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Wish to scope the secondary diode voltage on our offline Flyback SMPS
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David Hess:

--- Quote from: lordvader88 on September 21, 2018, 05:15:36 pm ---What is the reason for doing this? Whats wrong with the standard ground clip, is it that the wire acts like an antenna or something? Or the inductance of it or something ?
--- End quote ---

The standard ground clip has two problems:

1. The length adds inductance which causes ringing distorting the measurement.  This by itself may not be a problem up to 100MHz if care is used.

2. Magnetic flux within the area between the probe and probe ground lead will completely corrupt the signal.  A twisted pair or coaxial connection minimizes this problem by reducing the loop area.  If you want to test for this, short the ground lead to the probe tip and wave it around inside the probing area.

The 300MHz and 20MHz example shown below of some low frequency switching power supply waveforms at startup were measured using short RG-179 coaxial connections with coaxial to probe tip adapters and standard x10 probes.  Even with a bandwidth of 300MHz, only very low levels of switching noise is present and that might be only ground noise.  These measurements were impossible when using the ground leads on the probes.
ocset:
Thanks, we’re actually thinking of using a piece of coax, and putting a 9 Meg resistor at the end of it, in order to make a high bandwidth  scope probe.  Has anybody tried this? We could add a 15pF  cap in parallel to the 9Meg resistor too.
We wish to do this because we wish to have something thats solderable to the test point.
The "spring barrels" we find, are too fiddley and dont always give a good connection to the ground barrel.....(maybe its just us but thats how we are finding it)
capt bullshot:
Many people tried that, and there's an article with all the results:
http://w140.com/tekwiki/images/6/62/062-1146-00.pdf
Short form: What you propose is good for a 5MHz oscilloscope
dmills:
For this was 50 ohm mode invented, take a length of 50 ohm coax, stick a BNC on one end and connect the screen to your reference at the other end, a 4950 ohm resistor (or as close as you can find) in a small SMT package goes in series with the centre conductor right at the point you are probing, **STICK THE SCOPE IN 50 OHM MODE**, job done, one 100:1 probe with bandwidth for days. For a 10:1 equivalent use a 450 ohm resistor.

Build that right and you have more then enough bandwidth for any flyback secondary on the planet, and the parts are trivial enough that mounting them and a U.FL or such at likely points on prototypes is well worth the trouble at layout time, because it means you can just plug the scope in.

I do wish people would get with the fact that with modern devices EVERYTHING is pretty much RF, once you get this it all becomes much easier to deal with.

Regards, Dan.
fourtytwo42:

--- Quote from: dmills on October 21, 2018, 06:51:57 pm --- in a small SMT package goes in series with the centre conductor right at the point you are probing
--- End quote ---
Please be very careful with the breakdown voltage rating of said small resistor!
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