TEST & MEASUREMENT HANDBOOK
The complexity of wireless receiver tests ERIC HSU | KEYSIGHT TECHNOLOGIES , INC .
The crowded spectrum of today’s RF environment puts a premium on quantifying the performance of radio receivers.
THE DEMAND FOR wireless communications now challenges the physical limitations of today’s wireless communications systems. Interference can easily arise when systems operate in a crowded wireless environment using a shared spectrum. Signal congestion makes the process of designing, testing, and isolating system problems more complex. In the next few years, billions of devices will connect through many different and emerging wireless technologies. Each device may integrate with two or more wireless standards. With many wireless standards using the same unlicensed bands, device manufacturers must verify that neither co-channel nor adjacent-channel interference will degrade their designs. This situation presents challenges to device designers as design and verification testing becomes more complex, time-consuming, and expensive. For example, consider the most commonly used 2.4-GHz industrial, scientific, and medical (ISM) band, which includes wireless standards such as Bluetooth, WI-Fi, and ZigBee. These longtime standards enjoy broad support in both the integrated circuits (ICs) and integrated modules that are built into IoT devices. Co-existence in the unlicensed band comes with a price. Bluetooth uses the frequency-hopping spread spectrum (FHSS) technique, and Wi-Fi uses direct sequence spread spectrum (DSSS) and orthogonal frequencydivision multiplexing (OFDM) as a way to increase resistance to interference. Furthermore, Bluetooth enhanced the FHSS with the adaptive frequency hopping (AFH) to resist interference in the 2.4-GHz ISM band. Wi-Fi added the dynamic frequency selection (DFS) to avoid interference with radar signals in the 5-GHz band. Designers must take various interfering signals into account when evaluating the receiver performance of wireless IoT devices. Consider a digital radio receiver. First, the receiver must extract the RF signal in the presence of potential interference. A preselecting filter, the first component of the receiver, attenuates out-of-band signals received from the antenna. A low-noise amplifier (LNA) then boosts the desired signal level while minimally adding to the noise of the radio signal. Next, a mixer down-converts the RF
A real-time spectrum analysis at the 2.4 GHz ISM band illustrates the crowded spectrum in this band with multiple Bluetooth and Wi-Fi devices simultaneously enabled.
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DESIGN WORLD — EE NETWORK
6 • 2020
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