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DC coupled 2.7 GHz Active Probe Project - Now Available!
joeqsmith:
I agree, active probes certainly have their place. I would like to see your diff probe if you pull it off. When I investigated it, I could not locate parts to do the job. That has been a long time ago and I suspect based on your goals, you will be able to pull it off. That would be a handy probe to have on hand. Much of the high speed digital has been differential for some time.
Searching ebay, even junk 1GHz diff probes have an asking price of over $200. Not suggesting anyone would pay that. Looking for my D300, this one was about the cheapest I could find which at least appears complete and they offer returns.
https://www.ebay.com/itm/175614100115
Of course, that's about 4X the BW you are targeting but still, I think there is money to be made if you can keep the costs down.
When my wife and I were first married, I was needing a higher speed scope than we had at work. I rented a scope similar to that and had it shipped to our house so I could sort out how to run it before taking it to work. My wife was worried that something would happen. I think the cost was more than our house at the time. :-DD
hpw:
--- Quote from: lasmux on August 13, 2023, 08:06:03 pm ---On another topic, I was considering a differential version of the probe. I've been playing around in LTSPICE, and I could probably get over 1GHz of bandwidth, with the same 0.7pF tip capacitance, +/- 30V input dynamic range, but I suspect my CMRR wouldn't be great without a bit of work on matching the two inputs closely.
--- End quote ---
Yeah, as I currently use the AP034 diff probe (see picture) as nice using 0.1" pin headers for fixed or tips connections over the active HFP2500 or the crappy active Cal Test CT4121. Even the CT4121 as shielded front header still picks up RF waves.
Hp
tszaboo:
--- Quote from: lasmux on August 13, 2023, 08:06:03 pm ---I've seen that probe from Andrew Zonenberg before. He has some really nice other projects also. I think the main issue with that probe is the tip capacitance is actually quite high for a resistive probe, and the bandwidth stops at 2GHz. Not sure why it was necessary to use those Vishay FC0402 resistors. Under 2GHz I don't think a normal 0402 resistor would have been an issue, and they are miles cheaper.
--- End quote ---
I just looked at those FC0402 resistors and they are 5 USD in single pieces. :-DD
I really don't get it. 0402 SMD thin film inductor is going to set you back 2c, but if you want a resistor with the same technology, it's going to cost 300 times more.
joeqsmith:
S11 & S21 for the blade probe using the PNA. My homemade PCB waveguides are going to be useless. Starts out at 500 ohms as and drops off rather quickly reaching 100 ohms at 6 GHz. Mag isn't very stable (when holding all the parts in my hands, moving them around...). Then there is de-embedding the interconnect. Consider it all just a gross measurement to give us some idea how it behaves.
JohnG:
--- Quote from: lasmux on August 13, 2023, 08:06:03 pm ---
My original application was my actively quenched APD as a photon counting module, where I wanted to measure across a 1k ballast resistor restricting current through an APD. A passive probe had way too much capacitance and would have slowed down the response of the signal, a resistive probe would have made the ballast resistor ineffective and probably damaged the APD. An active probe was the only way that I could think of.
--- End quote ---
There is another way that might work. You make the 1k ballast part of your probe by putting a 953 ohm resistor in series with 50 ohm microstrip or similar transmission line right on the same PCB. Just don't forget to terminate the other end of the line if it is not connected to your scope. You get your intended loading and measurement at the same time. Note that this does not work as well if the ballast is connected to the positive bus, but sometimes it is possible to do something clever and make it work anyway, especially if you can put in a series dc blocking cap (if you don't need a dc response).
John
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