DEVELOPMENT AND IMPLEMENTATION OF DIGITALLY TUNED 88-108 MHz FM SIGNAL STRENGTH METER

Abstract

The digitally tuned signal strength meter was implemented to measure signal strength of frequency modulation (FM) radio stations in the frequency range of 88-108MHz.The received signal strength meter consists of the following units namely: Antenna, RF Amplifier, FM Receiver, Voltage control oscillator( VCO), phase lock loop, Audio Amplifier, Buffer Amplifier, microcontroller, and liquid crystal display. The operation of the received signal strength meter commences when the monopole antenna senses the available signal in the space around it and sends it to the low noise Amplifier (MAR-4) for amplification. The amplified radio frequency signal is demodulated using SA605 in conjunction with locking signal generated by the digital tuner. The locking signal in the range 88-108 MHz was generated using Phase lock loop (UMA1015A) and VCO (MAXIM2606). The PLL was designed to generate a standard frequency of 10 kHz this signal is multiplied by pulse counter to give 8800-10800. The preset pulse is fed to the phase detector, which produces an equivalent voltage (Vtune 0.4 – 2.4V) required by the VCO to produce actual frequency require for demodulation. The sensed signal was designed to lock with frequency from the VCO to produce IF of 10.7MHz. The signal was further filtered and fed into the audio amplifier (LM386). The demodulation I.C SA605 produced a voltage proportional to the signal strength of a radio station. The buffer amplifier (741) links the signal strength output of the demodulation I.C to the microcontroller (PIC 16F877). The microcontroller was programmed to control the switch or keypad, the PLL to display corresponding signal strength output in dBµ and frequency of the selected radio station in MHz on the LCD. The fabricated and Imported Signal strength meters were placed side by side to obtain signal strength of different transmitting stations in Ibadan over a couple of hours. The equipment compare favourably with the imported model with average correlation of 99.73%