Sorry to reply so late I wasn't on this list. DAC is not required since binary signals can be used as a source. So the key point is how to generate fast varying signal (in terms of slew rate) and how to capture them efficiently.
For the acquisition an ADC is definitely needed. The problem of the above method is that there are almost always reflections at end of cable (wether a fault is present or not). And I guess the analog integrator must be very fast and precise to work properly. Fast ADC's are not always so expansive it depends on parameters you care about. For example the AD9204 is only 5$, http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad9204/products/product.html(and can be ordered as sample for prototyping). Here the sampling rate is only 20Msps but the input signal bandwidth is 700MHz. I told about equivalent time sampling, this can be used here to sample the reflected signal at high frequency such as 1GHz just by generating a 20MHz clock signal. This is very precise, 10ns corresponds to about 10cm. The measurement is just longer since 50 acquisitions must be performed to achieve it. The only issue of equivalent time sampling is generating is making precise phase shift of the clock signal. With an FPGA this can be done quite easily using an FPGA but if a DSP is used I think this can be done externally using programmable delay line. Circuits such as http://www.maximintegrated.com/datasheet/index.mvp/id/2608 are provided for less than $10 (or sample). -- Adrien -------- Original Message -------- Subject: wire characterization ... done cheaply ? Date: Sat, 29 Dec 2012 16:07:03 -0300 From: Werner Almesberger <[email protected]> Reply-To: English Qi Hardware mailing list - support, developers, use cases and fun <[email protected]**hardware.com<[email protected]> > To: [email protected]**hardware.com<[email protected]> After watching Adrien Lelong's EHSM talk about bouncing signals into wires and figuring out the condition of the wire (broken, shorted, spliced, etc.) from the shape of the pattern that comes back, I wonder if there wouldn't be a cheaper way to accomplish such things. The basic setup is that you send a pulse with a DAC, then receive what comes back with an ADC, and then do some processing, either offline, or in real time with an FPGA, DSP, etc. One issue is that fast ADCs are pricy. Fast DACs are cheap. If all else fails, you can make one with a bunch of resistors. Now, I wonder if one couldn't accomplish similar things with a much simpler setup: instead of a fast ADC, you'd use a slow ADC. For signals within the ADC bandwidth, it would act as usual, which may be useful for some broad characterization. For signals much faster than the ADC bandwidth, the ADC could act on the integral of the signal, combined with a window function. The window function could just be a gate, operating at a speed similar to that of the DAC. For example, to determine when exactly a pulse begins coming back (it may come back as several pulses), one could open the gate from time 0 to an initial estimate t, and then, for a second measurement, from t to "infinity". In the simplest case (we just check if there's a signal or not) there would be four possible outcomes: Signal received in interval Interpretation 0 to t t to infinity ----------------------------- ------------------------------**-------- nothing nothing there is no reflection something nothing t is after the last major response nothing something t is before the first major response something something t is in the middle of the response(s) You'd then adjust t, e.g., with a binary search, until you find the point at which the first interval changes between "nothing" and "something". Interpreting the amount of energy seen by the ADC (i.e., the charge built up during integration), varying the window, varying the shape of the outgoing pulse, etc., would allow for a lot of additional degrees of freedom to further dissect the wire's response (alas, using math that's a bit beyond me). I wonder how much one could accomplish with such a setup. Even simple MCUs have peripheral speeds in the tens of MHz, so it should be possible to have fairly good time resolution with quite simple means. - Werner ______________________________**_________________ Qi Hardware Discussion List Mail to list (members only): [email protected]**hardware.com<[email protected]> Subscribe or Unsubscribe: http://lists.en.qi-hardware.** com/mailman/listinfo/**discussion<http://lists.en.qi-hardware.com/mailman/listinfo/discussion>
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