In this blog post, we’ll explore the principles behind how radios and TVs select and receive the desired broadcast signal from among various frequencies, the basic concepts of filtering, and how a simple low-pass filter works.
Today, many people watch TV or listen to the radio. To receive a broadcast, you must select and receive radio waves within a specific frequency band transmitted by a broadcast station. In the case of radio, each station uses a designated frequency band, so you must select that specific frequency to listen to the desired program. Although it is difficult to realize that TV also directly selects frequencies because it uses channel numbers, TV likewise receives broadcast signals using specific frequency bands of radio waves. To receive a broadcast, you must select and receive the signal from the desired frequency band among many frequencies, and the technology used in this process is filtering. By selectively receiving signals within a specific frequency band—among the many radio waves transmitted by broadcast stations—and suppressing the remaining signals, radios and TVs allow you to listen to the desired broadcast.
When you throw a pebble into water, ripples spread out in concentric circles. Frequency refers to the number of times a wave repeats at a constant interval, measured per second, and is expressed in hertz (Hz). Electromagnetic waves also exhibit wave properties similar to these, and the electromagnetic waves generated or utilized by electronic devices around us each have their own specific frequencies. Therefore, when receiving a broadcast, you must select the desired signal from among the frequency bands of the radio waves transmitted by the broadcasting station. The reason a radio can select and listen to a specific station among many is precisely because this principle of frequency selection and filtering is applied.
Filters are used when we want to select and listen to a specific frequency. While there are various types of filters in actual use and their designs are complex, they all share a similar basic principle: they allow signals within a specific frequency range to pass through while attenuating or blocking signals in other ranges. Let’s take the low-pass filter—one of the simplest types of filters—as an example. A low-pass filter is a circuit that allows signals in the low-frequency range to pass through while relatively attenuating signals in the high-frequency range.
A low-pass filter can be simply constructed using a resistor and a capacitor. For example, consider an RC low-pass filter with a 1 kΩ resistor and a 10 nF capacitor. A resistor is a component that impedes the flow of current, while a capacitor is a component that stores electrical energy. A capacitor consists of two conductors facing each other with an insulating material in between; while it makes it difficult for current to pass directly, it can store electrical energy. To illustrate this using an analogy involving people moving through a space, a resistor is similar to a narrow passageway that people cannot pass through all at once, while a capacitor can be thought of as a space where people can linger for a moment.
As the frequency of an AC signal increases, the capacitor responds more quickly to voltage changes, causing it to bypass the signal more effectively. Conversely, when the frequency is low, this effect is relatively smaller. Therefore, in a low-pass filter, low-frequency signals are transmitted relatively well to the output, while high-frequency signals are bypassed through the capacitor and are significantly attenuated at the output. To use a wave analogy: at high frequencies, the crests and troughs of the wave repeat rapidly, allowing the capacitor to effectively capture these rapid voltage changes; at low frequencies, since the changes are slow, the signal is transmitted more effectively toward the output. Ultimately, in a low-pass filter, the signal is attenuated more as the frequency increases, and it is relatively well preserved as the frequency decreases.
When a high-frequency signal passes through this circuit, it is significantly attenuated at the output, whereas a low-frequency signal experiences only relatively minor attenuation, allowing signals in the low-frequency range to pass through. If a specific frequency component of an electrical signal is maintained at a sufficient amplitude, the signal containing that component is transmitted to the output; however, if a specific frequency component is significantly attenuated by the filter, that component becomes weaker at the output. Low-pass filters utilize this characteristic to allow low-frequency components to pass through while attenuating high-frequency components. Thus, the fundamental role of a filter is to selectively allow signals in the desired frequency range to pass through or to attenuate signals in the unwanted frequency range.
