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DC coupled 2.7 GHz Active Probe Project - Now Available!
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lasmux:

--- Quote from: joeqsmith on June 20, 2024, 11:43:25 am ---
--- Quote from: lasmux on June 19, 2024, 03:32:51 pm ---
--- Quote from: joeqsmith on June 19, 2024, 01:15:11 pm ---You make no mention of lowering input impedance to support low voltage signals where drive strength is less of a concern.  Is this no longer being considered? 
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Yeah, I tried this, it makes no difference at all to the noise pickup. Not surprising in retrospect ...

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As we have discussed, for digital you could potentially sacrifice dynamic range as well to improve the SNR.  I have included the spreadsheet from my review which shows how the probes I have on-hand compare.   Even my old Tektronix probe was only rated to +/-6V.    Again, I'm not suggesting this would be a replacement for what you are offering but an extension. 

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That would require reducing the input resistive and capacitive voltage divider ratio, which isn't as simple as it sounds. There are two issues. As the divider ratio reduces, the ground half of the capacitor divider would need to become significantly smaller than the existing parasitic capacitance of that circuit node, which is problematic. I could compensate for this with a larger capacitance across the input resistor, but then that would increase the tip capacitance/loading of the probe, which I don't want to do. Secondly, the output would in general have larger voltage swings, so the slew rate limitation of the probe would become more pronounced.


--- Quote ---Other snips from the review showing my setup as well as my with and without touching the probe.  There's a fair bit of difference between what you show.
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Yeah I see. My assumption here is that in my setup when the probe is clamped, I have more coupling between the probe and signal grounds than you do, due to my specific setup, so the worst case when clamped isn't as bad. The probe being handheld further improves this response and improves the grounds coupling. In your case, with the copper tape added to better couple the probe and signal grounds as per the photo above, the specific local setup shouldn't matter as much and you should just get a good response without it being handheld. If you have time (and some copper tape handy), it would be interesting to see if it makes a difference in your setup also.
joeqsmith:
In real world use, adding a strap would be difficult, or at least it is not something I have ever considered when making a measurement.  Still, I'll set it up again and see what effect the strap has. 

Agree that it's not so simple to develop such a probe with good performance.  There's a reason we don't find them for $20 from China.  Not yet anyway. 
JohnG:
That the copper strap helps suggests that there might be common-mode pickup that is contributing to the problem. Maybe it's worth trying to loop the cable through a toroid for a few turns and see if that helps.

Just a thought.

John
lasmux:

--- Quote from: joeqsmith on June 20, 2024, 02:06:59 pm ---In real world use, adding a strap would be difficult, or at least it is not something I have ever considered when making a measurement.  Still, I'll set it up again and see what effect the strap has.
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I think it's easier than soldering in a in a probe ground, which is not unusual when using active probes apparently. Hence all the solderable pins included with the probe. Personally I always use it handheld.
Thanks very much. Looking forward to seeing if it makes a difference for you :)


--- Quote from: JohnG on June 20, 2024, 03:27:25 pm ---That the copper strap helps suggests that there might be common-mode pickup that is contributing to the problem. Maybe it's worth trying to loop the cable through a toroid for a few turns and see if that helps.

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I've just tried this now with a ferrite I had on hand, and I don't think it made any difference. I'll have another look later on.

I think another option may be a better low-inductance ground pin, such as a ground blade pin, or perhaps a pin made with some braid soldered to it. This would be a lot more difficult to make cheaply however.

nctnico:
At some point you'll need to move to soldered-in probes or add signal pickup points in the design.
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