Could you please post pics of your test setup?
Sure, see attached pics. 50 Ohm output goes thru 50 Ohm divider. One side goes into the Ref channel termed 50 Ohms, the other to the Tee and into the Test channel termed 50 Ohms. The short is just a wire soldered to the shell. The .025 Ohm current sense R is indeed axial, so some lead length is involved, as you can see.
I plan on cutting the shell flush with the center conductor. It will shorten the path length and I can then solder smaller parts to it.
To test the effect of cable length, I inserted a 16" extension BNC to each cable in turn, and sweeping the .025 Ohm R (to 10MHz) to see if there was any change. Nothing. So one nice thing about the shunt-thru method is, it is relatively insensitive to cable length (to 10MHz, anyway) . Very conducive to use with probes.
I might be able to replicate, so we can compare results. What is representative spacing on your boards?
I'd say 5 cm would be typical.
I would say that the inductive rise is not about limit of the test setup, but more of a DUT limit.
You may be right. I'm not sure yet. (Trust but verify...) The inputs on the MS420 go through several relays and RC trims before reaching the FET buffer. I remember comparing Z measurements with it to the HP 3570A years ago and I got better results with the HP. The 3570A input goes right into the FET. Very clean signal path. I attributed the better results to the input stage being much simpler. But I could be wrong.
Back in the day, Anritsu sold an impedance probe for the MS420, called the MA413A. It had a xfmr in the probe body right behind the tip. Rated impedance range was 1Ohm to 1MegOhm. So it seems they too got messy results at lower impedances, and so they just spec'ed it above the mess
Axial-leaded and low inductance don't go well in one sentence. Off course, it's connected. If the DUT is big, it requires big spacing, it's inductive(regardless of test probe spacing).
Yes, that's essentially saying the same thing I said last post. The spacing (of probe, test points, DUT) basically sets the upper freq limit.
The thing is, if it's THT board, there are no mOhms at 10MHz anyway....
Hmmm... really? Even with a good ground plane and distributed capacitance? I guess it's fair to say, that is why I want to do this measurement - to see what you already know
The other response I would make is: and therein lies the opportunity.
If the DUT was small, but your setup forced use of unnecessary big spacing, then we could talk about exces inductance of the test setup.
Yes, that is not the case. I know to avoid that...
Depending on noise performance of the analyzer, simple opamp based low noise pre-amp might help with the dynamic range.
I was thinking that yesterday. There's a 20-40dB level drop to the T channel (depending on the impedance). A 10MHz 20dB preamp could be put together.
Have you tried the semi-rigid test setup with small SMD resistor? To see where is the limit is?
I don't have any yet. But I'll be parting out a Tek 7000-series mainframe in the next couple days, it has an analog delay line made from coiled rigid coax and then I'll have a large supply of it
Do you have SMA on the back end of your DIY probe? I'll probably stay with BNC. It's convenient.
Soldering coaxial pigtails to vacant capacitor pads is ok an works surprisingly well.
Makes sense. I'll be doing that sometimes, for sure.
Ingun page is ok in EU.
Ah, ok. They might have someone in their basement hosting it for the US
Thanks for your input on this!