Telecommunication Basics Part 1: Filters

In December 2012, I was award an MS in ECE. My focus was in telecommunications. After learning this, an acquaintance asked me to explain what telecom is. I think I might have gotten the highlights of the field across to her, but it wasn't as easy as I had hoped. This series of posts is dedicated to explaining the basics of telecom as simply as possible.


Filters are at the heart of telecommunications. Without filters, there would be no internet, cell phones, AM/FM radio, or TV. Filters are electrical circuits that let you pick out a specific signal you're interested in and ignore everything else.

To illustrate how filters work, let's imagine a simple scenario. Alice the human and her dog Axel both know Morse code and want to send messages to Bob the human and his dog Buddy.

Alice and Axel are impatient; they want to send their messages at the same time. Alice gets a whistle and sends her message in Morse code. Axel does the same thing, but sends his message with a dog whistle. The dog whistle makes a sound at a frequency that is too high for humans to hear; however, both dogs and humans can hear the human whistle.

Since Bob cannot hear Axel's dog whistle, he is able to focus on Alice's message and decode it: "Alice says hi!" Buddy is out of luck. He can hear both whistles at the same time, so it's hard for him to make out what Axel is trying to tell hi.

The reason Alice's message gets through but Axel's doesn't is because human ears are low-pass filters. They filter out any sound that is too high in frequency. This let's Bob ignore Axel's message and easier decode Alice's message. Buddy's ears are also low-pass filters, but with a higher cutoff frequency. Buddy's ears are not tuned to accept Axel's message and reject Alice's message. He hears both messages, and they drown each other out.

When you change the station on your radio, you're actually changing the frequencies that your radio is filtering out. Tune in to station 101.5 MHz, and your radio filters out all the signals that are higher or lower in frequency. That's why you only hear one radio station at a time even though all the radio stations in your area are broadcasting simultaneously.

Without filters, only one message could be sent at a time. Filters allow us to send lots of different messages at the same time. This means are can use filters to send messages to lots of different people (like with cell phones) or we can send lots of data very quickly to one person by sending the first half and the last half of a message at the same time on two different frequencies.

Filters in Practice

If you look at the theoretical throughput for IEEE 802.11ac (otherwise known as WiFi), you'll notice the speed goes up with the channel width. From 20 MHz up to 160 MHz, the data rate is proportional to the channel width. This is because the wider channels let WiFi devices send more messages at the same time. It's just like adding more lanes to a highway.

Filters enable you to focus on the messages that are directed to you. Bandwidth determines how many simultaneous streams of data can be sent.

Photo of the Arecibo Observatory by Ed Ivanushkin