Small fixture - biggest thing going in favor is really low mounting inductance. You are usually able to solder SMD pat to it in a way that you do not add any(or very little) inductance. Short semi-rigid coax also helps with the braid error, but the biggest killer of braid error is the CM toroid.
I am fortunate (so far) to not have to worry about braid error...
If I was going to measure 0603 and smaller capacitors, I'd definitely go for soldering dut to the small fixture. I would say that key is the ratio of your fixture inductance to your DUT inductance.
That makes sense.
If your DUT is inductive as hell(IMHO all THT stuff compared to 0603), you don't really care for fixture inductance anyway.
I'm not so sure about that. The work I am preparing for involves modifications to existing boards, populated by a mix of SMD and THT parts. Cleaning up old messes, you might say. So I need to be able to characterize these THT parts in freq ranges where we know they do not behave well.
For regular part testing, I use component fixtures that we have for Bode. I take out the small fixture only when I need to measure really low impedance/low inductance stuff
I am hiding my jealousy
Can you post a close-up pic of the fixture?
Actually you can easily built fixture that you are dreaming about .
Small board, SMA on opposite edges, 50Ohm trace going in between. Mill-max pin receptacles for THT parts pins, in 50mil grid. See the picture.
Yes, I have some of the Mill-Max and thought of something like that, but I decided that the variable contact resistance of those connectors was not suitable for low-impedance stuff. And that's what I am concentrated on right now.
Above 1 Ohm is easy. You can do it without special fixtures, even. (Have you seen the YT video about Z measurement, soldering a radial 1nF cap with an axial 47 Ohm R and sweeping Z all the way to 65MHz? It looks so easy!) Low Z @ high freq is tough!
I used mill-max receptacles for transformer when I was developing my last SMPS - great stuff, you design tight layout from the start and mess with transformer design all day long. No soldering necessary(a tip from Mr.Ridley).
Yes, for those impedances, it's a viable setup.
And you could probably experiment with CM choke on the output as well, as I do not believe that for low MHz range you need CM choke with coaxial winding.
Fortunately I can skip the CM choke business. At least with the Anritsu...
Do you have access to machinist shop?
Only in my dreams
Just for the helluvit, last night I put together a fixture based on the "typical" way of measuring impedance; measuring voltage on either side of a series resistor (see below). Not useful for SMT at all, but good for radial parts. I didn't take pics or save a plot (will do so next time and post), but the results were a bit worse than the injection xfmr method. Even with short signal paths and careful grounding, there is excessive inductance in the setup. Using 1 Ohm reference R, measuring the axial 25mOhm R used previous, without short compensation, it was accurate to maybe 30kHz. Short comp extended it to 100kHz or so. Not very useful.
Before I abandon it, I am going to try moving the reference R into the ground leg and measure the current there. It's a lower-Z node so the L should be lower and usable bandwidth higher.