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In-circuit component testing

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rhb:
I've recently had an adventure with trying to diagnose and repair an instrument for which no service data is available.  Sadly, that is the future for much electronic repair work.

In this instance, I was concerned with testing capacitors in circuit.  After failing with the Arduino LCR tester based BSIDE ESR02 Pro and the Atlas Peak ESR70, I bought an EDS-88A II which arrived today.

I pulled a bunch of boards from the junk box and set to testing caps.  None of these instruments is able to reliably test capacitors in circuit. If there are significant parallel low impedance paths the simple tests being used fail.

On careful consideration, I think that a vector network analysis approach might succeed.  What I have in mind is a 3 receiver VNA equipped with shielded probes.  Touching the probe tips together sets the cable delay term.  If the probe tips are then placed at the leads of a particular part, all other parts in parallel will appear at larger phase delays.  Those delays are easily solved which in turn allows separating capacitor leakage from a parallel resistor or inductor.  So far as I know, no current tester does that.

If you have the skills to be able to identify the delays between arrival times  to much less than the resolution in the time domain, i.e understand that this is done by applying the shift theorem and linear fits in the frequency domain, I'd like to discuss the matter.  Either publicly as an OSSW/OSHW project, or if you prefer, privately as a commercial venture.  I'm not concerned with making money from this. I'm retired.  My concern is to make repairs easier or in some cases simply possible.

I'd like to add that what I am considering is *much* more sophisticated than an "octopus".  what I'm after is the same answer as you would get if you removed the part and put it on an LCR meter.

The nanoVNA design seems as if it would be adequate HW with some changes and more CPU.  A Pi CM3+ should be more than enough CPU capacity.

Have Fun!
Reg

Jay_Diddy_B:
Hi,
IMHO this is not possible.

Consider this simple test:



Two capacitors in parallel, one is 4.7uF and the other is 470nF

This the impedance plots from a VNA:



The bold trace is just the 4.7uF capacitor, the other trace is the parallel combination.

On the left side of the self resonance the impedance changes by 10%
On the right side of the resonance the inductance changes by factor of two. It is actually dropping from 1nH to 500pH

You tell with the VNA if the 470nF is fitted.

This example is just two capacitors …

The technique used is described in this thread:

https://www.eevblog.com/forum/projects/high-bandwidth-current-injector-for-impedance-measurements/msg3026052/#msg3026052

Regards,
Jay_Diddy_B

rhb:
Thanks for the reply, but the mathematics of the VNA software are inadequate for what I want to discuss.

If you look at a TDR of a pair of caps in parallel shunt across a transmission line, the closest cap will make a bump in the TDR before the farthest cap.  "Farthest" can be a few millimeters or less with adequate phase resolution.  High BW is not needed.

I've got a long term thread on TDR testing of RF connectors which shows every little inductance or capacitance change  in the impedance along a transmission line at fractional mm resolution.

https://www.eevblog.com/forum/rf-microwave/testing-rf-connectors-and-cables/

In the frequency domain your example is  two vectors at different quasi-linear phase shifts.  It's linear if the parts are ideal.  If the phase response of the parts is not linear, then it gets slightly more messy, but not impossible.

Of course, if the VNA probes are equidistant from the parts, you can't solve it.  I'm presuming that the probes are at a particular part.

Have Fun!
Reg

Jay_Diddy_B:

--- Quote from: rhb on May 19, 2020, 12:14:40 am ---Thanks for the reply, but the mathematics of the VNA software are inadequate for what I want to discuss.

If you look at a TDR of a pair of caps in parallel shunt across a transmission line, the closest cap will make a bump in the TDR before the farthest cap.  "Farthest" can be a few millimeters or less with adequate phase resolution.  High BW is not needed.


Snip ..
Have Fun!
Reg

--- End quote ---

I believe that this is wrong. You need very short very fast pulses to do high resolution TDR. Short fast pulses are high BW.

This means that you are trying to measure 'normal' components at microwave frequencies.

TDR can be modelled in LTspice:



I have the components 1ns apart, which is about 30cm.
I am using 1 \$\Omega\$ transmission line, because power distribution is normally done with low impedance planes.

This is the result of running that model:



The result with the open circuit and the electrolytic are the same, implying that the Electrolytic is not there at microwave frequencies.

Regards,
Jay_Diddy_B

 tdr model.JPG (53.98 kB. 747x673 - viewed 1385 times.)

rhb:
Please show the BW term in the shift theorem.

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