Avoid the Pitfalls Associated with IoT RF Receiver Testing
2019-09-11 | 4 min read
The demand for ubiquitous wireless communications challenges the physical limitations of current wireless communications systems. When IoT systems operate in a crowded wireless environment on the same shared spectrum, interference between the systems can occur. This makes the process of designing, testing, and isolating system problems more complex. Table 1 below illustrates the key IoT wireless standards and their performance characteristics.
Table 1. IoT wireless connectivity standards performance attributes
For example, consider the most commonly used 2.4-GHz ISM band, which includes wireless standards such as Bluetooth, WiFi, and ZigBee. These long-time standards enjoy broad support in both the ICs and integrated modules that are built into IoT devices. Figure 1 illustrates a crowded 2.4-GHz industrial, scientific, and medical (ISM) radio band with multiple Bluetooth and WiFi devices simultaneously enabled. Co-existence in the unlicensed band comes with a price. Bluetooth uses frequency-hopping spread spectrum (FHSS) technique, and WiFi uses direct sequence spread spectrum (DSSS) and orthogonal frequency-division multiplexing (OFDM) to increase resistance to interference, noise, and jamming.
Furthermore, Bluetooth enhanced the FHSS with the adaptive frequency hopping (AFH) to resist interference in the 2.4 GHz ISM band. WiFi added the dynamic frequency selection (DFS) to avoid interference with radar signals in the 5 GHz band. You need to take various interfering signals into account when you evaluate the receiver performance of wireless IoT devices.
Figure 1. Real-time spectrum analysis at 2.4-GHz ISM band
Receiver design is challenging because the wireless device needs to handle a wide variety of input signal conditions and these conditions are difficult to predict. In addition, you need to inject noise and various interfering signals in order to characterize the receiver’s performance. Most of the wireless specifications for receiver tests include minimum input sensitivity, dynamic range, adjacent and alternate channel selectivity, intermodulation immunity, spurious emissions, and blocking. To learn more about receiver testing, download the application note “Testing and Troubleshooting Digital RF Communications Receiver Designs.”
For long-distance wireless communications, the multipath signals may add up constructively or destructively at the receiver, and the Doppler effect causes frequency shift at the receiver as well. The effects of multipath and Doppler shift cause linear distortions that can be reduced with an adaptive equalizer of a receiver. In addition, systems’ channel coding and antenna diversity can also reduce the effects. Like the receiver test, test specifications indicate sensitivity or throughput tests under the specific channel conditions.
With multiple wireless standards using the same unlicensed bands, IoT device manufacturers need to verify that neither co-channel or adjacent-channel interference will adversely impact their designs. This situation presents challenges to device designers at a time when design and verification testing is growing more complex, time-consuming, and costly. Whether you are working on a single radio format or integrating multiple formats into an IoT device, easy access to the right test signals streamlines validation and helps ensure interoperability.