Amplitude modulation first allowed transmission of voice wirelessly using continuous wave (CW) transmission. Before CW transmission, we had damped wave transmissions which are oscillations that died out like a pendulum. Think of damped oscillations as not trying to keep swinging on a swing or not cranking your grandfather clock. Eventually, the oscillations cease. Continuous oscillations are like giving the pendulum a push to keep it going, pushing a friend on a swing or cranking your grandfather clock; they keep oscillating so long as energy is added to the system. An example of a damped oscillator is a spark gap transmitter. A continuous wave oscillator example is the Alexanderson alternator which was the device used to broadcast the first AM signal.
What is modulation? Modulation means that we are changing the signal. When you talk to someone, your voice is transmitted at certain frequencies that can only travel as far as you can be heard. In order to send a voice signal further, we have to modulate the signal to radio frequencies. The two types of modulation are AM and FM.
Now that we have a solid understanding of Amplitude Modulation, let’s see how it is implemented. We can model the circuit below as having five components: 1) source 2) amplifier 3) filter 4) oscillator 5) antenna. Theoretically the capacitors could be called a sixth component but their main purpose is for something known as AC coupling. The basic premise behind coupling is to remove DC voltage which sounds like noise at the output of the amplifier. The source in this case is a 3.5 mm stereo audio jack. The two stereo pins are tied together by being on the same row and the ground pin is grounded to the negative terminal of the battery (9V). The amplifier is no simple device. You can find the datasheet here. I found that the examples in the data sheet, specifically the AM radio power amplifier schematic, work well. If you want the details of how the LM386 was designed, here you go. The Zobel network and more specifically Boucherot cell helps to reduce noise if there were a loudspeaker at the output signal before it is fed into the crystal oscillator. You can get rid of this filter. Finally, the AM signal is sent through the antenna which is just a long wire.
After assembling the circuit, plugging in a source, and carefully tuning your radio to 1 MHz or 100 X 10 KHz, you can hear an audio signal being broadcast wirelessly. Here is the schematic I followed without the filter.
What is modulation? Modulation means that we are changing the signal. When you talk to someone, your voice is transmitted at certain frequencies that can only travel as far as you can be heard. In order to send a voice signal further, we have to modulate the signal to radio frequencies. The two types of modulation are AM and FM.
Now that we have a solid understanding of Amplitude Modulation, let’s see how it is implemented. We can model the circuit below as having five components: 1) source 2) amplifier 3) filter 4) oscillator 5) antenna. Theoretically the capacitors could be called a sixth component but their main purpose is for something known as AC coupling. The basic premise behind coupling is to remove DC voltage which sounds like noise at the output of the amplifier. The source in this case is a 3.5 mm stereo audio jack. The two stereo pins are tied together by being on the same row and the ground pin is grounded to the negative terminal of the battery (9V). The amplifier is no simple device. You can find the datasheet here. I found that the examples in the data sheet, specifically the AM radio power amplifier schematic, work well. If you want the details of how the LM386 was designed, here you go. The Zobel network and more specifically Boucherot cell helps to reduce noise if there were a loudspeaker at the output signal before it is fed into the crystal oscillator. You can get rid of this filter. Finally, the AM signal is sent through the antenna which is just a long wire.
After assembling the circuit, plugging in a source, and carefully tuning your radio to 1 MHz or 100 X 10 KHz, you can hear an audio signal being broadcast wirelessly. Here is the schematic I followed without the filter.