Voice Transmitter via AM Medium Wave Band

Figure 1 

The circuit was designed to operate a voice transmitter over the AM medium wave band under a limited power output.

• Amplitude Modulation – a method or technique of emphasizing and transmitting data or information via an alternating current radio carrier waveform; the process whereby the amplitude of a carrier wave is changed in accordance with a modulating wave which includes applications using sinusoidal carriers, pulse carriers, or any other form of carrier
• Transmitter – an electronic device that can produce or amplify a carrier wave signal, modulates it with a significant signal, and radiates the resulting signal from an antenna which are being utilized in television, telecommunications, and radio
• Medium Wave Band – a part of the medium frequency radio band used mainly for AM broadcasting with an allocated frequency range of 530 KHz to 1610 KHz, extending to 1700 KHz in the Western Hemisphere

There are two parts comprising this transmitter, the RF oscillator and the audio amplifier. The oscillator is the heart of the transmitter which is built around Q1. The circuit may be tuned to around 500 KHz to 1600 KHz using the tank circuit’s combination of inductor L1 and variable capacitor VC1. Thepieces of this circuit may be obtained from old MW radios. To produce oscillation, Q1 requires the regenerative feedback from the amplifier to make the feedback signal in phase with the input signal. This is attained by attaching the opposite ends of the tank circuit to the base and collector of Q1. The oscillation frequency is controlled by the variable capacitor while resistor R2 ensures that the oscillation and modulation signal will not be shunted to the ground, by increasing the input impedance, through the very low internal resistance of Q1. To make sure that the oscillation is passed from collector, to the emitter, and through the internal base emitter resistance of the transmitter by the capacitor C2, the signals are coupled or joined by capacitor C7 from the base to the top of inductor L1.

The audio amplifier consists of Q2 conducting at common emitter. The full gain of the amplifying stage is attained by the emitter resistor being decoupled by capacitor C5. To increase the transmitting range of the circuit, an antenna may be utilized or just a 30cm of wire at the collector while using the 4.7K ohm preset resistor will increase the quantity of AM modulation. Lastly, an electret condenser microphone (ECM) is used because it is more sensitive and responsive and well suited for any grade of sound.

The first amplitude modulated signal was transmitted in 1901 by a Canadian engineer named Reginald Fessenden. Amplitude Modulation (AM) is practically used in Am radio broadcasting on the long medium and short wave bands, single sideband transmission systems, vestigial sideband systems, frequency division multiplexing, time division multiplexing, phase discrimination multiplexing, for two way radio communication at VHF for aircraft, reduced carrier systems.

Amplitude modulation may be utilized not only in radio communications but also in transmitting human voice electronically, where the voice waves on both sides are modulating the voltage of the direct current loop connected to them by the telephone company. In AM radio, the voltage or amplitude of a carrier with a fixed center frequency by the station is varied by the analog audio signal. Amplitude modulation is also used in digital data, like quadrature amplitude modulation, where both amplitude and phase modulation are used to create different binary states for transmission. AM is also used in modulating the light waves in optical fibers.

Some of the advantages possessed by using amplitude modulation receiver are, simplicity of implementation, can be demodulated using a circuit consisting of very few components and they are cheap as no specialized components are needed. The disadvantages that one may encounter are inefficient in terms of its power usage, inefficient in terms of its use of bandwidth and prone to high levels of noise.

Source:www.zen22142.zen.co.uk/Circuits/rf/amtx.htm