Circuit Board Design

1 Overview of Entire system

The current EE design in regards to the "Magicoil" device consists of four elements: Power, Micro-controller, Signal Generation, and Lock-In/ADC. The Micro-controller is used to control signal generation, receive the return signal from the pick-up coils and communicate with external applications (Android App, PC). The lock-In amplier is for picking up that return signal, and can be modeled as a very tight band-pass lter followed by a very high gain amplier, and is typically used for applications where we are searching for very tiny signals (which is the goal here!). Both the Power and Signal Generation are outlined in further detail below:

2 Overview of Power System

To power the "Magicoil" we utilized a generic 24V wall adapter as our power source. This way we can keep the bulky transformers/LiPo batteries out of the device, making the "Magicoil" both lighter and smaller. This 24V input is then regulated down to 3.3V, 2.5V, -2.5V, 18V, and -18V. This was accomplished with a combination of switching and linear regulators. Careful consideration was made to select ICs that can supply the high current necessary for the signal generation system.

3 Overview of Signal Generation system

The signal generation is a complex system consisting of four stages. We begin with generating our sinusoidal signal. To accomplish this we used the ever-classic "Wien-Bridge Oscillator." This oscillator circuit has adjustable gain and frequency by varying three RC constants. To keep the oscillations from running out of control, a JFET acts as a variable resistor and automatically controls gain (This was classically done with a lightbulb!). Because the Signal generation system needs digitally variable frequency from 10k-100kHz (HF), we replaced the three RC resistors with digital potentiometers{ resistors that can be varied by our microcontroller. The Wien-Bridge was chosen over a DAC or DDS chip because it allowed us to complete more of the work as analog design instead of software, and there were some concerning limitations regarding the speed of a DAC. In testing our Wien-Bridge design also was able to produce sine waves with less than one percent total harmonic distortion, so they are incredibly pure when compared to that of a DAC.

The second stage of our signal generation system is a opamp acting as a inverting voltage follower. The gain of the the opamp is also controlled via another digital potentiometer. This stage allows us to vary the amplitude of our sinusoid up to 15V.

The third stage of our signal generator is a unity-gain high current opamp that can supply enough current to drive our HF and LF coils. And finally the fourth stage is the impedance matching stage where we attempt to cancel out the reactance of the coils by matching it with a capacitance such that our load is purely resistive. This way our signal will attenuate to a lesser degree over the specied frequency ranges. The impedance matching is centered at the middle frequency of both HF and LF requirements.