AN AM RADIO PROJECT IN A LOWER-LEVEL ECE CLASS

The complex impedance concept has traditionally been a challenge for an introductory ECE class. An appropriate laboratory exercise may support this concept and spark student interest in the subject matter. A logical choice is to present a "wireless" project as it naturally includes the concept of complex impedances. Building a basic radio receiver on the protoboard is a challenge due to instability of the RF low-noise amplifier within an unpredictable protoboard environment. Our numerous attempts to build an AM station receiver on the protoboard have indicated the following difficulties: 1. The circuit can be built by the instructor and by those skilled in the field from the class, but not by all class students. 2. The design is difficult to debug; sometimes the flawless circuit simply does not function. 3. One critical point is shorting out the power rail of the protoboard (connecting it to the ground plane of the protoboard). Since this ground plane is made of aluminum and is glued to the board, such a connection cannot work well for the standard board. We have tried drilling holes and/or putting clamps; it did not work well in all cases. 4. A laboratory room may be electrically shielded. In our case, the room is located inside the ECE building and has walls reinforced with a metal grid. In this paper, we describe an alternative laboratory project that does not make use of the low-noise amplifier. Instead, we boost the voltage received by a Faraday's coil using a shunt capacitor (parallel LCR circuit), purely passively. The RF part of the receiver circuit (coil antenna, matching capacitor, and an envelope detector with a zero bias Germanium diode) is purely passive too. Students build a short-range 500-1500kHz complete TX/RX link on every laboratory bench. A low-cost function generator (Instek), which is a standard bench component, is used for the transmitter. An external audio modulation is introduced from a computer or iPod. The laboratory material described was successfully implemented in an introductory ECE class with mixed major content.