Looking at return pulse in Frequency Domain

[Anonymous]

Eric,
Wouldn't looking at the return pulse with a spectrum analyser give you more precise information for different metals? I know from varying a pulse ever so slightly into my spectrum analyzer I see diferences that were not even noticable on my scope. Thank you for your kind help.
Randy Seden-Simi Valley,CA.

 

[Anonymous]

Hi Randy, with the help from Eric I try to make up a PI unit , where the amplified pulse response is sampled by a DSP. This unit would allow to transform the pulse response into the frequency domain (FFT) and hopefully there is a difference in the spectrum of different metals. I'll report the results of my investigations as soon as possible. I need some help how the coil shielding is correctly done, especially the connection from the coil-shield to the cable-shield and the system ground, perhaps Eric can help with posting a sketchily drawing of the correct arrangement.
Stefan

 

[Anonymous]

I have tried this before and did not have much luck. The problem is that the edge is where all the information is. The flat part of the curve contains mostly DC information. I have a Tektronix digital scope with a 14(effective) bit analog to digital converter that can sample from a few hundred to up to 1GS/s and that also does FFT's. I have even tried differentiating the edge to convert the edge to an impulse but this did not help much. If you guys have any ideas how I could do this better I would be very interested in doing an experiment

 

[Anonymous]

Stefan,
This sounds very exciting and cutting edge! I proposed this idea to Carl Moreland (he is an engineer with Analog Devices and has the web site "Geotechnology" at http://tthn.com/geotech/ that has a forum that is for people that like to build their own metal detectors and magnetometers,and has some execellant schematics for them as well). He mentioned that to use a dsp with a pi unit to have a GOOD log amp with alot of dynamic range as the pulse decays rapidly. Analog Devices does have a new one that would fill the requirement nicely. Will look forward to seeing what you come up with.
Randy Seden-Simi Valley,CA.

 

[Anonymous]

Hi Randy and All,
I would be very interested in the log amp you mentioned. Do you have the part number? This is a good way to go, either for DSP or normal analogue processing. Although linear amps are invariably used, the signal strength varies so dramatically with object range, as a result of the sixth power law, that they rapidly run into saturation. The other benefit of a log amplifier is that it is possible to enhance the differences in the decay curve between ferrous and non-ferrous metals. A non-ferrous object has a decay that is exponential in shape for later times i.e. times longer than one time constant. Run this through a log amp and you get a linear decay. The decay from a ferrous object when run through a log amp is anything but linear. If you take a long sample window of the signal and differentiate it, then non-ferrous objects come out as a constant reading, depending on the slope of the linear decay, which depends on the time constant. Hence an indicator could be calibrated for different coins etc. Ferrous objects could be distinguished by the fact that the reading changes during the sample window as the slope changes. Tomorrow I will try and post some log plots of different objects which were taken in 1968 using a wide band log amplifier (yes it was an IC!) which clearly shows the differences which one would be hard pushed to notice on a linear display.
So why isn