RF Signal Measurement with STMicro’s STEVAL Board

2017-10-17 | By All About Circuits

License: See Original Project

Courtesy of All About Circuits

There are a lot of invisible radio frequencies flying around. Let’s take a look and explore what they are and how they can be measured.

BOM:

• ST Microelectronic's STEVAL IDB1007V1 Development Board
• Smartphone with Bluetooth capabilities

Technology is a part of our everyday lives, and more and more devices utilize wireless technology with radio frequencies (RF). With all of these wireless devices, RF signals are all around connecting us to one another. How can you measure these signals? With a dev board and an app on one of your wireless devices, of course!

How Does it Work?

RF communication is used to wirelessly transfer data and information with electromagnetic radiation. Time-varying electrical signals generate electromagnetic energy which distributes in waves. This technology is used every day each time a phone is connected to a car or a wireless headset is connected to a gaming console.

In this project, we used ST’s STEVAL board, which makes it easy to measure the strength of their Bluetooth signal in a variety of environments to see how these signals interact with the outside world. Basically, it makes it much easier to visualize RF signals.

STEVAL-IDB1007V1

Bluetooth is one form of RF communication. Bluetooth Low Energy (BLE) is Bluetooth that provides RF communication with reduced power consumption. It commonly is used with devices that periodically transfer small amounts of data.

RF signals can be affected by all of the materials and objects in the world. It’s the reason that, for instance, service gets lost in a tunnel. A satellite signal doesn’t have the strength to permeate layers of earth and cement. It’s important to note that, even without obstacles, RF signals don’t travel indefinitely.

All electromagnetic radiation follows the inverse-square law. This law states that signal intensity decreases with the square of distance from the source. Basically, signals lose strength the further they travel. This applies to even the smallest increment.

To look at these signals, we’ll use STEVAL’s app by connecting to the board on our phone via Bluetooth and use its RSSI (received signal strength indicator) to test different signal strengths. When the board is right next to my phone I get a reading of -42 dBm, so I’ll use this number as my reference point. Moving the board a few inches away dropped the signal to -57 dBm, which reiterates the inverse-square law.

To further test its durability through materials, I tried putting the board in my shoe and stuff the shoe with clothing. Doing so gave a reading of -51 dBm, only around 10 dBm below my initial reading. Not too shabby. So I discovered that the signal seemed to travel well through porous materials. Now onto the next level: a Faraday cage. If you don’t live in a lab, a solid option is a household microwave.

After unplugging the microwave, I placed the board inside and closed the door tight. I expected to see an instant loss of communication so imagine my surprise to see a reading of -78 dBm. While this is a significant attenuation in signal, it’s clearly not a complete loss. I can’t be sure if this speaks more to the Bluetooth signal or my microwave’s lack of protection.

Inside the STEVAL app

With the help of the STEVAL app, we got to visualize the strength of these signals. Note that not all signals interact exactly like Bluetooth, the RF interferes with the world around us, lessening signal strengths, much like a phone in a tunnel.