Active Probes: Part #2
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- Опубліковано 10 лют 2025
- I finish building and testing three different active RF probes: a classic Bob Pease design, an Op Amp probe based on the OPA814, and finally a dual-gate FET probe using a BF998.
Circuit design for the Bob Pease probe created by Robert Pease
Circuit design for the OPA814 created by Steve Hageman
Circuit design for the BF998 probe created by David Jewsbury
No endorsement is made nor implied to any commercial products referenced in this episode.
Very interesting video. Personally, I'd enjoy seeing more in-depth videos such as this two part series on your probe experiments.
I have a couple of HP 1 GHz amplitude & phase measuring instruments I bought from an auction. Of course the active (converter) probe tips were broken. I went through all kinds of troubles with getting or making the luckily replaceable tips. The threads were quite odd, I never could determine if they were # 000, # 00, or some metric size. I did not even try plain copper, nor brass. I wanted something reasonably strong and chose tellurium copper. It worked, but was a bit of a challenge on my tiny lathe and its standard tooling. I made the sharp end and the flange O.K. But to make the threads, I bought several different dies, only finding that the smallest sizes are not usually cutting type, they are forming type, intended just for the soft brass in time pieces. No matter, somehow I got a few actually fitting spare tips made. I would like to find carbide tooling, but have not seen anywhere for the small size used in my tiny lathe.
Great video presentation and nice idea using the 1/2" brass for the shell. The 3D printed end caps are very clever. Also, your side by side comparison was very helpful in understanding the parameters of the probes.
Since you utilized Ebay to source the BF998's for Jewsbury's FET probe, would you be able to share what price and quantity you ordered? I suspect that you would rather not provide vendor information but there seems to be lots of vendors from China that are offering the BF998's for around 50 cents apiece. Is that about what you paid? Since you have already acted as a 'guinea pig' for these components, I would feel more confident about buying a 10 pack or 20 pack if I knew a bit more about your purchase experience.
I hope you create a follow up video regarding the grounding tip spring for the probes along with it being used for actual troubleshooting. Thank you for the wonderful videos you create and the knowledge you're willing to share.
Thanks for the feedback! I bought 10pcs for just under $10.
have you thought of using Old Gramophone needles for the probe? I find them amazing.
Those needles are steel, so you get the inductance and losses by already rather moderate frequency. And worse on MHz ranges. If you want more strength, your best option might be something like tellurium copper, or maybe the beryllium copper (which you might want to avoid due to supposed toxicity, though).
i played with the Hageman design a long time ago when the amazing OPA656 amp was still a thing. What I realized in kind of a dope slap moment is that Steve's real goal was to make 50 ohm SA input look like an oscilloscope input. It has that front end stuff so you could put a 10:1 standard scope probe on it and compensate it! I'd love to see you try that with your Siglent passive probe. I went in a different direction- I modified the design and used a 10 meg input R and no Cin and got the native FET input of the 656. It wasn't Agilent but it was pretty good, about a 1 pF IIRC. I never really cleaned up my design- I like your 1/2" brass and 3d printed ends. HP made some little adapter boxes for some of their gear as an accessory that did this- I have a 15709 that was an accessory for a QAM analyzer. Scope probe in, 50 ohm out. Discrete FET design similar to Pease circuit. Service manual/schematics can be found on line. I may tackle this again, I still have some old OPA656's and some newer grandchildren from Maxim and ADI. Nice job and good presentation- not too basic and not too detailed, kind of told me what I was curious about. Good luck.
Thanks for the detailed feedback!
@@levelupeelab - have you fed the Hageman circuit with your Rigol 10/1 probe?
Nice work as usual...BTW, if you ever need a replacement indicator roller for the HG-10 apparently the "new" Heathkit store has replacements. 73 - Dino KLØS
Thanks for the tip!
On eBay and components: In the case of the BF998 MOSFETs, these were made in large numbers for the cable and satellite industry, so you are pretty safe buying these.
The way to look at it for counterfeit/culls, is first how difficult is the device to make/fake. In the case of the BF998, they are a challenging manufacture just given what they are. Also, the culls would have been long ago destroyed, however anything is possible but unlikely.
The next consideration, is there a comparable component that can be relabeled as a substitute; again the BF998 is pretty safe.
Lastly, is this a component that is both scarce, does not have a good sub, and so is a make or break for a repair, and is expensive when found; this is the worst situation for dealing with counterfeits but usually not culls, due to age/missed-opportunity.
My suspicion is that you have a mismatch coupling to the BF998 because that is such a horrendous amount of dB variance.
I would try to set up a fixture to investigate the BF998 and see if you could learn anything from that.
I've used these before but never in an instrumentation setting.
Anyway, hats-off for the work and video--very enjoyable!
73...
Thanks for the feedback and tips! Good call on the mismatch coupling, I would agree that it's most likely something that I did in the geometry and layout.
Great learning for thanks for your kindness in sharing 73s wb7qxu