Zack Higgs
Move 3: What is a radio and how does it work?
As most people know, a radio is a device that enables people to communicate wirelessly from one place to another. Some common examples of radio technology are inside your phone, garage door opener, GPS, wifi, and many others. It is an important way people communicate. Without the use of it, we would not have many of the great technologies that we have today. The first radio was created in 1895 by Guglielmo Marconi, although I think that it is unfair to name him the true invented of the radio. Much of the work that was researched and tested on radios, were done by Nikola Tesla, who created what is called a Tesla Coil. A Tesla Coil is virtually a powerful radio that broadcasters at a set frequency. Tesla was the first to create the basis for what was and is still used in radios. I think that it is only right to call Tesla one of the key investors of the radio. Needless to say, the radio is truly one of the most useful inventions ever.
So how does a radio work? A radio has several different components that enable it to send information. The basic things are an amplifier, modulator, oscillator, frequency generator, filter, and an antenna. The combination of all of these can broadcast a signal at a specific frequency. A receiver has very similar components but are arranged differently. For example: Let us say we want to broadcast songs from our phone to a standard car radio receiver. The first step would be to amplify the signal from your phone. Then it is needed to modulate it to either FM or AM, the modulator will take the amplified signal and allow the FM or AM radio to be received. After modulation, it needs to be filtered to reduce the noise in the signal, and next it will need to be combined with another signal that will be broadcasted so you can tune into it on your radio. The frequency that is broadcasted on is called the carrier frequency. let's pick 89.3Khz because it is a free channel in the Racine area. So we need to generate a frequency at 89.3Khz, and there are multiple ways of doing this. After establishing our carrier frequency, the modulated audio signal needs to be combined with the carrier frequency using a mixer. Then that signal needs to be filtered again to reduce any noise that will interfere with the carrier frequency.
So far it is known that the frequency of tank circuit can be changed by tuning capacitance and inductance so that it resonates at a specific frequency. So how does a tank circuit actually resonate at a specific frequency? The only two things that go into determining the resonant frequency are capacitive and inductive reactance.
Capacitive reactance is basically the amount of electrical resistance that a capacitor has on the circuit. The relationship between resistance and frequency is not inversely exactly proportional, but generally, as the frequency increase, the resistance decreases. In order to understand why this happens, we need to understand how a capacitor works. Generally, a capacitor stores electrical energy over a period of time, and as time increases the charge decreases. So the DC voltage will drop as the time increase, and when a capacitor is fully charged the resistance is relatively low because the capacitor does not need to charge up. Because a tank circuit uses AC, the voltage is sinusoidal, so it is not constant and therefore the capacitor will alway have a resistance. As the frequency goes up the two peaks of the sin wave become closer together so the voltage is more constant across the capacitor. This means that the capacitor does not have the time to discharge, so it's resistance is much lower at a higher frequency. As the frequency goes down the wavelength of the sinusoidal wave increases, and the charge of the capacitor will drop more because of the increased time between peaks. This results in a higher energy loss because the capacitor needs to be charged back up to maximum in order to efficiently pass through. This is happening very fast, in the order of thousands of times per second.
Next is inductive reactance which is the inverse of capacitive reactance as seen below by the function XL, and unlike a capacitor it is linear. The inductor is simply a few turns of copper wire that create an inductive load, in the order of several henrys (henrys being the unit of inductance). At low frequencies, inductors behave like a closed circuit, a direct connection with nearly zero resistance, but as the frequency increase the resistance increases. This is because of how inductors react at a higher frequency, this results in an increase in resistance.
Because the two graphs are inverses they cross at one specific point and this corresponds to a specific frequency, as well as a specific impedance (resistance). At this frequency the resistance is the least, resulting in a dramatically higher voltage (using ohm's’ law). As the frequency approaches zero, for the graph of capacitive reactance, the resistance will become infinitely small. This also happens to the inductive reactance, as the frequency increases the inductance will approach infinity. So by using the intersection of the two graphs the resistance will be at its minimum value. When plotting a graph of voltage vs. frequency, there will be a large, dramatic rise in voltage. Then after the resonant frequency is reached the voltage will drop again, due to an increase in impedance. This is similar to the impedance vs. frequency graph, it results in a drop in impedance at the resonant frequency. The two graphs are inverses, so as the voltage rises the resistance drops and vice versa.
Resonance is one of the largest underlying principles that define radio technology, making it possible to have a number of different channels. Each channel resonants at a specific frequency. For example, if you wanted to listen to an FM channel at 102 MHz, then you would tune the capacitor in you care to a different value changing the resonance of the circuit, then your speakers would play what is on that channel.
