This is an extension to the Impedance measurements discussed earlier, see:

https://www.eevblog.com/forum/testgear/capacitive-impedance-plots-with-sds2104x-plus-bode-function/msg4342009/#msg4342009However decided to start another thread since some very interesting and unexpected results have shown up, actually started to add to the above until we made some measurements, you'll understand why in a few movements.

Here's an interesting configuration that many folks may have at hand if they have an LCR Bench meter.

Using a popular fixture for SMD that often are used with Bench LCR meters, add a AWG and DSO and you have an Impedance/Admittance Measuring device for SMD capable of generating plots!!

In the simplest form you just need a couple BNC adapters and cables. Connect the AWG output to the Fixture Force (H or L cur) and with the DSO CH1 to Fixture Sense (H or L pot), (use a BNC cable or Scope probe). Now set DSO Ch1 & Ch2 to 50 ohms and Ch2 to the other side of the SMD fixture (H or L) as shown. Set the Bode Plot parameters as required and the plot will show the voltage across the 50 Ch2 resistor divided by the voltage sensed on Ch1.

A little analysis that assumes the voltage on Ch2 is much smaller than Ch1 at the measurement of interest, Ch1 is the AWG output in this case.

I of DUT = VCh2/R, where R is Ch2 termination 50 ohms

DUT Z is ~ VCh1/I or (VCh1/VCh2)/R

VCh1/VCh2 which is inverse of dB version of screen Bode Plot Result (VCh2/VCh1) in dB, so

VCh1/VCh2 = 1/[10^(dB/20)], where dB is Bode Reading in dB

Then Z ~ R/[10^(dB/20) ]

and C ~ 1/(|Z|*2pi*F) where R is 50 ohms in this case (Ch2 termination)

C ~ [10^(dB/20)]/(R*2pi*F), where F is obviously the frequency of the measurement.

Note: We could have swapped Ch1 and Ch2 so that the Bode Plot was inverted but wanted to keep the display relative to Admittance.

Also note when making these measurement the "Quality" of the measurement for Hi Q capacitors is when the phase isn't too far from +90 degrees (remember this is admittance)!!

We ran these for starters:

1nF COG cap, which measured 992.8pF @ 10KHz (plot 115) and 993.2pF on Hioki IM3536 Lab LCR Meter

100pF COG measured 102.1pF @ 1MHz (plot 116) and 101.0 on IM3536

10pF COG measured 10.12pF @ 1MHz (117) and 9.943pF on IM3536

1pF COG measured 1.07pF @ 1MHz (118) and 1.015pF on IM3536

0.3pF COG measured 0.345pF @ 1MHz (120) and 0.3134pF on IM3536

I know this seems crazy to be able to measure these smaller caps with a DSO, but to actually get this performance we didn't expect and this was with no calibration or measurement correction. We did measure the short thur and got less than 0.1dB loss and for an open we got less than 0.05pF equivalent parasitic capacitance

Will admit that some time ago we did modify this fixture to improve the isolation and internal grounding!

And who said that accurate DSO Channel 50 ohm termination isn't useful, and low noise DSO front ends aren't necessary and DSO dynamic range doesn't matter

Anyway more to come later, and here's the really simply setup with only DSO, AWG, SMD Fixture and 3 BNC cables and thru adapters.

Best,