The other end lead of the audio jack connects back to the transformer and then connects to the positive terminal of the battery pack. The 1k ohm resistor connects to the transformer and passes through to a 8 ohm resistor that connects to one lead of a 3.5mm audio jack. The AM transmitter circuit diagram has a crystal oscillator that connects to an antenna on the 8-pin side, the negative terminal of a battery pack on the 7-pin side and a 1k ohm resistor on the 14-pin side. A wire connected to the oscillator output (pin 8) serves as the antenna for broadcasting the amplitude-modulated radio wave. The stepped up signal from the secondary coil of the transformer modulates the power to the oscillator chip (+ power at pin 14 and - power at pin 7). The transformer isolates the music player from the rest of the circuit, couples the music player and the crystal oscillator, and "steps up" the signal voltage from the music player in proportion to the ratio of 1 kΩ to 8 Ω. Figure 2 shows the connections you need to make to build the circuit. Plus it is a lot of fun when you actually have it working!īefore we get into the step-by-step instructions for building the circuit, we'll first go over the circuit design. Although the circuits used in radio stations for AM broadcasting are far more complicated, this nevertheless gives a basic idea of the concept behind a broadcast transmitter. You'll see that you can receive the signal through the air with an AM radio receiver. You can connect the circuit to the headphone jack of a portable music player (e.g. In this project, you will make a simple low-power broadcast circuit, using a crystal oscillator integrated circuit and an audio transformer. Note how the peaks of the output trace (its envelope) follow the form of the input signal. The bottom diagram shows the resulting output signal (blue). The signal is used to modulate the amplitude of the carrier wave. The top diagram shows a carrier wave at a set frequency and amplitude (green) and a signal to be broadcast (red). Illustration of amplitude modulation of a carrier wave by a signal. In the case of AM radiowaves the signal interferes with the carrier waves amplitude creating an output signal shown at the bottom of the diagram in blue.įigure 1. The top diagram pictured shows carrier waves in green and a signal in red. Information is sent over these carrier waves by modifying the carrier waves with broadcast signals. Radiowaves are broadcasted using carrier waves of uniform wavelength and amplitude. Two example graphs show the relationship between broadcast signals, carrier waves and output signals. Figure 1 shows graphically how amplitude modulation works. The "AM" in AM radio stands for "amplitude modulation," and the "FM" in FM radio stands for "frequency modulation." A radio receiver removes the carrier wave and restores the original signal (the voice or music). The modulation can change either the amplitude or the frequency of the carrier wave. The carrier wave is modulated (varied) in direct proportion to the signal (e.g., voice or music) that is to be transmitted. So how does a radio wave carry sounds such as voice or music to your radio receiver? The radio station broadcasts a carrier wave at the station's assigned frequency. AM radio signals are carried by medium frequency (MF) radio waves (530 to 1710 kilohertz (kHz) in North America, 530 to 1610 kHz elsewhere), and FM radio signals are carried by very high frequency (VHF) radio waves (88 to 108 megahertz (MHz)). Radio waves are divided into many sub-classifications based on frequency. Electromagnetic radiation at frequencies ranging from 3 hertz (Hz) to 300 gigahertz (GHz) are classified as radio waves. For example, electromagnetic radiation at frequencies between about 430 terahertz (THz) and 750 THz can be detected by the human eye and are perceived as light. In a vacuum, electromagnetic waves travel at the speed of light.Įlectromagnetic waves such as light, x-rays, and radio waves are classified by their frequency or wavelength. Electromagnetic radiation is a propagating wave in space with electric and magnetic components. When you listen to an AM or FM radio station, the sound that you hear is transmitted to your radio by the station using electromagnetic radiation as a carrier-radio waves. For example, light is electromagnetic radiation and so are x-rays. Electromagnetic radiation is all around us.
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