<agg>
azonenberg: I enjoyed your probe video, thanks for posting it! do you think a 50r-tip resistive probe might be a reasonable low cost option for probing a 1v1 supply? I don't mind the 11mA extra load, and seems it would give usefully less attenuation
<agg>
Obviously no offset control so can't get to super low ranges on the scope which might limit the utility
<agg>
But I was imagining just soldering a 50r smd resistor and coax to a spare decoupling cap spot
<azonenberg>
Yes, that is a possibility. There's a lot of tradeoffs between nothing and a really nice probe that may be good enough for a specific application
<azonenberg>
The other option is to try a 0.1 uF cap
<azonenberg>
which will AC couple and lose really low frequency transients as well as DC supply droop
<azonenberg>
but give you great high frequency performance
<azonenberg>
The dedicated power rail probe is essentially this plus a separate low bandwidth DC path
<azonenberg>
so if you don't actually *need* the DC path it might be good enough
<azonenberg>
Long term I plan to build an open hardware power rail probe
<azonenberg>
but i have my hands full with other kinds of probe in the meantime
<agg>
I'm trying to debug what causes the whole rail to collapse to 0 for a few ms if I turn on enough ffs in the fpga
<agg>
So having the DC content is probably going to be helpful
<agg>
But yea, I'll probably try adding a 0.1u and see how it compares
<agg>
Would you do just a 0.1u into the coax, no resistor on the pcb side?
<agg>
(so that divider is just c on top and r in the scope?)
<azonenberg>
Correct
<azonenberg>
So this eliminates all DC loading on the PSU and acts like a 50 ohm 1x probe
<agg>
Nice, thanks!
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<azonenberg>
also I'm still trying to find good leads for the AKL-PT5. I'm tentatively using SMPM connectors because they're push-on, small, and good to double digit GHz which is the BW range I'm targeting
<azonenberg>
but most SMPM cables are $$$$ and a lot of them are semi-rigid or just really stiff
<azonenberg>
I found one from sv microwave that looks nice and is nice and skinny (0.047"), less than half the diameter of the T-Flex 405 my current test cable uses
<azonenberg>
But i'm concerned about losses (it's a foot long, not sure if available shorter - i guess i can always measure and de-embed if it's only a few dB)
<azonenberg>
And it's also very much not cheap
<azonenberg>
i lose a lot of the benefit of a low cost probe head if it has to be used with a $133 cable
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<azonenberg>
So I'm actually contemplating using a probe head with a small amount of high frequency peaking to help compensate for cable loss
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<agg>
well, the resistive probe soldered to the 1v1 rail is incredible in terms of seeing the power rail
<agg>
but also the extra 100R load seems to prevent the rail dying
<agg>
so I guess problem solved, just leave the probe and scope attached :P
<azonenberg>
lol
<agg>
for example you can really see the smps going from light-load 100kHZ to actually-working 1.5MHz https://imgur.com/a/1SskBRU
<azonenberg>
This is why the dedicated power rail probes are 50K ohms. Much less invasive to the control loop
<agg>
at 1v1 the scope's internal offset is enough to see the power rail even at 2mV/V scale
<azonenberg>
And yeah the huge offset range of the powrer rail probes is more important for looking at things like 3.3V
<azonenberg>
or especially 5/12/24V intermediate rails
<agg>
the only thing on this 1v1 rail is the fpga, which goes from very low static current to currently about 600mA atm quite quickly
<azonenberg>
Have you tried the capacitive probe option yet?
<agg>
so I wouldn't have thought the extra 11mA makes _that_ much difference, but I wonder if having 11mA more load when it transitions from power-save to full regulation makes a difference
<agg>
not yet, got a bunch more playing around with this setup before I wanna resolder it
<azonenberg>
(also if you get really stuck and have a reproducible test setup you can send me a board and I'll see what the RP4030 makes of it)
<azonenberg>
Or if you just want to see how close your cheap setup got to the expensive option
<azonenberg>
In future versions of my probing talk I want to include test results of a bunch of low cost open hardware / DIY options compared to $$$$ commercial probes
<agg>
thanks for the offer! atm I'm hoping this won't be a huge issue, not least because it only happens when I put every single flipflop in a chain and run at 400MHz (on this ECP5)
<azonenberg>
My expectation is that they won't be quite as good but will be a tiny fraction of the cost
<azonenberg>
so much better value per $$ for 80-90% of applications
<agg>
yea, exactly
<agg>
certainly this setup is performing miles better than the rc probe, either handheld or the sensepeek ones
<agg>
i.e. I can actually resolve the 20mVpp ripple in power-save mode and the 3mV or so in regulation, and see the transition and the frequencies...
<agg>
it was all just a hash previously
<azonenberg>
Yep :D
<azonenberg>
and yeah the sensepeek probes are not half bad if you solder up your own low-L ground, they're on par with most other R-C divider probes at that point
<azonenberg>
but the stock long ground lead is awful as you can probably imagine
<agg>
yea, I have a little wire soldered on, but it's a bit fiddly
<azonenberg>
I really want to make both resistive and active differential probes for the platform
<agg>
which is a shame as the whole setup otherwise is great at removing fiddliness
<azonenberg>
I have a ~3 GHz resistive probe prototype that has been on ice for a couple of years now
<azonenberg>
sensepeek sent me some dev material then ghosted me
<azonenberg>
i need to reach out to them again but don't want to until I get time to actually spend on it
<agg>
yea, it would be very neat for sure
<agg>
resistive probe in that form factor seems like it wouldn't need to be mad expensive either
<azonenberg>
oh it wouldn't
<agg>
how would you handle grounding?
<azonenberg>
let me dig up pics of my prototype
<azonenberg>
That is one of the unsolved problems. Easy mode is to use the same long ground lead as before
<azonenberg>
but thanks to the lower capacitance get much better performance
<azonenberg>
i tested a rigid copper wire ground lead which worked great but was super fiddly
<azonenberg>
i have some ideas around pogos etc
<agg>
when you say long ground lead, I don't recall what mine even came with but I usually use a very short f-f jumper to a sp10 probe
<agg>
obviously still something like 4" of ground
<agg>
but perhaps a bit better than the croc clip on a handheld rc