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# The aim of this project is to design, build and test a simple AM radio receiver, which can receive only one radio station, namely Virgin AM (1.215MHz), with the budget constrain of &amp;pound;6.30.

Extracts from this document...

Introduction

Introduction

The aim of this project is to design, build and test a simple AM radio receiver, which can receive only one radio station, namely Virgin AM (1.215MHz), with the budget constrain of £6.30.

This paper explains the circuitry and design choices for the radio. It consists of 6 different sections: Signal detection, frequency selection, signal amplification, signal demodulation, audio amplification and voltage biasing. For the complete schematic of the circuit, a breakdown of components required and expected cost, please refer to the appendix.

Practical Design

1. Aerial and buffering

A wire aerial of length 50 cm will be used as a detector of the signal. The induced voltage will be: VE = Vl/2 = 0.25mV. Also, the aerial will be buffered with an n-channel JFET with unity gain to increase the impedance of the aerial to prevent it from affecting the radio signal.

To prevent any damage to the circuitry by accidentally shorting to DC supply, a small capacitor, C0 is coupled with the aerial. This capacitance should have an impedance lower than that of free space (= 370Ω) at the frequency of the radio signal (=1.215 MHz) to reduce signal loss. I have chosen C0 to be 0.1 μF. It has a relatively small impedance of 80Ω.

Middle

v will be placed in parallel with the inductor in order to adjust the resonant frequency around the 1.215 MHz region, and the frequency at which the signal overlaps the least can be isolated. The resonant frequency, ωo = 1 / 2π√(LC). Choosing L1 = 47 μH gives C1 = 365pF. The largest trimmer capacitor available is 65pF, and there has to be connected in parallel by a fixed capacitance of 330pF.

3. Signal Amplification

The tuned circuit is connected to the collector of a common emitter amplifier as shown in figure 3. The base of T2 is connected to the output of the input stage. The gain of the common emitter amplifier is given by G ≈ ZRC/ (re + R1//Zc) where ZRC is the impedance of the tuned circuit at resonance and the emitter resistance re =25mV/Ic. At signal frequency, capacitor C2 bypasses resistor R1 and its impedance is given by Zc2 ≈ re/10. So, the gain, G ≈ 10 (ZRC/ re).

Resistor R1 limits the quiescent emitter current to make it smaller than 1mA. Under quiescent conditions, the emitter voltage of T2, VE = (1.55-0.7) = 0.85V. Thus, R1 is selected to be 2.8kΩ for a quiescent emitter current of ≈0.4mA. C2 is chosen to have a value of 0.1 μF so that Zc2 = 8.23 ≈ re

Conclusion

out = 6V, R7 = 1.2kΩ, and for Is = 5mA and a zener with a reverse breakdown voltage of 3V is required. For Vout = 4V, R7 = 800Ω and Is = 5mA, a zener with a reverse breakdown voltage of 5V is required. C7 acts as a decoupling capacitor which has lower impedance at the signal frequency and at noise frequency (50 KHz) than the circuit impedance (≈ 1kΩ). C7 is chosen to be 100nF.

Conclusion

With a budget limit, a radio that costs was designed, and does not exceed the budget of £6.30. However the actual component values may differ from the calculated and amendments will have to be made during the testing and building stage for the radio to clearly receive the desired radio station (Virgin AM at 1.215 MHz).

References:

1. Microelectronics Circuits 4th Edition, Sedra & Smith
2. 1st year Electronics Lab Booklet, Spring Term 2004

Appendix: The Final Circuit Design

This student written piece of work is one of many that can be found in our GCSE Waves section.

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