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 i?? 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 (= 370? ) at the frequency of the radio signal (=1. 215 MHz) to reduce signal loss.
I have chosen C0 to be 0. 1 i?? F. It has a relatively small impedance of 80?. Resistor R3 is to hold the gate voltage at essentially 0V and conduct a gate current of a few nA to earth while still maintaining high input impedance. A 300k? resistor is used for this purpose. The IdVgs characteristic of the FET is given by Id = k (Vgs- VT)2. (For IDSS = 4mA, VT = -4V and a channel current bias, Id of 1. 5mA) K = IDSS / VT2 Id = k (Vgs- VT)2 = 4mA / (-4)2 VGS = i?? V (Id / K) + VT = 0. 25 mAV-2 VGS = i?? V (1. 5mA / 0. 25mAV-2) + (-4) VGS = -1. 55V or -6. 45V (not applicable)
This makes the source voltage, Vs = Vout = 1. 55V. Value of R2 = VS / Id = 1. 55 / 1. 5mA =1033. 3? i?? 1K?. 2. The Tuned Circuit The tuned circuit can pick out a desired radio station frequency by resonating and that particular frequency (in this case, 1. 215 MHz). The bandwidth of the signal should be 8 KHz to support an audio signal of up to 4KHz. Limiting the bandwidth in this way ensures that there are no overlapping frequencies from other radio stations transmitting at nearby frequencies. Since the quality factor, Q = frequency/bandwidth, the ideal Q would be: 1. 125 MHz/ 8KHz i??
152. In figure 2, Q is mainly the natural Q of the inductor, since the Q for the capacitor is extremely high. Choosing the inductor to be 47 i?? H gives us a Q factor of i?? 120. This value is lower than the ideal 152, and this increases the bandwidth of the circuit to 9. 4 KHz. A trimmer capacitor, Cv 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, ? o = 1 / 2? V(LC). Choosing L1 = 47 i?? H gives C1 = 365pF.