Wi-Spy’s approach to spectrum analysis is a bit different than other spectrum analyzers. Regular spectrum analyzers like Agilent and Tektronix contain Super Heterodyne Receivers to measure an entire RF band at whatever arbitrary resolution you’d like. This is extremely powerful… and expensive. The Airmagnet Spectrum XT uses a Wi-Fi radio to measure a 20 MHz band at twice the resolution of the channel subcarriers (156.3 KHz). This provides a fixed resolution aimed at measuring 802.11n signals.
Wi-Spy, on the other hand, uses a narrowband RF receiver to scan across the RF band in tiny steps. These steps can be anywhere from 25 KHz to 400 KHz wide. While Wi-Spy can’t scan at arbitrary resolutions like an Agilent spectrum analyzer, it has about a dozen resolution bandwidth settings ranging from about 50 Khz to over 600 KHz (exact values depend on Wi-Spy model and RF band). Currently Chanalyzer automatically adjusts the Wi-Spy settings when you change frequency bands, but finer-grained control is coming soon in Chanalyzer Pro and Chanalyzer Lab (currently in private beta).
Here are screenshots showing the 2.4 GHz band at the smallest and largest step sizes of the Wi-Spy 2.4x.
The variable step size and resolution bandwidth allow Wi-Spy to be optimized for different bandwidths and signal types from 802.11n to Bluetooth and Zigbee. The default Wi-Spy settings scan the 2.4 GHz band using a step size of 333.3 KHz. This is ideal for detecting Bluetooth, which uses 1 MHz channels. For scanning the entire 5 GHz band, Wi-Spy DBx uses a large step size to decrease the sweep time (time it takes for each scan of the RF band). As you zoom into smaller sections of bandwidth, Wi-Spy shrinks its step size to provide higher resolution. For detailed Wi-Fi scanning, Wi-Spy can be configured to scan at six times the resolution of the channel subcarriers!
Here’s a high resolution scan of a 40 MHz 802.11n channel.
This powerful hardware configurability is one of the many reasons why wireless professionals choose Wi-Spy more often than any other spectrum analyzer.
By Ryan Woodings (Metageek “Chief Geek”), reproduced with permission