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| (ultra)acoustic in circuit component level crack detector? |
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| rhb:
I'd forgotten about lasers. You might be able to get the BW required using a laser pulse as the source and an interferometer as the receiver. But the resulting system would be *very* complex. There is no escape from the wavelength vs resolution problem. That's the fundamentals of the wave equation at work. |
| rhb:
This is *really* cool. I am astonished at the experimental result. Setup is a 5 nS pulse at 10 MHz repetition rate from my 33622A and my MSO2204EA. The first image is the SG connected to the scope via a 50 ohm thru. Visually it looks perfectly Gaussian and zero phase. DS0123.PNG The next image is the SG pulse fed to the DSO with the FFT turned on. We see a classic comb spike series every 10 MHz from DC to Nyquist at 500 MHz and a clear picture of the antialias filter amplitude response. DS0125.PNG The 3rd image is with the SG and the scope connected to the bare xtal. It has now has a minimum phase very broadband filter applied which results in the asymmetric waveform. Bode wrote the classic paper on this. It's important to note that the ringing I expected at the fundamental of the xtal is not seen. This was a real surprise for me and suggests that a 10 cent xtal may work as a piezo transducer to very high frequencies. This is the shortest pulse I can produce. I now have an excuse to buy one of Leo's 100 pS long pulse generators DS0116.PNG I then attached a 0.55 mm thick piece of plastic between two xtals with super glue. In this instance the source and receiver are the same xtal. In accordance with theory, we see ringing as the pulse travels through the plastic disk and xtal, hits the air-xtal interface and is reflected with a negative reflection coefficient. It then travels through the disk until it hits the opposite air-xtal interface and is reflected again. DS0130.PNG The last image is the direct arrival through the disk detected by the 2nd xtal. All the other stuff is reflection off the air-quartz interface at the back side of the xtals. DS0128.PNG The single xtal reflection response suggests that a simple probe that one pressed against a part might actually work. I'll repeat the tests later with my LeCroy, but I'm beginning to think that coppercone's idea might actually work. after all. |
| coppercone2:
hmm I have a 50KHz 70ps wave generator (old tektronix unit). Too bad I am on vacation or I would experiment with this. I also have a nice VNA meant for categorizing crystals as a matter of fact. what you did is clever. I would like to see this work. I wonder how you can couple tweezers to a part. I wonder if a lossy deformable material would work better despite the attenuation then a hokey contact between hard tweezers and a hard part. god the box of crystals I got off ebay a long time ago has been bugging me, I literally used 1 part from it. one of those purchases which made me question keeping components at all. salvation at last? also may be of some interest about crystals |
| coppercone2:
hopefully i can link the ode to joy within 2 days? :clap: |
| rhb:
I don't think tweezers are needed. I think a pencil like probe that one pressed down on top of the part is the most attractive option. The resonance frequency of a cracked part will be higher than a part which is not cracked. There's really not a lot more information to be gained from the transmission response. In fact, it was proved over 40 years ago that given the transmission response of a stack of layers you could derive the reflection response and vice versa. However, this would only apply to cracks parallel to the PCB plane. But it should make a very good way to locate cracked SMD solder joints. Here's a photo of the DUT used. It's just two 40 MHz 3rd overtone xtals with the cans removed and a piece of thin plastic glued between them. I bought 100 for $10 long ago to use to build xtal filters. The minimum phase response of the bare xtal assures me that the ringing with a single sensor is a measure of the travel time thickness of the sandwich. I made a similar plot before I glued the 2nd sensor on, but failed to transfer it to the PC. I find it interesting that a 1 Meg load on the 2nd xtal modifies the response of the first xtal. |
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