PI TX Settling

[Carl-NC]

Yes, it appears the biggest improvement comes from a target tau that matches the TX width. I ran a few other taus to verify the peak seems to be 100us.

Here are the improvements between the fast (TX-2us) pulse and the slow (TX-ramp) pulse:

1us: 1%
10us: 10%
100us: 37%
1000us: 2.5%

Note that for the ramped TX current, the long-term target response is a steady-state eddy current. This is expected, as the eddy response is proportional to the derivative of the magnetic field. This creates an offset eddy that subtracts from the turn-off eddy, as people have correctly argued. For a 1us target, the steady-state eddy current is small because of the higher effective resistance of the material (and/or skin effect), so the offset is so small compared to the flat-top responses (TX-2us & TX-20us) that it makes no difference.

For the 10us target, all 3 eddy responses have flattened out but the higher conductivity yields a larger proportional offset for the TX-ramp response.

As the target taus approaches 100us, none of the target eddy responses have settled out, but TX-2us and TX-20us are clearly on a positive trajectory while TX-ramp is still heading south. This creates the largest relative offset between the fast and slow cases.

By the time the target tau hits 1000us, the eddy responses are so slow that all 3 are just following the TX current and all are close to the same offset eddy value, which happens to be an offset almost exactly equal to the turn-off eddy step. What this means is that such a target should have almost no detectable response.

[Qiaozhi]

From the results, it appears that flat-topping the coil current gives the most improvement for the 100us target tau. But even then the difference between the sets of results is not large. I will attempt to create a similar SPICE simulation for comparison.

[Carl-NC]

Introduction

When you turn on a standard PI transmitter, the coil current rises exponentially with a tau of L/R, where L is the coil inductance and R is the total equivalent series resistance. A typical setup is a TX width of 100us, L=300us, and R=1-3 ohms. This gives a tau of 100-300us, so if the TX width is only 100us then the coil current will still be rising at the turn-off point.

Eric Foster has long argued that the coil current should be flat-topped at turn-off; a still-rising TX current will continue to induce reverse-eddies in the target, and those reverse eddies will subtract from the forward eddies at turn-off and reduce the overall target signal. While I've never doubted that this happens, for years I've considered that the effect is not significant enough to worry about. My reasoning was that the dI/dt of the rising turn-on (typically 10mA/us) is far less than the dI/dt of the turn-off collapse (typically 1A/us). But I never spent any time doing an objective investigation.

Circuit Setup

The first stage of this investigation is in Spice. I concurrently ran 3 transmitters with 3 different turn-on characteristics. The first was with no series coil R other than the MOSFET switch (IRF740); therefore it has a coil current that is very close to a linear ramp. At 100us it peaks at 1.52A which is the peak used for the other transmitters. The second TX has a series R of 15 ohms to achieve a tau of about 20us, which means that at 100us is should be settled to 5 tau. Its drive voltage is set to 23.8V to give a flat-top current of 1.52A. The third TX has a series R of 150 ohms for a tau of 2us, and a drive voltage of 229V.

Each TX coil is coupled to its own target modeled by an inductor & resistor. The target tau is L/R; the L was fixed to 100uH and the R was adjusted for desired tau.

Each target is coupled to its own RX coil which drives a simple MAX410 non-inverting preamp stage.

The sim was run for target taus of 1, 10, 100, and 1000us. For each target tau sim, I plotted the TX current (always the same for each sim), the target eddy current response, and the preamp output voltage. Each plot shows the result for the ramp TX (blue), the 20us TX (green), and the 2us TX (red).

Sim Results

Target tau of 1us:



Target tau of 10us:



Target tau of 100us:



Target tau of 1000us:



To be continued...