Electronics > Projects, Designs, and Technical Stuff
In-circuit component testing
rhb:
--- Quote from: virtualparticles on June 04, 2020, 08:47:15 pm ---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
--- End quote ---
Thank you for supplying prior art.
That's *exactly* what I have in mind. When I get some free time I'm going to build a microstrip circuit with parts in parallel and hit it with my 11801. That will show each part along the stripline separately. And if you can do it in the time domain, you can do it in the frequency domain.
I'll then use a VNA and FFT to demonstrate doing it with a VNA using the same hardware.
Parts that can take 500+ V charges on capacitors are pricey ($12/each) so I may relax that back to 50-100 V.
Have Fun!
Reg
hendorog:
--- Quote from: rhb on June 04, 2020, 10:22:19 pm ---
--- Quote from: virtualparticles on June 04, 2020, 08:47:15 pm ---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
--- End quote ---
Thank you for supplying prior art.
That's *exactly* what I have in mind. When I get some free time I'm going to build a microstrip circuit with parts in parallel and hit it with my 11801. That will show each part along the stripline separately. And if you can do it in the time domain, you can do it in the frequency domain.
I'll then use a VNA and FFT to demonstrate doing it with a VNA using the same hardware.
Parts that can take 500+ V charges on capacitors are pricey ($12/each) so I may relax that back to 50-100 V.
Have Fun!
Reg
--- End quote ---
If I'm understanding correctly - big if.
Your comment about enhancing time domain resolution typically seen on a VNA - as it is really limited by the ability to resolve phase and not frequency - could be used with time domain gating to allow the probe effects to be removed.
And the same approach should work with the individual components - in the time domain the only interesting component is the one at the end of the probe. Anything earlier is fixture, and anything later is some other component. So you gate the time domain and then transform back to frequency.
rhb:
That's a good summary of my point. There is a significant time delay between the reflections from different parts and that can be used to distinguish the part at the probe tips from everything else. Touching the probe tips together gives the delay time to the tips. Gate out the distant parts and a capacitor between the probe tips has a series capacitance and resistance with a leakage resistance in parallel.
With a 16 bit ADC and a 500 kHz sweep, the phase resolution will allow excluding parts more than 3 mm away. One would need to sweep higher for modern SMD stuff, but for older thru hole gear even 250 kHz would suffice.
The low frequency end of the sweep gives a measure of the leakage resistance, the slope gives the capacitance and the high frequency end gives the ESR. A perfect capacitor is a constant slope. A flat section at low frequency is the leakage and a flat section at high frequency is the ESR.
If one precomputes the values as a function of frequency for a range of values, then a basis pursuit will find the optimal choice of the 3 values. As we are simply going for good/bad that doesn't need to be especially precise. The parallel leakage resistance should be zero if the cap is good. The ESR varies with working voltage and capacitance, but if the display shows ESR for measured capacitance as a function of working voltage with the measured ESR shown, a quick look at the cap or knowledge of the circuit will tell the user if it's good or bad.
Have Fun!
Reg
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