Contributor
timestamp: September 14, 2023, 06:23:40 pm
content: Quote from: Doctorandus_P on September 14, 2023, 03:56:43 pmThat's what's known as a Z0 probe, see the links in Reply #1, #2 and also tggzzz's page which contains a number of links...https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/There are many Z0 probe threads on here too, eg.https://www.eevblog.com/forum/projects/lo-z-probe/A x20 Z0 probe, 1k input (950R tip + 50R scope termination), is a useful compromise for many CMOS logic level applications. It's useful to either have several probes with different tip resistors and a common 50R through terminator (assuming that the scope doesn't have an inbuilt 50R option), or easily changeable tip resisors - useful as it's common to solder the tip to the board. I built a pair with IC socket inserts at the tips to make swapping the resistor a plug-in process.QuoteThe only time you would probe (preferably solder) the board with 50R coax directly (with 50R scope termination), without a tip resistor, is if you have a 50R source impedance signal. This gives an effective x2 probe with the 50R scope termination. The source shouldn't see the cable capacitance load because, as you say, it is a transmission line.

author: Berni
author_rank: Super Contributor
timestamp: September 15, 2023, 12:20:30 pm
content: Quote from: Doctorandus_P on September 14, 2023, 03:56:43 pmBecause the OP asked for 1x and it is in the thread title.Do have a sampling scope with a 3.5mm connector for input. Did make a Z0 probe before and also used it on a spectrum analiser,works great.I like to use some HP 2.5Ghz active probes that i scored used for a decent price, so i dont have much use for these DIY Z0 probes. The low C loading is really nice when you can't get a nice tight ground path

author: EPAIII
author_rank: Super Contributor
timestamp: September 17, 2023, 06:48:56 am
content: The original question was about a 1X probe made with ordinary coaxial cable, perhaps RG-58 or RG-59. From what I know about Z0 probes, they are not 1X. More like 10X or 20X and made for scopes with 50 Ohm inputs to handle very high frequencies, not the 1 MOhm input of most scopes.But perhaps a plain piece of RG-58, 50 Ohm coax would be a 1X, Z0 probe.

author: Gyro
author_rank: Super Contributor
timestamp: September 17, 2023, 10:41:33 am
content: Quote from: EPAIII on September 17, 2023, 06:48:56 amIn technical terms, it wouldn't be a X1 Z0 probe. Z0 probes are always terminated at the characteristic impedance of the cable (hence the 50R) and act as transmission lines. You will never get a truly X1 Z0 probe as there will always be some circuit source impedance that will act as a divider with the low impedance scope termination.A 'non Z0' piece of coax, will of course work into the a 1M scope input for low frequency and audio (unless you happen to have an opamp output that objects to the specific capacitance of the cable for instance). That's basically what the bog standard coax clip lead cables and cheap scope probes, switched to X1 are.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 17, 2023, 02:50:49 pm
content: QuoteBut, any typical cheap probe is capable at least 100 MHz in 1:10 mode. SMPS signals are quite large in amplitude, so you are fine with 1:10 and you normally don't need 1:1. So, you really need only battery powered oscilloscope with frequency range of (idk) 20 MHz and thats it.It can be much easier to buy some small tablet battery powered oscilloscope than make good reliable 1:1 probe (which can be easy killed by high voltage).