Electronics > Projects, Designs, and Technical Stuff
1GHz active probe project feedback
lasmux:
Hey,
I've built a single-ended active oscilloscope probe to analyse one of my other projects (an active quenched single-photon APD which will get its own post at some point), and it's turned out pretty well. I'm considering setting up shop and flogging it on Ebay. It would be really cool to get some feedback and comments! Hopefully I'm not totally barking up the wrong tree.
Specification
Bandwidth (-3dB): DC - 1.15GHz
Probe capacitance: <1pF
Input impedance: 1MOhm
Output impedance: 50Ohm
Attenuation: 20:1 into 50Ohm, 10:1 into 1MOhm
Rise time: 500ps measured on my 500MHz oscilloscope. It's probably faster than this.
Measurable voltage range: +/-15V
Absolute maximum input voltage: +/- 50V
(simplified schematic for now)
This device is based on an OPA858 high speed FET input OP-AMP. I have it powered with a 9V battery, and have also a low voltage indicator LED. The probes are swappable gold plated pins, so you can have either sprung-loaded pins, rigid pins, flexible wires, or even a soldered connection to the DUT.
(data points >100kHz measured on a nanoVNA, low frequency data points added manually by checking amplitude response on an oscilloscope)
100MHz square wave.
(captured on my Agilent 54111D 500MHz oscilloscope, square wave generated by nanoVNA)
Here the white trace is the signal directly measured by the oscilloscope, and the red trace is the probe measuring the same signal
I'm going to look into a nice enclosure next. I'm working to keep the cost down of the unit, but it'll probably end up costing around £140 as I don't think I'll get much volume of sales which just makes everything more expensive. And I have to calibrate every device myself. So the probe is quite expensive, but still vastly cheaper than any other active probes out there. And actually much cheaper than any GHz passive probes also.
Some specific questions:
Would it be preferable to sacrifice a bit of bandwidth to reduce the peaking at 800MHz? It's only 1dB but maybe that's too much? Although it could be fakery caused by the nanoVNA too!!
Would my test results on my nanoVNA be accurate enough for someone to buy this? The device is calibrated but it's obviously not a professional piece of equipment.
Thanks
macaba:
Active probes are an interest of mine, on my todo list to try, so I wanted to congratulate you on a project well done. :-+
I follow azonenberg's work in this area, he has posted his adventures on trying to get a well behaved response, so it might be worth taking a look at his twitter/mastodon account for ideas.
jwet:
I agree with the last poster- nice work. Not to detract and partially out of jealousy- I offer my 2 cents.
I worked on something similar with an OPA656, basically a slower cousin of the 858. I was never able to get very far past 150 Mhz, which was actually fine for my needs but I expected to get 500 Mhz. I don't know how you arrived at your input impedance numbers- this is easy in simulation but much harder in the real world. Something to try is to look at S11 looking into the input with your VNA. At 1 Ghz, 1 pf looks like 160 ohms but its very difficult to get anything like this in the real world. In my design, the input got squirelly on the smith chart over a couple hundred Mhz- things like loops and kinks indicating resonances. Hi Z RF probes like the HP85024 got fractions of a pF input C which is kind of what you need- this generally takes hybrid type design on exotic substrates which is why they cost. What you're competing with don't forget is just a 950 ohm resistor on the end of a coax- it has 20 db of attentuation too but probably is a higher impedanace overall in the end. See what your probe S11 looks like. Hopefully I'm wrong. The S21 plot you show looks excellent!
lasmux:
--- Quote from: macaba on December 02, 2022, 06:14:21 pm ---Active probes are an interest of mine, on my todo list to try, so I wanted to congratulate you on a project well done. :-+
I follow azonenberg's work in this area, he has posted his adventures on trying to get a well behaved response, so it might be worth taking a look at his twitter/mastodon account for ideas.
--- End quote ---
Thanks for the encouragement! I'll check out Azonenberg, from a cursery google, he looks like he knows his stuff!
--- Quote from: jwet on December 02, 2022, 06:42:04 pm ---I worked on something similar with an OPA656, basically a slower cousin of the 858. I was never able to get very far past 150 Mhz, which was actually fine for my needs but I expected to get 500 Mhz. I don't know how you arrived at your input impedance numbers- this is easy in simulation but much harder in the real world. Something to try is to look at S11 looking into the input with your VNA. At 1 Ghz, 1 pf looks like 160 ohms but its very difficult to get anything like this in the real world.
--- End quote ---
Ah, yes I looked at the OPA656 a while back. A very nice amp.
--- Quote ---I agree with the last poster- nice work.
--- End quote ---
Thanks ^-^
I just ran a couple of S11 measurements and I get around 110 Ohms at 1GHz, so not quite the 160, but not super far off. I'll see if I can improve that with some tweaks. I noticed that the N2795A active probe (1pF also) is also around 150 Ohms at 1GHz according to its datasheet, so I think that's a good target to shoot for.
And the smith chart
To be honest I don't have a lot of experience with interpreting smith charts, maybe should do some more reading... I think I see a 1.8pF load at 1GHz (it's around 1pF at 500MHz), but I don't quite understand the implications of the shape.
Mechatrommer:
i'm also working myself on such a thing, just at very slow pace, many other projects to take care of... its a pity OPA858 only available from digikey, the shipping cost is suck blood.
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