Changes

* Multiple-frequency designs drive the three transmitter coils simultaneously, with sinewaves at three distinct frequencies. This improves signal-to-noise ratio by lengthening measurement time.

* Multiple-frequency designs drive the three transmitter coils simultaneously, with sinewaves at three distinct frequencies. This improves signal-to-noise ratio by lengthening measurement time.

* Operating frequencies are typically 30 Hz to 15000 Hz.  1000 Hz, 1300 Hz, and 1600 Hz are a good starting point. Higher frequencies give higher induced voltages. Lower frequencies reduce error-causing eddy-current effects.

* Operating frequencies are typically 30 Hz to 15000 Hz.  1000 Hz, 1300 Hz, and 1600 Hz are a good starting point. Higher frequencies give higher induced voltages. Lower frequencies reduce error-causing eddy-current effects.

* The transmitter coils are usually series tuned with capacitors.

* The transmitter coils are usually series tuned with capacitors.

* The transmitter-coil currents must be measured. The currents vary slowly due to coil heating, so currents can be measured periodically.

* The transmitter-coil currents must be measured. The currents vary slowly due to coil heating, so currents can be measured periodically.

* Some designs use DC pulses to drive the transmitter coils, instead of AC frequencies. This simplifies driver design, but makes receiver signal recovery more difficult. Pulse-driven transmitter coils must be driven sequentially.

* Some designs use DC pulses to drive the transmitter coils, instead of AC frequencies. This simplifies driver design, but makes receiver signal recovery more difficult. DC pulse-driven transmitter coils must be driven sequentially.

* Data-acquisition electronics measures the currents in the three transmitter coils, and measures the voltages induced in the three receiver coils.

* Data-acquisition electronics measures the currents in the three transmitter coils, and measures the voltages induced in the three receiver coils.