Wednesday, March 29, 2023

How Not to Run Hot: Overcoming Thermal Challenges in Wi-Fi Front-End Designs (Part 1)

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  • Higher modulation schemes. To help achieve higher capacity and data rates, Wi-Fi designs are moving from 256 QAM to 1024 QAM modulation schemes. With 1024 QAM modulation, there are more bits per symbol — 10 bits per symbol versus 8 bits in 256 QAM. However, as the data rate increases, EVM on the RFFE becomes a principal concern. The constellation is so dense in 1024 QAM that the processor must use sophisticated system decoding to distinguish each point. When the processor works harder, the unit device heat increases.
  • How the RFFE performance affects the overall current draw on the system processor. Poor RF front-end performance means the processor will have to work harder to meet overall system requirements. Working the processor more increases system hardware heat.

#2: What about the RF switch and low noise amplifier (LNA)?

In the switch, insertion loss can also generate excess heat. When insertion loss increases and signal strength is lowered, the PA works harder to compensate and push higher outputs, which degrades efficiency. And less efficiency means more heat from the device. Using high-linearity, low-loss switches keeps the insertion loss within specifications across the entire band.

Receive throughput is highly dependent on the LNA gain and noise figure. Although the LNA doesn’t contribute significantly to heat generation, the effect of heat on the LNA can drastically affect throughput. Heat degrades the noise figure, and depending on the circuit design and choice of wafer technology, the compensation for this can lead a designer to a specific solution.

#3: Finally, the filters

RF filters drift to the left or the right due to changes in temperature, as shown in the following SAW vs. BAW figure. These shifts can cause high insertion loss on the band edges, which could cause a low gain or POUT response from the RFFE. If the filter drifts too much (as shown in the SAW figure), then the PA pushes more power output to compensate for the insertion loss. This increases current and decreases system efficiency.

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Using filters with high insertion loss can decrease linearity and increase the RF chain POUT. One big advantage of Qorvo’s LowDrift™ bulk acoustic wave (BAW) filters is their stability over temperature shifts. Diplexers, bandpass filters and coexistence filters that use BAW technology with lower temperature drift help mitigate insertion loss, and lead to good product thermals.



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