Simple DIY 2GHz differential probe - layout ok? probe tip? - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 05, 2021, 03:26:47 pm
content: I have tried to make this board small to make a FR4 board usable and to keep the signal path short.But will it work, assuming the components are the right type and value?I had to make an 90 deg angle from shield to board and back again.EDIT: The idea is to make it function as a "in between" unit with a passive resistive 540 Ohm coax (10X) like the one in Dave's latest video about passive probes.

author: ledtester
author_rank: Super Contributor
timestamp: February 06, 2021, 09:39:14 am
content: QuoteDo you have the capability to test it?

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 06, 2021, 10:35:22 am
content: Thanks for replying! :-)No unfortunately not but the smd components was used to a similar project by someone else.I have tried to "compress" the board, add the connectors, routed the board with 50 Ohm traces, and I am hoping to use FR4 instead of something way more expensive.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: February 10, 2021, 09:11:18 am
content: I think 2GHz is a high frequency, but it is still valid for FR4.More confusing is the very close location of the resistors and the transition holes.As well as the lack of matching capacities.

author: KT88
author_rank: Frequent Contributor
timestamp: February 10, 2021, 10:17:33 am
content: It won't be a true diffential probe as the inputs are very different in impedance due to the geometry.I agree on S. Petrukhin's concerns about the resistor placement.Also the input capacitance will be too high for 2GHz and the input impedance would suffer from lack of compensation and frequency depenent coupling between both inputs.Coax cable with coax connectors are not really siutable for differential signals (except you use two of them) - the design will suffer from severe (frequency depenent-) mode-conversion. This means you won't be able to tell which signal component is differential and which is related to common-mode signals.Differential probes are usually very symmetrical to avoid any uneven loading of the inputs.Last but not least: For frequencies beyond a few hundred MHz an input capacitance below 1pF is desirable.CheersAndreas

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 18, 2021, 11:48:36 pm
content: Sorry hadn't got any message of any reply.I have tried to change this designhttps://archive.org/stream/ElektorMagazine/Elektornonlinear.ir2015-07_08#page/n103/mode/1upto make my version.The traces in the in and out is of equal length for each side. The BNC should be made for up to 10 GHz.

author: rfclown
author_rank: Frequent Contributor
timestamp: February 19, 2021, 05:16:18 am
content: Quote from: FriedMule on February 18, 2021, 11:48:36 pmBNC was designed for <= 2GHz.Ditch the BNC on the input, it isn't differential. Notice the design you're referencing doesn't have a single ended connector at the input. His output connection isn't good in that he has a long lead length on the coaxial pigtail connection.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: February 19, 2021, 08:00:16 am
content: Quote from: FriedMule on February 18, 2021, 11:48:36 pmThe differential input must be symmetrical, both of its conductors must be same type, same length, and run parallel. In BNC, this is not the case, it is a coaxial connector - it is a pipe that has a shell inside which a conductor. You can consider your probe isolated, but not differential. You can use it to measure signals relative to the Gnd of the test circuit, but you can't use it to measure a truly differential signal, for example, at some resistor, at the input of the OPAMP, etc., by connecting the shield to one of them. Do you understand the difference?

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 19, 2021, 01:28:48 pm
content: Thanks a lot!! Your both's explanation, combined with the picture, did make it a lot easier to understand — do I hope:-)I have made two new images where I have ignored the connectors and everything else, just to make it differential.If that is correctly made, then can I add the missing parts.Just to summon up my idea from the other comments I have made:My idea was to have a scope where I could either connect the standard probes or connect the differential probes.I am thinking of using some sort of resistive passive probe, maybe of a type like the one Dave showed in one of his last videos.In the "out" end do I then need a way to get the signal to BNC in on the scope.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: February 19, 2021, 03:56:03 pm
content: Do not forget to correctly calculate the gaps according to the input voltage and correctly select the voltage resistors.

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 19, 2021, 04:20:20 pm
content: I am planning on using the probe to low-ish voltage. Below 250V. So the gap can be tiny?The component values should be the same as in the schematic in the article. :-)

author: S. Petrukhin
author_rank: Super Contributor
timestamp: February 19, 2021, 04:48:42 pm
content: If you cover it with a protective varnish, it will be more reliable. The characteristics of the resistors on the circuit do not indicate their permissible voltage, however, you should check it in the datasheet.

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 19, 2021, 04:55:35 pm
content: All I have is this article, I try to follow it as long it's not deemed wrong by you or someone else, the voltage should just be a matter of component selection and max for the center chip.I have removed the silkscreen, is that a great idea?Sorry for asking about something you have said several times, but do you think it may work or do I have to make bigger spacing, smaller/bigger components?Anything else that have to be corrected before going on with connectors?

author: dietert1
author_rank: Super Contributor
timestamp: February 19, 2021, 05:40:12 pm
content: When i started making differential probes for our 6 GHz DSO i found that coupling from probe input to amplifier input was largely capacitive above 1 GHz. This is the reason for using multiple input resistors and they should not be folded but stretched.On the output side, you need to terminate to Gnd one of the outputs and feed the other one to the scope.At the time i bought a USB test pulse generator from Leo Bodnar to check response down to about 50 psec.Regards, Dieter

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 20, 2021, 07:18:57 pm
content: I think I do understand everything you write, and I am sure you are right, but when we are talking about 2GHz instead of 6GHz, could it not be that the bad quality board has more influence on the signal than folding the resistors? I am thinking that many do talk about 90 deg angles do not matter that much if the signal is below 2GHz.The original article I have taken this from, do maybe contain some errors, but the design was tested to about 2GHz. :-)

author: S. Petrukhin
author_rank: Super Contributor
timestamp: February 20, 2021, 07:29:08 pm
content: Everything in the world is relative.Any ordinary material, any arrangement of elements on the board will not completely block the signal, maybe not even distort it beyond recognition, but will affect the shape, amplitude etc. Parasitic links can create ringing, distortion of front, amplitude, and phase.If you only want to evaluate the signal, see only the fact of its passage and some general estimates, then you can be brave.Important measurements require good equipment that is properly designed and tested, and is good if calibrated.In any case, only their bumps hurt. Experiment-it's useful. But be attentive and careful not to get an explosion.

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 20, 2021, 08:17:58 pm
content: LOL:-)To be honest, do I know that I would never be able to make anything nowhere near a professional product that cost thousands of dollars. But that does in no way mean I am not trying to get it as good as possible.As stated before do I hope on making a differential probe that somewhat mach the one in the article in performance.I do listen to your advices more than I follow the article, but I do also think the author may have some reasons for his design choices. :-)

author: dietert1
author_rank: Super Contributor
timestamp: February 20, 2021, 09:18:12 pm
content: The two input resistors should be in a linear chain instead of folded. The reason isn't that "the signals don't want to go around the corner". The reason is that with two resistors you will have half the parasitic capacitance as coupling (except if you fold them like your design). If you use three resistors in a chain you will have 1/3 of the parasitic coupling.Let's try some numbers. If one of the input resistors has 5 KOhm and a parasitic capacitance of 0.1 pF then the time constant will be 0.5 nsec. That gives you a useful bandwidth of about 700 MHz. If you divide that resistor into three similar SMD resistors in a chain arrangement, you can make it work up to 2 GHz.And don't put a ground plane underneath those resistors...Regards, Dieter

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 21, 2021, 12:42:49 am
content: I have removed the ground plane from under the three resistors, yellow color plane.The two of the three resistors is straightened out, did not know how to handle the first resistor that is connected to the chip in both ends.I have added my drawing of the schematic in the magazine, I am wondering on why there is used a 50 Ohm and a 100 Ohm resistor on the output. Would a balun not be a better solution?EDIT: forgot to say that I am wondering on why plus and minus is switched around on the schematic?

author: dietert1
author_rank: Super Contributor
timestamp: February 21, 2021, 07:17:37 am
content: The parasitic capacitance of the feedback resistors is less critical since they are lower in resistance. In the end one would try to get a balanced capacitive divider.Don't forget the 50 Ohm impedance of the coaxial transmission line to the scope input. Then you have two 100 Ohm resistors on the amplifier outputs.There are solutions using kind of crossover with two transmission bands. Then a balun can be used for the RF band. I'd leave that for engineers with GHz RF simulation and design software.Even the "simple" linear amplifier will probably give you several board revisions until working well.Regards, Dieter

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 21, 2021, 08:31:25 am
content: Oh, yes the coaxial of 50 Ohm, and the shielding of about 0 Ohm)Please allow me this somewhat ignorant question:-)As the components are placed now (forget about missing connections and so on) would it not be possible to say if it will work? We know the type of board, components, placement and layers. :-)

author: dietert1
author_rank: Super Contributor
timestamp: February 21, 2021, 09:31:31 am
content: If you got the amplifier pins correct it will work. But you can't expect flat transmission right from the start. That may need tuning. For example you might have to try different ratios of R4/R6 and R5/R7. That's not a board revision.Also i would turn the capacitor pairs by 90° so the hot ends of all four capacitors are close to the amplifier. And the +/- 5 V supply lines should include small ferrites of 10 to 50 uH, so the amplifier doesn't radiate on the supply wires. I would also include diodes into the supply lines - for safety during initial experiments. Building the thing and then losing it due to confusion of + and - supplies is a certain risk with DIY.Regards, Dieter

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 21, 2021, 01:19:52 pm
content: I am glad for all that help!! :-)What I do not understand is that the author of the article has made that many "less than ideal" decisions?How did he get the great results if folded resistors, rotate caps and long lead is a bad solution?