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In-circuit component testing

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RoGeorge:
The idea of measuring in circuit components based on reflectometry is very nice, and might come handy in some very narrow domains of application.   :-+

However, I don't think it will be practical for day to day measurements, and certainly not at $100 or so BOM. 




--- Quote from: rhb on May 19, 2020, 01:05:16 pm ---the limitation is *phase resolution*, not BW
--- End quote ---

Phase resolution is the same as time resolution.  Small time corresponds to high frequency because f=1/T, therefore high bandwidth is a must.

- in math: The method might appear to be working on paper, because in math the signal processing is made with infinite bandwidth (precision), and also without noise or other limitation imposed by the real world.
- in physics: For the real world, everything is also a filter (and since we don't know the characteristic of those filters, that implies loosing some information about the studied signal).
- in engineering: Measurements are altered by noise, sampling frequency are limited by technology, digital processiing (computing) can be done with a limited precision only.

rhb:

The time resolution is (frequency * 360) / phase_resolution_in degrees for a single frequency.  If you are measuring the phase delay by a linear fit to the phase in the frequency domain across multiple frequencies you can achieve better time resolution than the phase resolution at any one frequency or the Nyquist of the sampling in the time domain even in the presence of zero mean Gaussian noise.

The math is only infinite precision if you assume that.  One is quite at liberty to make  calculations under assumptions of finite precision.  That is often very necessary.

Noise is certainly an issue, but  I know a few ways to address that.  Probe design is probably the hard part.

*By design* the GPS signal at the antenna is around 56 dB below the noise floor of the analog front end.  Processing gain takes care of it.

BTW it's spelled "losing" not "loosing".  Pet peeve of mine.

Have Fun!
Reg

RoGeorge:
Thank you for pointing out the spelling error, I was not aware about "loosing" vs "losing".

Googled it, and the funny thing is Cambridge dictionary has the sentence "This would eliminate the risk of loosing important information." as an example https://dictionary.cambridge.org/dictionary/english/loosing .  That is almost identical with my "that implies loosing some information about the studied signal"   ;D , but I guess Cambridge example is about unintentionally broadcasting confidential information, while I was trying to say that some info will be lost.


Back to the measuring idea, there might be yet another limitation, at the physics level.

Let's assume we know all the parameters of the probing cable, and we have a very fast ADC.  At one side of the probe is a generator+ADC, at the other side is the component, let's say a capacitor we want to measure, soldered on a PCB.
* 1.  We generate a signal, let's say a step voltage because it's easy to visualize it's evolution in time.
* 2.  The step wavefront will start to propagate through the transmission line, and will arrive at the capacitor's terminals.
* 3.  At this moment a fraction of the signal will travel back, and another fraction will continue to travel through the PCB, and will met other components.
* 4.  When the wavefront in the PCB will hit the most nearby component next to our probed C, that component will reflect some energy back, just as the probed capacitor did, except this new reflection from the nearby component will be a little late and lower in amplitude.
* 5.  To simplify calculation, let's say the velocity factor in the PCB is 1, and the next nearby PCB component is a SMD at 1.5mm distance from the probed capacitor.  The delay between the probed C and the next reflection will be about 10ps.
* 6.  That means after about 10ps other reflections from other components will start polluting our signal reflected by the probed capacitor.  In other words, we need to keep only first 10ps and discard the rest.  Since the velocity factor will be less than 1, let's be generous and instead of only 10ps we will give it 100ps before the signal reflected by C will be flooded by other reflections from the other components soldered on the same PCB traces.
* 7.  Even with a 100ps window of useful signal instead of 10ps, acquisition and conversion won't be easy.  Maybe a sampling head can deal with that.  Let's park this problem for a moment, because there is a much bigger one.
* 8.  In the first 10...100ps we calculated above, there is not enough time for the signal to travel inside the probed capacitor's plates.  In other words, there is no capacitor yet to reflect back the wavefront.  The reflection will start to happen only after the capacitor's plates are traveled by the wavefront.
* 9.  I didn't calculate the velocity factor inside the capacitor, but I suspect it will be much lower than the velocity factor in PCB traces.  Chances are nearby resistors will reflect faster than the wavefront will travel through the C plates and back.
Electrolytic capacitors will be even slower in reflecting the signal, because they are made from very long Al foil plates coiled as a cylinder.  This means we will never know if the reflected signal we see is from the probed capacitor or from some other components on the PCB.

rhb:
"That's a fine glory." - Humpty Dumpty

What does "loose"mean?  What does "lose" mean?

I have a BA in English lit.  The reference you cited  merely reflects the demise of competence with the English language.  It merely cites common incorrect usage collected by a web scraper.  Read the disclaimer.

"Loosing" is to "let loose,  let free,  unhinder etc"  *Very* different.  "Losing"  is to suffering a loss.

Oxford is the custodian of the English language in the form of the  Oxford English Dictionary aka the OED. 

I suggest the rest be left until I have the  time to make a circuit and subject it to the tender mercies of my 11801/SD-24. I am *not* suggesting that TDR alone can solve the problem.  What I'm saying is that the addition of TDR can.  Even if the reflections overlap, basis pursuit can resolve them.

Have Fun!
Reg

virtualparticles:
You can do some interesting tests using a VNA. There are three modes which can be used for this. Simple shunt, Trans-shunt and series. Here is an article I wrote which describes these measurements.

https://www.signalintegrityjournal.com/blogs/8-for-good-measure/post/1344-using-a-vna-for-power-plane-impedance-analysis

It is possible to distinguish between the capacitor and a resistor if one applies a little logic and observes over frequency.

Thanks!

Brian

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