RF Foundations
The Electromagnetic Spectrum
Electromagnetism
Imagine a simple DC circuit. We have a battery, bulb, and a switch. In the off position, electrons are stationary and generating an electrical field. The battery’s chemical reaction has pushed excess electrons to the (-), leaving the (+) with a deficit. When the circuit closes, the electric field pushes the electrons through the circuit, and back to the (+) side of the battery. As these electrons move through these electric fields, their motion creates a stable magnetic field. The creation of magnetic fields via electron movement and vice versa is the basis of electromagnetism. The faster they move, or radiate, the higher the frequency. Starting in order lowest to highest for non-ionizing radiation we have radio, microwave, infrared, and visible light. For ionizing radiation(can remove electrons from atoms,changing DNA)we have ultraviolet, followed by x-ray, and gamma.
Now replace your battery with a generator. Instead of a one way flow of electrons, they’re rapidly changing direction. They’re vibrating, radiating. They aren’t flowing negative to positive, there is no drift, they’re bouncing and transferring energy electron to electron. Depending on how quickly this directional change occurs determines frequency or cycle(HZ). Like in our DC circuit, when that electric field changes, so does our corresponding magnetic field. Instead of electrons moving side to side, see them as rising up and down, like an actual wave. The more precise these ossiclating electrons move, the cleaner the attached wave.
Sound behaves similarly. A pure sine wave is a dull, consistent tone, because it’s operating at a pure consistent frequency. When you pick an Open A(110hz) on an electric guitar, the string vibrates, which disturbs the copper wire wrapped magnets in the pickup, creating an electric field. The guitar is a generator. The vibrating string creates our first frequency(110hz)called our fundamental, followed by a series of additional frequencies called overtones due to resonance from the instrument’s makeup. These overtones are what differentiate one instrument from another. A guitar from a piano, the saxophone from a fiddle, or a guitar from another guitar. Overtones are our fundamental’s voice. Without overtones, sound has no defining character or timbre.
Analog Modulation, Frequency, and Amplitude
Theoretically, you could hook an electric guitar up to an antenna and “transmit” an electromagnetic signal. The guitar would just have to be massive. Pickups only generate a few millivolts and even with that, we have no way to tune a guitar to an antenna’s matched frequency consistently(remember those overtones). We need a way to carry, amplify, and tune that electrical series of waves coming from the guitar lead. To transmit effectively we need a carrier wave, a high frequency, pure sine wave that can be tuned precisely to transmit the signal. So, we start transmitting this high frequency wave, then our guitar signal gets inputed into this wave and modulates it, creating a series of extra frequencies. These extra frequencies are knownn as sidebands. Sidebands are to a carrier wave what overtones are to a fundamental. They’re extra frequencies which give the carrier it’s voice. Without sidebands, we would just have a sine wave.
Amplitude Modulation
Amplitude Modulation relies on input adjusting the wave’s amplitude, peak to peak. Our carrier is at a set amplitude/frequency and whenever we pick our note, the carrier and input combine, like a guitar string vibrating off the body of the guitar, creating sidebands wwith varying frequencies. These sidebands, upper and lower, these then interfere with our carrier wave, adjusting the amplitude. Our receiver will tune to the carrier frequency, filter out unwanted frequencies, and measure the shape of the changes of the carrier’s amplitude(envelope), which will be decoded for our output. Since our signal is dependent on amplitiude and not frequency, AM is more susceptible to interference from other electromagnetic waves(motors, generators, lightning, powerlines, etc.). The major benefit with AM is it’s longer transmission range at a cost of quality.
Frequency Modulation
Frequency modulation relies on the input adjusting the wave’s frequency, not it’s amplitude. Our carrier sits at a constant amplitude, but when we play our note, it causes the sine wave to compress and stretch, changing how quickly it osscilates. The frequency changes depending on the input, but the overall amplitude stays fixed. The receiver locks on the carrier’s center frequency, filters out everything else, and tracks the rate of fluctuation in the wave’s frequency. That variation is decoded to reconstruct our original signal. Unlike AM, FM isn’t as prone to EMI. Due to it relying on frequency to carry the signal, the only thing interfering would be nearby frequency’s within that range and overpowering(high amplitidude) the signal. Due to FM’s short wavelength(high frequency)it is known for having higher quality at the cost of range.