View Full Version : Time Domain Reflectometry
05-23-2008, 10:03 PM
Has anyone developed an object for this? I would like to use it to build a sensor for the measurement of·sand moisture for a concrete batching system at work. Is it even possible with the prop?
A bit of info:
05-23-2008, 11:01 PM
We use TDR's all the time to measure coaxial cable lengths and check them for problems. What you describe sounds more like ultra-wideband radar...
Anyway, I think the main problem would be fast data aquisition. You'd need some kind of external ADC chip in the MHz range and a pre-amp for it...
05-23-2008, 11:05 PM
From wikipedia also:
TDR is used to determine moisture content in soil and porous media, where over the last two decades substantial advances have been made; including in soils, grains and food stuffs, and in sediments. The key to TDR’s success is its ability to accurately determine the permittivity (dielectric constant) of a material from wave propagation, and the fact that there is a strong relationship between the permittivity of a material and its water content, as demonstrated in the pioneering works of Hoekstra and Delaney (1974) and Topp et al. (1980).
TDR is news to me, but have a look: http://en.wikipedia.org/wiki/Earth's_field_NMR
applications and methods are discussed in chapter 3 of
This work was done in Laos! as a master's thesis.
The Prop clearly has the frequency synthesis and analysis capabilities. The cost would approach the cost of a proto board with a couple of hand wired coils... + some a/d and amplification + your time.
Potential applications are relatively unlimited.
05-26-2008, 02:42 AM
Thanks for taking the time to look that up rjo_ but i'm not sure talking about the same thing :)
05-26-2008, 06:16 AM
It occurred to me that one could measure propagation delay to a certain resolution using a feedback mode of the counters. Feed one end of the transmission line from the Bpin, while the other end feeds the Apin. The counter module makes the state of Bpin the inverse of Apin, so with the propagation delay it forms an oscillator, and in the NCO mode the counter frequency would be the variable of interest. For best results, the end of the cable would be terminated get good matching to Zo. To compensate for signal attenuation, the receiving end could be biased actively or passively near threshold.
The propagation on a cable with a dielectric constant =3 is about 0.1 meter per nanosecond, so 2.5 meters of cable would yield a 25 nanosecond delay, and that would be the limit of resolution of the Prop running with an 80mHz Clkfreq.
If a transmission line of sufficient fixed length were set up through moist soil or a slurry, then the variable dielectric constant of up to around 80 at saturation would lead to a variable propagation delay, to indicate moisture content.
With a sophisticated instrument like the Tektronix model 1502 cable tester, the TDR measurement can yield a continuum of information. In cable testing, allows the operator to see reflections that occur due to discontinuities from connectors or damage all along the length of the cable, each of which creates a reflection. When such an instrument is applied to moisture measurement, it can "see" variations of dielectric constant along the length of the transmission line, from which moisture content can be profiled as a function of depth. I don't know how the Prop could reach that level of sophistication, but, hey! maybe with clever directional couplers and rf magic etc.