Have you ever wondered exactly what is going on in the 2.4GHz WiFi and ISM band around your house. What channel is it best to set your wireless router to? Why are you getting such poor performance across your WiFi network? Is your neighbour on the same frequency?
Just what is out there? This neat little gadget will sniff the airwaves and give you a graph of the signal strength vs frequency across the entire band. It connects to your computer by USB 2.0 and with the companion Windows software you can display the spectrum or save the raw data to an Excel compatible file for some more number crunching.
The 2.4GHz Band
The 2.4GHz ISM (Industrial Scientific Medical) band is often called the WiFi band because it is used for WiFi networking (ie, 802.11 b/g/n). This band is unlicensed, meaning that you and anyone can transmit on it. As a result it has been used by a multitude of products including video transmitters, portable telephones, Bluetooth devices, wireless keyboards, toys and so on. Because you cannot see what is going in the band on you can experience strange behaviour from your wireless gadget. All of a sudden your wireless keyboard skips characters, is it because someone is using a portable phone on the same frequency?
The biggest victim is WiFi networking. This needs a lot of bandwidth, is always transmitting and is sensitive to interference. This is why people often cannot get a decent range from their wireless network and give up in disgust.
This scanner will draw a graph on your computer screen showing you the activity across the band and indicate the best frequencies to use. If you use a laptop you can also wander around and identify the culprits that are clogging the airwaves.
How It Works
Internally the scanner is very simple. It just contains a radio receiver and a microcontroller…
The radio receiver is the Cypress CYWUSB6935 Radio SoC (System on a Chip). This is a complete low power radio transmitter/receiver chip for the 2.4GHz band and is controlled by a microcontroller over a synchronous serial (SPI) interface. The microcontroller can write to various registers in the chip to set things like operating frequency and can read other registers to retrieve data from the chip.
This chip is designed to operate over the 2.4GHz band and has the ability to listen on a frequency for any other devices that may be already using the frequency. This is to help the microcontroller select a suitably free frequency before transmitting. The chip reports the signal level as a number typically up to 30, with zero representing no signal. We use this facility in this project – simply put, the microcontroller instructs the module to step to a frequency and measure the signal level at that frequency, when done it steps the chip to the next frequency and instructs it to measure the signal level there. And so on, right across the band.
We actually do not use the transmit/receive function, which is normally the chip’s main purpose in life.
The microcontroller used in this project is the Microchip PIC18F2550 which integrates the complete USB 2.0 functionality. The microcontroller sets the radio chip to a frequency, reads the signal level from the chip, stores the value in its internal memory and steps on to the next frequency. This continues until the complete 2.4GHz band is covered. The 18F2550 then sends the data off to your computer using USB and your computer, using custom software, displays the resultant spectrum.
Physically the scanner is just a small box hanging on the end of a USB cable.