2.1 The EM Spectrum
The Electromagnetic Spectrum
Now that we have introduced the time and frequency domains, let’s take a closer look at electromagnetic radiation and the electromagnetic spectrum.
Electromagnetic Radiation is a form of energy that is carried by synchronized oscillating electric and magnetic fields. Electromagnetic radiation is unique in that its actions can be explained by theories that are based on both waves and particles. Electromagnetic radiation also travels without a medium. Waves on the ocean require water in order to exist. Sound waves require air to propagate. Neither of these waves can travel through a vacuum. But, electromagnetic waves can. In fact, they travel through the vacuum of space at the speed of light.
Recall that a wave can be described by its frequency of oscillation. Electromagnetic waves are no different and they cover quite a broad range of frequencies.
Fun Fact: There are no known physical limits on maximum and minimum frequencies in nature.
Frequencies are grouped into bands based on similarities in their physical traits or specific applications. Some frequency bands travel through the Earth’s atmosphere with less loss. Some are more useful for a particular application and are set aside for experimentation, while some bands have more than one purpose. Two common frequency bands to note are light and radio.
Visible Light is defined as electromagnetic radiation having wavelengths from 400 to 700nm (1nm is 1x10-9m). This is equivalent to frequencies from 5x1014Hz to 1x1015Hz, although wavelengths are traditionally used when discussing light. Electromagnetic radiation having wavelengths (or frequencies) in this band are visible to a human observer.
Radio frequency (RF) band of electromagnetic waves have frequencies from 8.3kHz (104 Hz) to 300GHz (1011Hz).
The full electromagnetic spectrum and the RF band are shown below:
The RF band is useful for many industries and applications:
- Direct audio communications (cell phones, mobile radios, FM radio, AM radio)
- Device communications (wireless keyboard, WiFi hotspot, game controller)
- Interplanetary Studies (giant radio telescope at the Arecibo Observatory in Puerto Rico, SETI Institute in California, etc.)
Within the RF band, specific frequencies are dedicated to communication and broadcast open to anyone:
- Citizens Band (CB)
- Industrial, Science, and Medical Band (ISM)
- Other unlicensed communications bands.
Others, like Amateur Radio Band (a.k.a Ham Radio), commercial AM and FM radio, and TV are licensed channels that are specifically allocated or rented by individuals or corporations for a particular use. Licensed broadcast channels are monitored very closely by the national government and the channel licensee in order to ensure that the broadcasts maintain certain content and physical transmission criteria. In the USA, the Radio Spectrum is regulated by the Federal Communications Commission (FCC).
Below is a short video showing wired and wireless signal transmission
Video: Wireless RF_OBSDownload
Electromagnetic Interference (EMI)
Electromagnetic Interference is also important to understand. Many devices, classified as intentional radiators, are designed to transmit and receive RF signals e.g. FM radios, WiFi routers, wireless keyboards. However other devices are not specifically intended to create RF signals. These are classified as unintentional radiators and are the primary source of electromagnetic interference (EMI).
EMI is RF noise. An unintentional radiator creates RF radiation that is not intended to communicate, control, or deliver any relevant information. Some designs exhibit less noise than others, but all manufacturers are concerned with unwanted RF Noise. In order to control and maintain a safe operating environment, governments regulate the amount of acceptable EMI that a design or product can have.
Note: Products outside of the limits set forth by the regulations can bring heavy financial penalties to offending individuals or companies.
When performing experiments and development with RF, it is very important to understand the requirements of working within a specific frequency band. If you are working within a licensed or restricted band, make sure to research how to do that safely and work within the regulations for that band.
Below is a short video showing what interference can look like on a Spectrum Analyzer
A filter is a useful component in many designs. The primary goals of a filter are to remove unwanted frequencies and to enhance desired frequencies from an input signal. Here are some common filters and their descriptions:
Low pass - Allow frequencies below a certain value to pass through, and reject higher frequencies. This can be used to remove high frequency noise from a signal.
High pass - Allow frequencies above a certain value to pass through, and reject lower frequencies. This can be used to remove low frequency components from the input signal.
Band pass - Allow a certain frequency range, or band, to pass through the filter and reject those frequencies that are outside of the operating frequency band. This type of filter will allow a band of frequencies to pass through the filter with little to no changes while drastically lessening the amplitude of signals outside of the operating band.
Notch - Reject those frequencies that are inside of the operating frequency band and reject those within the operating band of the filter. This type of filter will allow all frequencies outside of the operating band to pass through the filter with little to no changes while drastically lessening the amplitude of signals having frequencies inside the band. (opposite of a Band pass filter).
Our previous discussions on the time/frequency domains and the electromagnetic spectrum provide a base for knowledge of RF. In the following sections, we introduce basic RF signal generation and measurement instrumentation with a focus on typical RF component tests, broadcast/radio monitoring, and EMI.