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Products => Test Equipment => Topic started by: jebiga on February 12, 2023, 07:31:02 am

Title: Hands free, low inductance, low circuit loading scope measurement techniques
Post by: jebiga on February 12, 2023, 07:31:02 am

I have been trying to find an effective probing solution for a project that I am working on. The project has a mix of analog and digital signals. While I have made do with the usual probing techniques (holding a probe to a test pad and using an alligator clip somewhere on the PCB for ground, or using a clip-on spring to ground the probe), I would really like to find a better solution for probing, and so far this search has been fruitless. In a previous thread I asked for advice about using small surface mount RF coax connectors, but this has some obvious issues that I have realized with the help of some kind forum members.

Given this, I would like to ask if there are any existing probing solutions that meet the following criteria. I realize that I'm unlikely to find a solution that meets all of them, but I am something of a noob so hearing people's suggestions and being able to go do my own research is helpful.

• 100MHz of bandwidth, at least.
• Low inductance. Ground clips are convenient but the >75mm ground wire has a noticable effect on signal quality when viewed on the scope.
• Suitable for probing 3.3V digital buses (I2C, SPI, UART), 3.3V PWM signals (motor step drive lines), current-sense resistor voltages, maybe some light analog work like photodiode signal conditioning circuits for characterization.
• Hands-free. Sometimes it's necessary to make long measurements of a signal and holding the probe in place is not adequate. PCBite and similar tools are nice, but not foolproof because they can fall if moved. A surface-mount connector would be ideal to meet this criteria.
• Relatively small size. The SWJ connectors that Murata produces[1] are 1.4x1.2mm, which is compact enough to sprinkle around a board. It is also nice to be able to probe more than one signal at a time, a small connector is important.
• Low circuit loading. A Z0 probe seems to be a good solution for digital signals, but if I want to make analog measurements it would be nice to minimize the effect my probing solution has on the circuit.

I find it somewhat confusing that there are no existing tools for this job. To me, the problem seems simple: just extend the signal-carrying conductor of a 10X probe to a surface-mount connector, and add a direct ground connection with an ultra short path to minimize inductance. Why doesn't a small version of such a connector exist? I know that Keysight and Tek have some solderable probe adapters[2], but the price is ludicrous and they're not compact at all. There must be something I am missing and I would appreciate if someone could clue me in or suggest a solution.

Thank you!

[1]: https://www.digikey.com/en/products/detail/murata-electronics/MM8830-2600RJ4/10706299 (https://www.digikey.com/en/products/detail/murata-electronics/MM8830-2600RJ4/10706299)
[2]: https://www.keysight.com/us/en/product/N4864A/vertical-probe-tip-to-pcb-adapters.html (https://www.keysight.com/us/en/product/N4864A/vertical-probe-tip-to-pcb-adapters.html)

Title: Re: Hands free, low inductance, low circuit loading scope measurement techniques
Post by: tautech on February 12, 2023, 08:01:10 am

Simplest solution is to make provision for SMD test points when doing the design however if it’s not too cramped you can drop them onto traces if they’re not too narrow later. Most convenient size are in 0805.
I’m a fan of these in prototypes and they’re darn easy to recycle numerous times.
https://www.keyelco.com/category.cfm/Test-Points-Surface-Mount/Micro-Miniature/p/523/id/741 (https://www.keyelco.com/category.cfm/Test-Points-Surface-Mount/Micro-Miniature/p/523/id/741)

Title: Re: Hands free, low inductance, low circuit loading scope measurement techniques
Post by: jebiga on February 12, 2023, 08:19:59 am

I have seen those Keystone test points used, and while they are useful, I don't believe they're perfect. In particular, they can be quite large, and there is no easy way to affix a probe to them unless you use one of those little grabber clips. If you do that, you need to figure out how to ground the probe. Perhaps another grabber clip to a second test point that's grounded? Seems suboptimal...

A connector that has both ground and signal built in, along with an adapter to BNC would be ideal, but it doesn't seem like anything of the sort exists. Basically, a non-RF version of this would be the Platonic ideal:

Title: Re: Hands free, low inductance, low circuit loading scope measurement techniques
Post by: tautech on February 12, 2023, 08:38:44 am

In 0805 size scope probes hook nicely onto Keystone test points.
Pretty easy to use and you can drop a 0805 footprint in most anywhere it’s not high density but CAD programs have a little fit and throw violations having both pads on the same net until you specify they can be.

There are circular post and ring types of test points however they can also make fine antennae.  :horse:

But you can go smaller with logic grabbers but then you need weigh up convenience as the flying leads when using them add more complexity.
This sort of setup that clips onto scope probes:
http://www.pintek.com.tw/customer/pintek/product/CP-3501R-PRO-b.jpg (http://www.pintek.com.tw/customer/pintek/product/CP-3501R-PRO-b.jpg)

Keystone are just easy.

Title: Re: Hands free, low inductance, low circuit loading scope measurement techniques
Post by: tautech on February 12, 2023, 09:05:16 am

SMB will be most convenient but when you look at the big picture it becomes obvious why Keystone are market leaders.
Otherwise build a bed of nails setup.