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What options are out there for high sampling rate (>250 MHz) DAQ on a DIY budget
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MasterT:

--- Quote from: rhb on June 01, 2018, 12:19:13 pm ---The waves will all reflect at the edges and faces of the plate.  This is what makes it ring. 

--- End quote ---
Correct. It also means, that sampling has to be done before first reflection hits nearest sensor, and distorts all phase propagation diagram.  Don't see how 25 mm comes into picture, but if sensor is positioned 12.5 mm to the edge of the plate, than 25 mm distance converted to time is sampling window frame.
 Though 1-5 MHz ADC is not gonna to work, at least 256 samples imply about 50 MHz sampling rate. The more samples, the better accuracy.
I read, that stm32f429 has GPIO-DMA capability, transferring data with close to required speed. Simply connecting parallel 8-bits TLC5540 seems good point to start research.
rhb:

--- Quote from: MasterT on June 01, 2018, 01:26:01 pm ---
 Though 1-5 MHz ADC is not gonna to work, at least 256 samples imply about 50 MHz sampling rate. The more samples, the better accuracy.
I read, that stm32f429 has GPIO-DMA capability, transferring data with close to required speed. Simply connecting parallel 8-bits TLC5540 seems good point to start research.

--- End quote ---

Rock cores are typically measured in the 0.8-1.2 MHz range.  There is absolutely no need for a 50 MS/S or faster ADC.  And at 500 samples per second it is routine to be able to measure the multiplexer skew from channel to channel.  On a 48 channel system that's about 40 microseconds.   And most systems are more like 4800 channels now though also with more ADCs.  You just have to know how.

The proper way to do this is to acquire samples of reasonable length, e.g. 4096, perform an FFT, do a linear fit to the phase after performing pair wise cross correlations in the frequency domain.  Having four or more sensors will improve the result as triangulations can be done with various permutations.  Four sensors allows 4 combinations of 3 sensors.  So under Gaussian assumptions the error will be cut in 1/2 by averaging the four solutions.  Placing sensors in the corners should reduce the reflection problem, but one would need to consider the problem for a good bit.  The behavior is quite complex.

I don't mind pointing to the literature, but I have no desire to read it.  It's several days of serious work to get your head around the mathematical physics of a problem like this.
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