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DC coupled 2.7 GHz Active Probe Project - Now Available!

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KungFuJosh:

--- Quote from: lasmux on July 27, 2023, 12:20:34 am ---I really hated setting up that website lol. The woocommerce plugin is just rammed full of features that make figuring out how to do something simple, very difficult. I really just need to watch a few tutorials on youtube and I'm sure it'll all be easy.

--- End quote ---

If you're only selling 2 products, you can skip woocommerce and just use PayPal buttons. If you plan on expanding later, that's a different story.

lasmux:
I had another look around, there's a little bit of testing of Zo probes done by... you!
https://www.eevblog.com/forum/testgear/fifty-ohm-probes/msg607659/#msg607659
And some others:
https://www.eevblog.com/forum/projects/lo-z-probe/msg801477/#msg801477
There's also this one:
http://jahonen.kapsi.fi/Electronics/DIY%201k%20probe/

Everyone seems to get very different results. Not quite sure what's going on. I think my active probes are probably better performing up to their stated bandwidth, but lower bandwidth potential than a well built Zo probe. Although getting a linear response across the bandwidth seems quite tricky on a Zo probe.


--- Quote from: KungFuJosh on July 27, 2023, 01:44:41 am ---
--- Quote from: lasmux on July 27, 2023, 12:20:34 am ---I really hated setting up that website lol. The woocommerce plugin is just rammed full of features that make figuring out how to do something simple, very difficult. I really just need to watch a few tutorials on youtube and I'm sure it'll all be easy.

--- End quote ---

If you're only selling 2 products, you can skip woocommerce and just use PayPal buttons. If you plan on expanding later, that's a different story.

--- End quote ---
I might go with this. Thanks for the suggestion.

joeqsmith:

--- Quote from: lasmux on July 27, 2023, 06:48:17 am ---I had another look around, there's a little bit of testing of Zo probes done by... you!
..
Everyone seems to get very different results. Not quite sure what's going on. I think my active probes are probably better performing up to their stated bandwidth, but lower bandwidth potential than a well built Zo probe. Although getting a linear response across the bandwidth seems quite tricky on a Zo probe.
--- End quote ---

Indeed, I have tried various configurations when making resistive probes.  Some better than others.   You can image that component selection and construction is going to have a major effect as you move beyond a GHz. 

Another from 2017 where I was playing with a cheap demo board.  The resistive elements were placed onto a board. 

https://www.eevblog.com/forum/microcontrollers/typical-speed-of-fpgas/msg1279898/#msg1279898

Even my LeCroy PP061 probes I mentioned are not good for these higher frequencies.  The diff probes I have are good for about 4GHz.  I also have the PP066 which are a 7.5GHz resistive probe.     

https://cdn.teledynelecroy.com/files/manuals/pp066-rp4030-user-manual.pdf

joeqsmith:
It would be nice if probes did not load down the circuit you are wanting to test.  I started out building one of Bob Pease's active FET probes.  From memory, maybe 100MHz BW.   It saved me a few times as a young engineer when the 10X probes we had were presenting too much load for my circuits.   For home, I eventually bought a Tektronix P6202 active probe.  This served me well until I started playing with faster circuits.   

Capture1 lists the higher BW probes I have for my hobby use along with the basic specs.  I included yours as a reference. 

I have a LeCroy coplanar waveguide test board and used this as the thru for the LiteVNA.   I then attached both a LeCroy PP061 and PP005 probe to a scope and used them to probe the board.   I was never able to locate a manual or data for the PP061.

Capture2 is looking at S21.  The old 10x probe presents less of a load than the resistive probe until the cross over point at 70MHz. 

Capture3  is looking at S11.  The impedance the VNA sees with the probe attached.  This is not the probe, but the probe in parallel with port 2.   Typically I would have some driver and load and want to probe between them. 

I think to run this sort of test I would want to use my old PNA.  Some of my probes are limited to about -4dBm.  And I would want to use the unknown thru model for that waveguide.   

***
Added photo of my old Tektronix active probe along with with the probes for my old WaveMaster.   

lasmux:
You have a lot of probes!

Thanks for this data. Interesting. I guess with a lot of the active probes, their manufacturer specific interface makes it very difficult to actually measure their response. Obviously you're able to measure the impedance. Regarding capture 2... it looks like it's true that passive probes don't help signal integrity  ::) The same is also true of certain Ukrainian active probes that can be found on Ebay these days. I bought one because it was cheap, and the signal loading was not pretty. Response linearity was apalling too. Not bad for $25, but not that useful either.

I personally have used three methods to measure the impedance.
1. I use a SMA T piece on the VNA port 1, with a 50 ohm termination. I then place the probe in the other side of the T piece. This is better than using the port 2 as termination as I think there will be worse reflection artefacts by not measuring near the termination point.
2. I use a custom built 50 ohm load which has had it's resistors exposed, and measure directly across those.
3. A custom 'open' SMA termination which I can measure directly across.

1 and 2 tend to give fairly close results. 3 tends to give lower impedance values at lower frequencies for some reason. I'm not sure if this is because it's trying to measure impedances much much higher than 50 ohms. Maybe it gets less accurate. I'm not sure.

For bandwidth measurements, I use the same setup method 1/2, but then have the probe output measured by the VNA port 2.

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