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Yet another scope recommendation

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tggzzz:

--- Quote from: BenjiBango on June 06, 2024, 08:49:12 am ---
--- Quote from: tggzzz on June 06, 2024, 08:17:34 am ---
Have a look at these pictures of "temporary high speed scope probe “sockets”" (scroll down to that phrase)
https://entertaininghacks.wordpress.com/2020/07/22/prototyping-circuits-easy-cheap-fast-reliable-techniques/

The only type of probe it makes sense to solder to a circuit is a homebrew Z0 resistive divider, but that requires 50ohm termination at the oscilloscope. (Well I suppose soldering a piece of coax is OK for low-level audio signals)

--- End quote ---

I think I didn't provide enough context with that. I use the "Probe-To-BNC" adapter to connect a thin coax cable (RG178 or somethig) which will then be soldered at the point of interest. Which might not be quite as good as the method in your link, but less bulky.

--- End quote ---

If you do that then there will be reflections causing the displayed voltage to be frequency dependent. The frequency of the nulls depends on the length of the piece of coax, shorter->higher frequency of first null. Use an online VSWR calculator to find those frequencies; better simulate it with a Spice or any of the online equivalents.

BenjiBango:

--- Quote from: tggzzz on June 06, 2024, 09:11:51 pm ---If you do that then there will be reflections causing the displayed voltage to be frequency dependent. The frequency of the nulls depends on the length of the piece of coax, shorter->higher frequency of first null. Use an online VSWR calculator to find those frequencies; better simulate it with a Spice or any of the online equivalents.

--- End quote ---

Didn't thought of this to be honest. I just compared it with the ground spring and lead, and the results was more of the range of the spring, but without needing a third arm to hold everything in place :)
But I will have a look into that, thanks.

tggzzz:

--- Quote from: BenjiBango on June 07, 2024, 07:14:40 am ---
--- Quote from: tggzzz on June 06, 2024, 09:11:51 pm ---If you do that then there will be reflections causing the displayed voltage to be frequency dependent. The frequency of the nulls depends on the length of the piece of coax, shorter->higher frequency of first null. Use an online VSWR calculator to find those frequencies; better simulate it with a Spice or any of the online equivalents.

--- End quote ---

Didn't thought of this to be honest. I just compared it with the ground spring and lead, and the results was more of the range of the spring, but without needing a third arm to hold everything in place :)
But I will have a look into that, thanks.

--- End quote ---

The other, obvious, point is that coax will load your circuit. For low frequencies that will appear to be a capacitance, for high frequencies it will be the characteristic impedance, 50ohms.

For a standard "high" impedance *10 probe, it is worth calculating the tip impedance (i.e. R//C) at relevant frequencies. Relevant for a digital signal is determined solely by risetime.

Then calculate the same for a "low" impedance *10 resistive divider probe. Note the quotation marks.

Have a look at the references in
https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

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