Tweet
I wanted to quickly see what's offered by the differential scheme proposed here so I knocked up the ridiculously quick (less than 10 minutes to make) and dirty (it's not twisted pairs and it's dripping with capacitance) coil....
...Still, it does appear to show the principle in action. In case it's not clear just looking at the photo, the ribbon ends are rejoined with a 2 conductor offset forming a continuous coiled pair. As per the bifilar scheme the - at the top is joined to the + at the bottom so becoming the common (green). The remaining ends become the + and - to the TX/RX circuit (red, blue). The diameter is 30cm and the pairs make 18 turns. Not sure how this "solenoid" winding compares to the flat (radial?) winding that's been shown though.
So on to the initial testing with a scrap of PCB foil at 15CM...
The white trace is the reference output (from differential amplifier G=500) with no target. Ch1 is the output with the copper foil shown above. Ch2 shows the end of the TX pulse/start of flyback. Ch3,Ch4 are the flyback at each end of the coil. My frontend circuit is basicaly the same as this one except my differential amp is discrete (3xOP37 in classic instrumentation configuration). It's a little way off of critical damping, due in part I suspect, to the resonances in the flyback - probably a result of the awful parasitic capacitance of the ribbon cable. Still, it settle out at around 12us so isn't a write-off.
So now for the money shot - 1UKPound at the same range...
Far less response but highly repeatable and relatively undisturbed by environmental effects. There appears to be merit in this technique
Regards to moodz and all the other contributors to this project so far!
...It seems to have gone a little quiet lately which is why I've jumped in here...
...Still, it does appear to show the principle in action. In case it's not clear just looking at the photo, the ribbon ends are rejoined with a 2 conductor offset forming a continuous coiled pair. As per the bifilar scheme the - at the top is joined to the + at the bottom so becoming the common (green). The remaining ends become the + and - to the TX/RX circuit (red, blue). The diameter is 30cm and the pairs make 18 turns. Not sure how this "solenoid" winding compares to the flat (radial?) winding that's been shown though.
So on to the initial testing with a scrap of PCB foil at 15CM...
The white trace is the reference output (from differential amplifier G=500) with no target. Ch1 is the output with the copper foil shown above. Ch2 shows the end of the TX pulse/start of flyback. Ch3,Ch4 are the flyback at each end of the coil. My frontend circuit is basicaly the same as this one except my differential amp is discrete (3xOP37 in classic instrumentation configuration). It's a little way off of critical damping, due in part I suspect, to the resonances in the flyback - probably a result of the awful parasitic capacitance of the ribbon cable. Still, it settle out at around 12us so isn't a write-off.
So now for the money shot - 1UKPound at the same range...
Far less response but highly repeatable and relatively undisturbed by environmental effects. There appears to be merit in this technique
Regards to moodz and all the other contributors to this project so far!...It seems to have gone a little quiet lately which is why I've jumped in here...
Comment