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DC coupled 2.7 GHz Active Probe Project - Now Available!

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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 ---
You made me do this.  ;D
But yeah, you piqued my interest, I designed this small board to be able to measure passives with the VNA. I was always disappointed by the inductors in RF matching, and wanted a way to measure them, this gave me the extra push to make this.
My issue now, how do I calibrate the VNA to 50 Ohm on a resistor, and then measure the DUT which is a resistor.

joeqsmith:
Why have components mounted on the thru, open and short?  For the load, why wouldn't you use a standard?  Are you concerned the length of the load will made a big difference?

tszaboo:

--- Quote from: joeqsmith on August 17, 2023, 01:17:04 pm ---Why have components mounted on the thru, open and short?  For the load, why wouldn't you use a standard?  Are you concerned the length of the load will made a big difference?

--- End quote ---
I don't intend to mount those components, but I placed the ~3mm long transmission line and the component footprint anyway.
I guess I could use an FC0402 50Ohm as the load. Normally the length shouldn't make a difference, but from what I've seen, the closer you can make the calibration look like the real world condition, the better.
Plus, I only have a female cal kit for for SMA (at work).
But you know what, it's an interesting question as well. I'll calibrate it without the components on it and mount some 0402s and see if it makes a difference.

nctnico:

--- Quote from: tszaboo on August 17, 2023, 04:46:52 pm ---
--- Quote from: joeqsmith on August 17, 2023, 01:17:04 pm ---Why have components mounted on the thru, open and short?  For the load, why wouldn't you use a standard?  Are you concerned the length of the load will made a big difference?

--- End quote ---
I don't intend to mount those components, but I placed the ~3mm long transmission line and the component footprint anyway.
I guess I could use an FC0402 50Ohm as the load. Normally the length shouldn't make a difference, but from what I've seen, the closer you can make the calibration look like the real world condition, the better.
Plus, I only have a female cal kit for for SMA (at work).
But you know what, it's an interesting question as well. I'll calibrate it without the components on it and mount some 0402s and see if it makes a difference.

--- End quote ---
With an OSL you will be calibrating at the connector. So whatever is different (like the trace not having a perfect 50 Ohm impedance and FR4 loss factor) for the real measurement will show up. Then again, I'm missing the DUT position for a reflection based measurement so how useful are the OSL parts on your board anyway?

But it does make sense to calibrate the board from an OSL calibration in order to determine whether the error is as expected c.q. up to what frequency your board is useable. But the through measurement will be the most useful anyway because the error is low over a much wider frequency range.

joeqsmith:
Board would be taken out and reference plane would be at the end of the transmission line.   I assumed they were making multiple boards and populated the test parts on the second one.   :-//

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