Products > Test Equipment
DC coupled 2.7 GHz Active Probe Project - Now Available!
lasmux:
--- Quote from: mawyatt on July 24, 2023, 02:05:48 pm ---@ lasmux,
Nice project, well done :-+
We didn't know about the MJF printing, that case looks very nice indeed!!
You certainly picked up on the Fusion 360, and produced a nice rendering as well!!
A question, why the selection of 26dB rather than 20dB attenuation?? Guessing limits of the +-2.5V supply rails?
--- End quote ---
Thanks for the encouraging words :)
Yes, MJF 3D printing is amazing. Combined with vibro-polishing I feel it sets a new standard for low-volume projects.
I could have made it x10 into the 50 ohm, but I would have had to double the gain of the op amp which would have reduced the bandwidth somewhat. Also it would have reduced the measurable voltage range to +/-7V due to the limited rails, as you alluded to.
lasmux:
To add. I also couldn't reduce the input voltage divider as it would have also meant reducing C1 by half (see schematic above) to maintain the capacitor divider ratio. C1 is a combination of an 0402 capacitor, and parasitic capacitance to ground. The 0402 capacitor part of C1 would have likely needed to be a smaller value than the smallest 0402 capacitor available.
lasmux:
Yes, theoretically, it's a 10:1 probe if you put it into a 1Mohm oscilloscope, but then with a 1 metre coax cable, you're going to get reflection artefacts causing issues over 20MHz or so. Typically for 50 ohm terminated active probes, they say the attenuation as measured across a 50 ohm terminated scope.
JohnG:
This looks like a really nice and affordable probe.
I have a question regarding large signal performance. The datasheet alludes to something that sounds like slew rate limiting that may affect the large-signal bandwidth. This is important to me because my primary application would be a scope probe. Have you done anything to look as this, or can you find someone with a high-bandwidth scope and source to look at it? I'm happy to volunteer, but I bet there are folks who would do the same and are much closer to you.
Thanks,
John
lasmux:
--- Quote from: JohnG on July 25, 2023, 02:16:29 pm ---This looks like a really nice and affordable probe.
I have a question regarding large signal performance. The datasheet alludes to something that sounds like slew rate limiting that may affect the large-signal bandwidth. This is important to me because my primary application would be a scope probe. Have you done anything to look as this, or can you find someone with a high-bandwidth scope and source to look at it? I'm happy to volunteer, but I bet there are folks who would do the same and are much closer to you.
--- End quote ---
Thanks!
The slew rate of the output is limited to 2V/ns, so for example a 10V rising edge input would be divided by 10x due to the probe attenuation to 1V (before 50 ohm termination), so due to the slew rate limitation would have an additional 500ps rise time (I think) (edit. around a 600ps rise, see post below). You'd have to have quite the unusual signal to be generating this kind of signal though. In my spice simulations this kind of input starts to make step responses/square waves look a little trapezoid. I don't have a high amplitidue pulse generator (or fast enough oscilloscope) to test this properly. In general, I don't know what the rise time is for more sensible signals as my oscilloscope just isn't fast enough (500MHz).
Note though that other active probes such as the Keysight N2796A 2GHz probe also limit their dynamic range at higher frequencies. This is a screenshot from their datasheet:
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